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doc/drawvg-reference.texi
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@documentencoding UTF-8

@settitle drawvg - Language Reference
@titlepage
@center @titlefont{drawvg - Language Reference}
@end titlepage

@top

@contents

@macro codeexample {block}
@cartouche Example
\block\
@end cartouche
@end macro

@macro vgscmd {name}
@ref{cmd_\name\,,@code{\name\}}
@end macro

@chapter Introduction

drawvg (@emph{draw vector graphics}) is a language to draw
two-dimensional graphics on top of video frames. It is not intended to
be used as a general-purpose language. Since its scope is limited, it
prioritizes being concise and easy to use.

For example, using the
@uref{https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API,Canvas
API} we can render a triangle running this code in a Web browser:

@example
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");

ctx.beginPath();
ctx.moveTo(125, 50);
ctx.lineTo(100, 100);
ctx.lineTo(150, 100);
ctx.closePath();
ctx.stroke();
@end example

The same triangle can be written with this drawvg script:

@example
moveto 125 50
lineto 100 100 150 100
closepath
stroke
@end example

It can be shortened using the aliases for @vgscmd{moveto}, @vgscmd{lineto},
and @vgscmd{closepath}:

@example
M 125 50
L 100 100 150 100
Z
stroke
@end example

Both newlines (@code{U+000A}) and spaces (@code{U+0020}) can be used
interchangeably as delimiters, so multiple commands can appear on the
same line:

@example
M 125 50 L 100 100 150 100 Z
stroke
@end example

@macro ffexprs
@ref{Expression Evaluation,,FFmpeg expressions,ffmpeg-utils}
@end macro

Finally, drawvg can use @ffexprs{} and frame metadata in command arguments. In
this example, we are using the variables @var{w} (frame width) and @var{h}
(frame height) to create a circle in the middle of the frame.

@example
circle (w / 2) (h / 2) (w / 3)
stroke
@end example

Many commands are a direct equivalent to a function in the
@uref{https://www.cairographics.org/,Cairo graphics library}. For such
commands, the reference below provides a link to the related Cairo
documentation.

@chapter Syntax

@macro svgpathlink
@uref{https://developer.mozilla.org/en-US/docs/Web/SVG/Reference/Element/path,SVG's @code{<path>}}
@end macro

The syntax is heavily inspired by languages like
@uref{https://imagemagick.org/script/magick-vector-graphics.php,Magick
Vector Graphics}, or @svgpathlink{}. Many command names are taken from
@uref{https://en.wikipedia.org/wiki/PostScript,PostScript}.

@section Structure

A drawvg script consists of a series of commands to describe 2D
graphics.

A command is an identifier (like @vgscmd{setcolor} or @vgscmd{lineto})
followed by its arguments. Each item in the code (command name,
arguments, etc.) is separated by any of the following characters:

@itemize
@item Space (@code{' '})
@item Comma (@code{','})
@item Newline (@code{'\n'})
@item Tabs (@code{'\t'})
@item Return (@code{'\r'})
@end itemize

The beginning of the item indicates how it will be interpreted:

@table @r
@item @code{//}
Comment
@item @code{0}, @dots{}, @code{9}, @code{+}, @code{-}
Number literal
@item @code{(}
Expression
@item @code{@{}, @code{@}}
Block delimiters
@item Anything else
Name of a command, a color, etc.
@end table

@section Comments

Comments start with two slashes (@code{//}), and stop at the end of the
line (either a @code{\n}, or the end of the script).

@example
circle 100 100 50 // this is ignored
fill

// this is also ignored
@end example

@code{//} must appear after a space, or at the beginning of the line. If
@code{//} is preceded by any non-blank character, the parser will
consider @code{//} as part of the previous item.

For example, in this script:

@example
circle 10 10 50// something
@end example

The parser throws an error because it tries to parse @code{50//} as a
number literal.

@section Commands

The way commands are parsed is inspired by @svgpathlink{}:

@itemize
@item
Every command in the script starts with its name, and it is followed by
zero or more arguments.

@item
There are no explicit delimiters between commands or arguments.

Most programming languages expect characters like parenthesis, commas,
or semicolons, to separate items. For example:

@example
moveto(10, 10); lineto(20, 30);
@end example

The equivalent in drawvg is:

@example
moveto 10 10 lineto 20 30
@end example

@item
If the command has no arguments (like @vgscmd{closepath} or
@vgscmd{stroke}), the next command starts at the next item.

@end itemize

@codeexample{
In the next script there are 4 different commands:

@example
newpath rect 10 20 30 40 setcolor teal fill
@end example

@enumerate
@item
@vgscmd{newpath} requires no arguments.

@item
@vgscmd{rect} requires 4 arguments, so it takes the next 4 numbers.

@item
@vgscmd{setcolor} requires 1 argument, so it takes the word @code{teal}.

@item
@vgscmd{fill} requires no arguments.
@end enumerate
}

@subsection Single-Letter Aliases

Most commands in @svgpathlink{} are also present in drawvg. For some of them,
there is an alias to a longer name:

@itemize
@item @vgscmd{curveto} for @vgscmd{C}.
@item @vgscmd{rcurveto} for @vgscmd{c}.
@item @vgscmd{lineto} for @vgscmd{L}.
@item @vgscmd{rlineto} for @vgscmd{l}.
@item @vgscmd{moveto} for @vgscmd{M}.
@item @vgscmd{rmoveto} for @vgscmd{m}.
@item @vgscmd{closepath} for @vgscmd{Z}, @vgscmd{z}.
@end itemize

Other commands only exist in a single-letter form:

@itemize
@item @vgscmd{H}, @vgscmd{h}
@item @vgscmd{Q}, @vgscmd{q}
@item @vgscmd{S}, @vgscmd{s}
@item @vgscmd{V}, @vgscmd{v}
@item @vgscmd{T}, @vgscmd{t}
@end itemize

This makes it possible to use a path in SVG to create the same shape in
a drawvg script.

@anchor{implicit commands}
@subsection Implicit Commands

For many commands, the name can be omitted when it is used multiple
times in successive calls.

In the reference below, these commands has a @emph{Can be Implicit} note
in their signature.

@codeexample {
For example, in this script:

@example
M 50 50
l 10 10
l 10 -10
l 10 10
l 10 -10
l 10 10
stroke
@end example

After the first call to @vgscmd{l} (alias to @vgscmd{rlineto}), the command
can be executed without the name, so it can be written as:

@example
M 50 50
l 10 10 10 -10 10 10 10 -10 10 10
stroke
@end example
}

To reuse the same command (@vgscmd{l}, in the previous example), the
parser checks if the item after the last argument is a numeric value,
like a number literal or a FFmpeg expression.

@codeexample{
In this example:

@example
l 10 20 30 40 stroke
@end example

@vgscmd{l} requires 2 arguments, and can be implicit, so the parser
performs this operation:

@enumerate

@item
Takes the two next items (@code{10} and @code{20}) and emits the first
instruction.

@item
Checks if the item after @code{20} is a numeric value. Since it is
@code{30}, it takes @code{30} and @code{40} and emits the second
instruction (@code{l 30 40}).

@item
Checks if the next item after @code{40} is a numeric value, but it is a
command (@vgscmd{stroke}), so it stops reusing @vgscmd{l}.

@end enumerate
}

This is another feature taken from @svgpathlink{}. An important difference with
SVG is that the separator between items is always required. In SVG, it can be
omitted in some cases. For example, the expression @code{m1-2} is equivalent to
@code{m 1 -2} in SVG, but a syntax error in drawvg.

@section Arguments

Most commands expect numeric arguments, like number literals, variable
names, or expressions.

@vgscmd{setcolor} and @vgscmd{colorstop} expect a color.

@vgscmd{setlinecap} and @vgscmd{setlinejoin} expect a constant value.

@subsection Number Literals

A number literal is an item in the script that represents a constant
value. Any item that starts with a decimal digit (between @code{0} and
@code{9}), a @code{-} or a @code{+}, is interpreted as a number literal.

The value is parsed with
@uref{https://ffmpeg.org/doxygen/trunk/eval_8c.html#a7d21905c92ee5af0bb529d2daf8cb7c3,@code{av_strtod}}.
It supports the prefix @code{0x} to write a value with hexadecimal
digits, and
@uref{https://ffmpeg.org/ffmpeg-utils.html#:~:text=The%20evaluator%20also%20recognizes%20the%20International%20System%20unit%20prefixes,many
units} (like @code{K} or @code{GiB}).

In the next example, all literals represent the same value:

@example
10000
1e4
10K
0x2710
@end example

@subsection Expressions

@ffexprs{} can be used as arguments for any command that expects a numeric
argument. The expression must be enclosed in parenthesis.

@codeexample {
The variables @var{w} and @var{h} represent the width and height of the
frame. We can compute the center of the frame by dividing them by @code{2}:

@example
M (w / 2) (h / 2)
@end example

They can also contain parenthesis (to group operations, to call functions,
etc):

@example
moveto
    ((w + 10) / 2)      // x
    (h / (2 * cos(t)))  // y
@end example
}

The variables @var{n} and @var{t} can be used to compute a value that changes
over time.

@codeexample {
To draw a circle oscillating from left to right, we can use an
expression based on @code{sin(t)} for the @code{x} coordinate:

@example
circle
    (w / 2 + sin(2 * t) * w / 4)  // x
    (h / 2)                       // y
    (w / 5)                       // radius

stroke
@end example
}

Expressions can be split in multiple lines, but they can't contain
comments within them.

@example
moveto   // This is a comment.
    (w   // This is part of the expression, not a comment.
     + h)
@end example

@subsection Variable Names

When an expression is only a reference to a variable, the parenthesis
can be omitted, and the item is just the variable name.

@codeexample {
The next 3 expressions are equivalent: in all cases, they create a
rectangle covering the whole frame.

@example
rect (0) (0) (w) (h)

rect 0 0 w h

rect (0) 0 (w) h
@end example
}

It is possible to create a variable with the same name of a command, and
then use it as an argument. In the previous example, the item @var{h} is a
reference to a variable (frame height), but in other contexts it may be
a command (@vgscmd{h}).

For @ref{implicit commands}, the parser prioritizes
commands over variable names when it has to determine if the command is
reused.

@codeexample {
In this example, the variable @var{c} is used as the first argument in two
calls to @vgscmd{l}. However, only the first one is valid, because in the
second call the parser recognizes @vgscmd{c} as a command.

@example
setvar c 5
l c 10 c 15
@end example

This issue can be fixed by surrounding the start of the second call with
parenthesis:

@example
setvar c 5
l c 10 (c) 15
@end example
}

@anchor{Colors}
@subsection Colors

The color to stroke and to fill paths can be set with @vgscmd{setcolor}.
Its argument has the same syntax for colors in FFmpeg:

@itemize
@item
A @ref{Color,,predefined color name,ffmpeg-utils}.

@item
In @code{#RRGGBB} format.

@item
Optionally, an @code{@@a} suffix can be added to set the alpha value,
where @code{a} is a number between @code{0} and @code{1}.
@end itemize

@example
circle 70 70 60
setcolor #FF0000
fill

circle 170 170 60
setcolor blue@@0.5
fill
@end example

The color can be a variable name. In that case, it must be assigned with
@vgscmd{defrgba}, @vgscmd{defhsla}, or @vgscmd{setvar} and a color.

@example
circle 70 70 60
setvar CustomGreen #22FF44
setcolor CustomGreen
fill

circle 170 170 60
defhsla CustomBlue 200 0.7 0.5 1
setcolor CustomBlue
fill
@end example

The commands @vgscmd{setrgba} and @vgscmd{sethsla} allow setting colors using
expressions. Similar to @vgscmd{defrgba} and @vgscmd{defhsla}, but with no
intermediate variable.

@subsection Constants

The argument for @vgscmd{setlinecap} and @vgscmd{setlinejoin} is an
identifier referring to a constant value.

@example
setlinecap round
@end example

@chapter Guide

@section Paths

A path is a complex shape, composed by lines and curves, that can be
used to fill a region, to stroke an outline, or to establish a clip
region.

In order to draw anything on top of a video frame, first we have to
define a path, and then use @vgscmd{stroke} or @vgscmd{fill}.

The
@uref{https://developer.mozilla.org/en-US/docs/Web/SVG/Tutorials/SVG_from_scratch/Paths,tutorial
on paths in MDN} is a good introduction to the topic. It is focused on
@svgpathlink{}, but the same concepts can be applied in drawvg.

@anchor{current point}
@subsection Current Point

Some commands require a @emph{current point}. Initially, the
@emph{current point} is set to
@uref{https://en.wikipedia.org/wiki/NaN,@code{NaN}}. It is initialized
with @vgscmd{M} or @vgscmd{moveto}. Other commands, like @vgscmd{lineto} or
@vgscmd{curveto}, updates the @emph{current point} to the new end of the
shape.

The @emph{current point} can be cleared with @vgscmd{newpath}. Commands
that clear the path, like @vgscmd{stroke} or @vgscmd{fill}, also clear the
@emph{current point}.

@codeexample {
@vgscmd{rlineto} uses coordinates relative to the @emph{current point}.

Given this script:

@example
moveto 20 100
rlineto 150 -90
rlineto -50 200
closepath
stroke
@end example

These are the coordinates of the @emph{current point} after executing
each command:

@multitable @columnfractions .5 .5
@headitem Command @tab Current Point
@item @code{moveto 20 100} @tab @code{20, 100}
@item @code{rlineto 150 -90} @tab @code{170, 10}
@item @code{rlineto -10 50} @tab @code{140, 210}
@item @code{closepath} @tab @code{20, 100}
@end multitable

The same script can be written with single-letter aliases:

@example
M 20 100 l 150 -90 -50 200 z stroke
@end example
}

@subsection Defining a Shape

A path is defined by adding lines, curves, or basic shapes.

@itemize
@item Basic shapes

@itemize
@item @vgscmd{circle}
@item @vgscmd{ellipse}
@item @vgscmd{rect}
@item @vgscmd{roundedrect}
@end itemize

@item
Lines
@itemize
@item @vgscmd{M}, @vgscmd{moveto}
@item @vgscmd{m}, @vgscmd{rmoveto}
@item @vgscmd{H}, @vgscmd{h}
@item @vgscmd{V}, @vgscmd{v}
@item @vgscmd{L}, @vgscmd{lineto}
@item @vgscmd{l}, @vgscmd{rlineto}
@item @vgscmd{Z}, @vgscmd{z}, @vgscmd{closepath}
@end itemize

@item
Curves
@itemize
@item @vgscmd{arc}, @vgscmd{arcn}
@item @vgscmd{C}, @vgscmd{curveto},
@item @vgscmd{c}, @vgscmd{rcurveto}
@item @vgscmd{Q}, @vgscmd{q}
@item @vgscmd{S}, @vgscmd{s}
@item @vgscmd{T}, @vgscmd{t}
@end itemize

@end itemize

Single-letter commands are taken from @svgpathlink{}.

@anchor{fill rules}
@subsection Fill

The region within the shape defined by a path can be filled with
@vgscmd{fill} or @vgscmd{eofill}. Each command uses a different
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-fill-rule-t,fill
rule}:

@itemize
@item
@vgscmd{fill} uses the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#CAIRO-FILL-RULE-WINDING:CAPS,winding
rule}, also known as
@uref{https://en.wikipedia.org/wiki/Nonzero-rule,nonzero rule}.
@item
@vgscmd{eofill} uses the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#CAIRO-FILL-RULE-EVEN-ODD:CAPS,even--odd
rule}.
@end itemize

@codeexample{
This script shows the difference between the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#CAIRO-FILL-RULE-WINDING:CAPS,winding}
and
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#CAIRO-FILL-RULE-EVEN-ODD:CAPS,even--odd}
rules:

@example
rect 50 10 100 60
circle 150 70 40
setcolor seagreen
fill

rect 50 130 100 60
circle 150 190 40
setcolor skyblue
eofill
@end example
}

@subsection Stroke

@vgscmd{stroke} draws a line around the shape defined by the path. The
stroke can be configured with different commands:

@itemize
@item @vgscmd{setdash}
@item @vgscmd{setdashoffset}
@item @vgscmd{setlinecap}
@item @vgscmd{setlinejoin}
@item @vgscmd{setlinewidth}
@item @vgscmd{resetdash}
@end itemize

@codeexample{
This example use @vgscmd{setdashoffset} to animate the stroke:

@example
moveto 0 0
lineto w h

setlinecap round
setdash 50 50
setlinewidth 20
setdashoffset (hypot(w, h) * t / -3)
setcolor seagreen

stroke
@end example
}

@subsection Clip

A @uref{https://en.wikipedia.org/wiki/Clipping_(computer_graphics),clip
region} can be established with @vgscmd{clip} and @vgscmd{eoclip}.

If there is an active clip region, the new clip region will be the
intersection between the existing one and the path. @vgscmd{resetclip}
reset the clip region to the whole frame.

@vgscmd{eoclip} uses the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#CAIRO-FILL-RULE-EVEN-ODD:CAPS,even--odd
rule} to compute the clip region.

@codeexample{
@example
rect 50 50 100 200
clip

circle 30 30 150
setcolor seagreen
fill

// Draw outside the clip region.
resetclip
circle 30 30 150
setlinewidth 3
setcolor skyblue
stroke
@end example
}

@subsection Preserving Paths

The path is cleared after any operation on it, like @vgscmd{fill} or
@vgscmd{stroke}. To reuse the same path in multiple operations,
@vgscmd{preserve} must be called before them.

@codeexample{
In this example, each path is used twice.

@example
circle 120 120 50
setcolor seagreen
preserve stroke
clip

circle 100 100 50
setcolor skyblue
preserve fill
setcolor tomato
stroke
@end example
}

@section Variables

A drawvg can use some variables, provided by the interpreter, to compute
values in @ffexprs{}:

@table @var
@item cx
X coordinate of the @ref{current point}.

@item cy
Y coordinate of the @ref{current point}.

@item w
Width, in pixels, of the frame.

@item h
Height, in pixels, of the frame.

@item i
The loop counter in repeat blocks.

@item n
Frame number.

@item t
Timestamp, in seconds.

@item ts
Timestamp, in seconds, of the first frame.

@item duration
Duration, in seconds, of the frame.
@end table

@anchor{User Variables}
@subsection User Variables

New variables can be created with the @vgscmd{setvar} command. It
associates a name with a numeric value.

The name must follow these rules:

@itemize
@item
It must start with an ASCII letter or an underscore (@code{_}).

@item
It can contain only ASCII letters, underscores, and digits.

@item
It must not match the name of a variable provided by the interpreter
(like @var{w} or @var{t}).
@end itemize

The same variable can be assigned multiple times.

@codeexample{
In this example, the result of an expression is stored in a variable
with the name @var{progress}. Then, it is used for the @var{x} and
@var{width} arguments of @vgscmd{rect}.

@example
setvar progress (w * (pow(mod(t / 2 + 0.5, 1), 2.5)))

rect ((w - progress) / 2) 0 progress h

setcolor darkblue
fill
@end example
}

Currently, a script can contain only 20 different variable names, but
this limit can be modified in the future.

@anchor{current pattern}
@section Patterns

The pattern for fill and stroke operations can be either a solid color,
or a gradient.

@itemize
@item Solid colors.

@itemize
@item @vgscmd{setcolor}
@item @vgscmd{sethsla}
@item @vgscmd{setrgba}
@end itemize

@item Gradients.

@itemize
@item @vgscmd{lineargrad}
@item @vgscmd{radialgrad}
@end itemize

@end itemize

The pattern is not cleared after being used in a fill or stroke
operation, but it is replaced by any command that sets a new pattern.

@subsection Gradients

To configure a gradient, first call to @vgscmd{lineargrad} or
@vgscmd{radialgrad}, and then add color stops by calling @vgscmd{colorstop}
for each stop.

@codeexample{
In this example, the whole frame is filled with a linear gradient:

@example
lineargrad 0 0 w h
colorstop 0 skyblue
colorstop 1 darkblue

rect 0 0 w h
fill
@end example

In this example, a radial gradient is used to simulate a sphere:

@example
radialgrad 90 90 5 120 120 100
colorstop 0.0 #90DDFF
colorstop 0.9 #000030
colorstop 1.0 #000000

rect 0 0 w h
fill
@end example
}

@subsection Variables

@vgscmd{setcolor} and @vgscmd{colorstop} accept a variable name as the
argument. The variable must be assigned with @vgscmd{defrgba},
@vgscmd{defhsla}, or @vgscmd{setvar} and a color.

@codeexample{
@example
// Use color #1020FF, alpha = 50%
setvar someblue #1020FF@@0.5

setcolor someblue

rect 30 30 120 120
fill

rect 90 90 120 120
fill
@end example
}

If a variable has the same name of a @ref{Color,,known color,ffmpeg-utils}, the
variable has preference, and will be used instead of the predefined color.

@codeexample{
@example
setcolor teal
rect 30 30 120 120
fill

setvar teal #70AAAA
setcolor teal         // Use the new color for `teal`.
rect 90 90 120 120
fill
@end example
}

@vgscmd{defrgba} and @vgscmd{defhsla} assign a color to a variable, by providing
an expression for each color component:

@itemize
@item
For @vgscmd{defrgba}: @emph{red}, @emph{green}, @emph{blue}, and
@emph{alpha}.

@item
For @vgscmd{defhsla}: @emph{hue}, @emph{saturation}, @emph{lightness}, and
@emph{alpha}.
@end itemize

Each color component must be in range @code{0} to @code{1}, except
@emph{hue}, which is @code{0} to @code{360}.

@codeexample{
@example
defrgba colorA 1 0.5 0.25 1     // colorA = RGB(255, 127, 63)
defhsla colorB 200 0.75 0.25 1  // colorB = HSL(200, 75%, 25%)

rect 0 0 (w / 2) h
setcolor colorA
fill

rect (w / 2) 0 (w / 2) h
setcolor colorB
fill
@end example
}

@anchor{transformation matrix}
@section Transformations

The coordinates for each command can be scaled, rotated, and translated,
by using the following commands:

@itemize
@item @vgscmd{rotate}
@item @vgscmd{scale}
@item @vgscmd{scalexy}
@item @vgscmd{translate}
@end itemize

The transformations are applied when the command is executed. They have
no effect on the existing path, only on the new segments added to it.

They are done by updating the
@uref{https://www.cairographics.org/manual/cairo-Transformations.html,current
transformation matrix} in the Cairo context. To reset the matrix to its
original state, before any transformation, use @vgscmd{resetmatrix}.

The transform origin for scale and rotation is initially at @code{0, 0},
but it can be adjusted with @vgscmd{translate}.

@codeexample{
@example
// Map (0, 0) as the center of the frame.
translate (w / 2) (h / 2)

// Scale the space as if the frame is 1x1 pixel.
scalexy w h

// Draw multiple lines with the same arguments,
// but each one on a different rotation.
repeat 10 @{
    rotate (PI / 10)
    M -0.25 0
    H 0.25
@}

// Reset transformations, so the scale does not
// affect stroke.
resetmatrix

stroke
@end example
}

@anchor{State Stack}
@section State Stack

The state of a drawvg script contains all parameters used for drawing
operations, like the current color, the transformation matrix, the
stroke configuration, etc.

The @vgscmd{save} command pushes a snapshot of the state to an internal
stack. Later, @vgscmd{restore} pops the latest snapshot from the stack,
and uses it as the new state.

The parameters that can be saved and restored are:

@itemize
@item
Pattern for stroke and fill operations.

@itemize
@item @vgscmd{lineargrad}
@item @vgscmd{radialgrad}
@item @vgscmd{setrgba}
@item @vgscmd{setcolor}
@item @vgscmd{sethsla}
@end itemize

@item Transformation matrix.

@itemize
@item @vgscmd{resetmatrix}
@item @vgscmd{rotate}
@item @vgscmd{scale}
@item @vgscmd{scalexy}
@item @vgscmd{translate}
@end itemize

@item Stroke configuration.

@itemize
@item @vgscmd{setdash}
@item @vgscmd{setdashoffset}
@item @vgscmd{setlinecap}
@item @vgscmd{setlinejoin}
@item @vgscmd{setlinewidth}
@end itemize

@item
Clip region

@itemize
@item @vgscmd{clip}
@item @vgscmd{resetclip}
@end itemize

@end itemize

@anchor{Frame Metadata}
@section Frame Metadata

Some FFmpeg filters add metadata to frames. The command
@vgscmd{getmetadata} can read metadata items containing a numeric value,
and store it in a variable that can be used for command arguments.

@codeexample{
The @code{cropdetect} filter computes the parameters to remove empty
regions around the video. These parameters are accessible in the
@code{lavfi.cropdetect} keys of the frame metadata.

@example
// Get metadata from cropdetect filter and store it
// in `cd*` variables.
getmetadata cdx lavfi.cropdetect.x
getmetadata cdy lavfi.cropdetect.y
getmetadata cdw lavfi.cropdetect.w
getmetadata cdh lavfi.cropdetect.h

rect cdx cdy cdw cdh
setcolor yellow@@0.5
setlinewidth 10
stroke
@end example

To test the script, copy it to a @code{drawcropdetect.vgs} file, and
then execute a command like this:

@example
ffplay -i example-video.webm -vf 'cropdetect, drawvg=file=drawcropdetect.vgs'
@end example
}

@section @code{if} / @code{repeat} Statements

There is limited support for control flow statements: only @vgscmd{if} and
@vgscmd{repeat}.

Both commands receive two arguments: an expression and a block.

@example
if (condition) @{
    // commands
@}

repeat (count) @{
    // commands
@}
@end example

@vgscmd{if} executes its block if the result of @code{(condition)} is not
zero.

@vgscmd{repeat} executes its block the number of times specified by
@code{(count)}. In each iteration, the variable @var{i} is used as a
@uref{https://en.wikipedia.org/wiki/For_loop#Loop_counters,loop
counter}.

If the result of the expression is not a finite number (like
@uref{https://en.wikipedia.org/wiki/NaN,@code{NaN}}) the block is not
executed.

@anchor{comp-operators}
@subsection Comparison and Logical Operators

@ffexprs{} only supports arithmetic operators (like @code{+} for addition).
Comparison operators (like @code{!=}) are supported via functions, while
logical operators (like @code{&&} for @code{AND}) can be emulated with
arithmetic operations.

@multitable @columnfractions .5 .5
@headitem Expression @tab FFmpeg Equivalent
@item @code{x = y} @tab @code{eq(x, y)}
@item @code{x < y} @tab @code{lt(x, y)}
@item @code{x > y} @tab @code{gt(x, y)}
@item @code{x ≤ y} @tab @code{lte(x, y)}
@item @code{x ≥ y} @tab @code{gte(x, y)}
@item @code{a ≤ x ≤ b} @tab @code{between(x, a, b)}
@end multitable

Logical operators can be emulated with multiplication (for @code{AND}),
or addition (for @code{OR}):

@multitable @columnfractions .5 .5
@headitem Expression @tab FFmpeg Equivalent
@item @code{x OR y} @tab @code{x + y}
@item @code{x AND y} @tab @code{x * y}
@end multitable

@codeexample{
In other programming languages, a code like this:

@example
if (x > y && z != 1) @{
    // …
@}
@end example

Can be written for drawvg like this:

@example
if (gt(x, y) * not(eq(z, 1))) @{
    // …
@}
@end example
}

@subsection Early Exit
@vgscmd{break} causes a @vgscmd{repeat} loop to be terminated immediately.

If it is executed outside a @vgscmd{repeat} block, it terminates the whole
script, or the current procedure.

@codeexample{
In this example, we are using the @ref{func-randomg,@code{randomg}} function
to draw a line with random segments.

The loop can be executed @code{500} times, but it is interrupted if the X
coordinate of the @ref{current point} (@var{cx}) exceeds the frame width
(@var{w}). The @ref{current point} is updated after each call to
@vgscmd{rlineto}.

@example
moveto 0 0

repeat 500 @{
    rlineto
        (randomg(0) * 15)
        (randomg(0) * 20)

    if (gt(cx, w)) @{
        break
    @}
@}

stroke
@end example
}

@anchor{Procedures}
@section Procedures

A procedure is a name associated with a block that can be executed
multiple times. It can take between 0 and 6 parameters.

@vgscmd{proc} is used to set the parameter names and the block for a
procedure:

@example
proc p0 @{
    // …
@}

proc p1 param1 param2 @{
    // …
@}
@end example

Inside the block, the arguments can be accessed as regular variables:

@example
proc square center_x center_y side @{
    rect
        (center_x - side / 2) (center_y - side / 2)
        side side
@}
@end example

@vgscmd{call} executes the block assigned to the procedure name. It
requires the name of the procedure, and the value for each parameter
defined in the call to @vgscmd{proc}.

@example
call p0

call p1 1 2

call square (w / 2) (h / 2) (w / t)
@end example

@codeexample{
In this example, the procedure @code{zigzag} draws multiple lines from
the @ref{current point}.

@example
setvar len (w / 10)
setlinewidth 5

proc zigzag @{
    repeat 10 @{
        l len len len (-len)
    @}

    stroke
@}

setcolor #40C0FF
M 0 60
call zigzag

setcolor #00AABB
M 0 120
call zigzag

setcolor #20F0B7
M 0 180
call zigzag
@end example

The color and the Y coordinate of the starting point can be sent as
procedure arguments:

@example
setvar len (w / 10)
setlinewidth 5

proc zigzag color y @{
    setcolor color

    M 0 y
    repeat 10 @{
        l len len len (-len)
    @}

    stroke
@}

call zigzag #40C0FF 60
call zigzag #00AABB 120
call zigzag #20F0B7 180
@end example
}

When the procedure returns, the value of the variable for each argument
is restored to the value it had before calling the procedure. Changes in
other variables (with @vgscmd{setvar}, @vgscmd{getmetadata}, @vgscmd{defhsla},
and @vgscmd{defrgba}) are preserved.

@codeexample{
In the next example, the variable @var{A} has the value @code{0} before
calling the procedure @var{P}. During the execution of @var{P},
@code{A} is @code{1}, but after it, @var{A} is @code{0} again.

@example
setvar A 0

proc P A @{
    print A
@}

print A
call P 1
print A
@end example

It writes the following messages:

@verbatim
[7:7] A = 0.000
[4:8] A = 1.000
[9:7] A = 0.000
@end verbatim
}

@vgscmd{break} causes the script to leave the current procedure, similar
to the
@uref{https://en.wikipedia.org/wiki/Return_statement,@code{return}
statement} in other programming languages, unless it is called within a
@vgscmd{repeat} loop.

The body of the procedure must be defined with @vgscmd{proc} @emph{before}
using @vgscmd{call}.

@codeexample{
In this example, when the procedure @code{notyet} is called, its body
has not yet defined, so the execution fails with the error
@code{Missing body for procedure 'notyet'}.

@example
call notyet

proc notyet @{
    // ...
@}
@end example
}

A procedure can be redefined by other calls to @vgscmd{proc} with the same
name. In such case, @vgscmd{call} invokes the last assigned block.

@codeexample{
In this example, the procedure @code{example} has two different blocks.

@example
proc example @{
    // block1
@}

call example    // executes block1

proc example @{
    // block2
@}

call example    // executes block2
@end example
}

@section Functions in Expressions

There are some functions specific to drawvg available in @ffexprs{}.

@subsection Function @code{p}

@code{p(x, y)} returns the color of the pixel at coordinates @code{x, y}, as a
@code{0xRRGGBBAA} value. It can be assigned to a variable, so the color can be
available for @vgscmd{setcolor} and @vgscmd{colorstop} commands.

If a single expression contains multiple calls to the function, it must return
the value of the last call in order to use it as a color.

@codeexample{
In this example, the first call to @code{p(0, 0)} is stored in the variable
@var{0} of the expression. Then, the same expression makes a second call to
@code{p(1, 1)}, and finally it returns the value in the variable @var{0}.

@example
setvar pixel (st(0, p(0, 0)); p(1, 1); ld(0))
@end example

Since the result of the expression is not the last call to @code{p}, the
variable @var{pixel} can not be used as a color, but it still can be used as
a numeric @code{0xRRGGBBAA} value.
}

If the coordinates are outside the frame, or any of the arguments is not
a finite number (like
@uref{https://en.wikipedia.org/wiki/NaN,@code{NaN}}), the function
returns @code{NaN}.

The @ref{transformation matrix} is applied to the
arguments. To use the original frame coordinates, call
@vgscmd{resetmatrix} between @vgscmd{save} and @vgscmd{restore}:

@example
save
resetmatrix
setvar pixel (p(0, 0))    // top-left pixel of the frame.
restore

setcolor pixel
@end example

Bitwise operations can be used to extract individual color components:

@example
setvar pixel (p(x, y))

if (not(isnan(pixel))) @{
    setvar px_red   (pixel / 0x1000000)
    setvar px_green (bitand(pixel / 0x10000, 0xFF))
    setvar px_blue  (bitand(pixel / 0x100, 0xFF))
    setvar px_alpha (bitand(pixel, 0xFF))
@}
@end example

@subsection Function @code{pathlen}

@code{pathlen(n)} computes the length of the current path, by adding the
length of each line segment returned by
@uref{https://www.cairographics.org/manual/cairo-Paths.html#cairo-copy-path-flat,@code{cairo_copy_path_flat}}.

The function expects an argument @var{n}, as the maximum number of line
segments to add to the length, or @code{0} to add all segments.

@codeexample{
In this example, @code{pathlen} is used to animate the stroke of a
spiral, in a 5 seconds loop.

@example
M (w / 2) (h / 2)

setvar a -1
repeat 16 @{
    rcurveto
        (a * 2 / 3) 0
        (a * 2 / 3) (a)
        0 (a)

    setvar a (-sgn(a) * (abs(a) + 10))
@}

setlinewidth 3
setdash
    (pathlen(0) * (1 - mod(t / 5, 1)))
    1e6

setcolor teal
stroke
@end example
}

@anchor{func-randomg}
@subsection Function @code{randomg}

@code{randomg(idx)} is similar to the @code{random(idx)} function,
available in @ffexprs{}, but its state is global to the frame, instead
of specific to each expression.

To understand the difference, we need to dive into how
@code{random(idx)} works inside a drawvg script.

First, each expression in FFmpeg has a set of 10 internal variables,
which can be written with @code{st(idx, value)}, and can be read with
@code{ld(idx)}. @var{idx} is a value between @code{0} and @code{9}.
These variables are initialized to @code{0}.

When a drawvg script is parsed, each expression is compiled with
@uref{https://ffmpeg.org/doxygen/8.0/eval_8h.html#ad3bf8f3330d1fd139de2ca156c313f34,@code{av_expr_parse}},
from @uref{https://ffmpeg.org/libavutil.html,libavutil}, and these
compiled expressions are reused for every frame. The changes in the
internal variables (with @code{st(idx, value)}) are visible between
frames, but they are not shared between expressions.

@codeexample{
In this example, the expression for the X coordinate updates its
internal variable @code{0} in every frame:

@example
circle
    (st(0, mod(ld(0) + 15, w))) // X
    120                         // Y
    (ld(0) + 20)                // radius

fill
@end example

@code{st(idx, value)} returns the updated value, so it can be used as
the result of the expression.

The radius is not affected because its internal variable (from
@code{ld(0)}) is not updated by the other expression.

Also, note that this example is just to show how internal variables are
kept between frames. A better approach to create this animation is to
use the variables n or t:

@example
circle (mod(n * 15, w)) 120 20
fill
@end example
}

The function @code{random(idx)} returns a
@uref{https://en.wikipedia.org/wiki/Pseudorandom_number_generator,pseudorandom}
value between @code{0} and @code{1}. @var{idx} is the internal variable
that is used both as the seed and to keep the state of the number
generator.

@codeexample{
The next example uses @code{random(0)} to generate a random value for
the center of a circle:

@example
circle
    (random(0) * w)
    (random(0) * h)
    10

fill
@end example

The circle in every frame is at a different position, but always on the
diagonal line of the frame. This happens because the values for the
coordinates X and Y are identical, since both number generators use the
same seed.

To distribute the circles over the whole frame we need different seeds
for each expression. This can be achieved by writing a non-zero value
(like @code{0xF0F0}) to the internal variable of one of expressions, but
only when its value is @code{0}:

@example
circle
    (random(0) * w)
    (st(0, if(ld(0), ld(0), 0xF0F0)); random(0) * h)
    10

fill
@end example

This approach is only useful if we need completely different positions
in each frame. In the next example, random values are used to distribute
many circles over the frame, but the position is fixed. The only change
over time is the fill color:

@example
repeat 20 @{
    circle
        (st(0, i + 1e5); random(0) * w)
        (st(0, i + 1e10); random(0) * h)
        10
@}

sethsla (t * 60) 0.5 0.5 1
preserve fill

setcolor black@@0.5
setlinewidth 1
stroke
@end example

This is achieved by using a precomputed state before calling @code{random(0)}.
The variable @var{i}, updated by @vgscmd{repeat}, is needed to compute
different states in each iteration.
}

The @code{randomg(idx)} function, which is specific to drawvg scripts,
is similar to @code{random(idx)}, but intended to solve the previous
problems:

@itemize
@item All frames have the same seed.
@item The state is shared between expressions.
@end itemize

The parameter @var{idx} has two uses:

@itemize
@item
The last two bits are the index of an internal state, so it is possible
to have 4 different number generators.

@item
The first call to @code{randomg} with a specific index will use the
argument as the seed for the number generator in that index.
@end itemize

In a script like this:

@example
M (randomg(0xFF1)) (randomg(0xFF0))
l (randomg(0xAA1)) (randomg(0xFF0))
@end example

There are 4 calls to @code{randomg}:

@enumerate
@item
The first call, with the argument @code{0xFF1}, uses the internal state
at index @code{1} (because @code{0xFF1} modulo @code{4} is @code{1}).

Since this is the first use of that index, the number generator is
initialized with the seed @code{0xFF1}.

@item
The second call has the same behaviour: it initializes the state at
index @code{0} with the value @code{0xFF0}.

@item
The third call has the argument @code{0xAA1}, and it uses index
@code{1}. Since that state is already initialized (with the seed
@code{0xFF1}), the value @code{0xAA1} is ignored, and it returns the
next number.

@end enumerate

@codeexample{
This example renders a simple rain animation, moving lines from top to
bottom.

@code{randomg} is used to distribute the lines over the frame, and to
apply different speeds to each one.

@example
rect 0 0 w h
setcolor midnightblue
fill

setcolor white

repeat 50 @{
    setvar offset (t * (randomg(0) + 1))

    moveto
        (mod(randomg(0) + offset / 6, 1) * w)
        (mod(randomg(0) + offset, 1) * h)

    rlineto 6 36

    setlinewidth (randomg(1) / 2 + 0.2)
    stroke
@}
@end example
}

@section Tracing with @code{print}

It is possible to trace the execution of a drawvg script by printing the
value of an expression, either with the @vgscmd{print} command, or with
the print function.

In both cases, the values are written to the FFmpeg log.

Printing expressions may have a noticeable impact on the performance, so
it is preferable to use it only when necessary.

@subsection Function print

The function @code{print(t)} writes the value of t, and returns its
argument.

@codeexample{
Given a line line this:

@example
M (sin(2 * PI * t) * w) 0
@end example

We can see the values of @code{sin(2 * PI * t)} by surrounding it with a
call to @code{print()}:

@example
M (print(sin(2 * PI * t)) * w) 0
@end example

Executing this script with a 1 second / 8 FPS video shows the expected
values for the sine function.

@verbatim
$ ffmpeg \
    -f lavfi \
    -i 'color=r=8:d=1, drawvg=M (print(sin(2 * PI * t)) * w) 0' \
    -f null /dev/null \
  |& grep 'Eval @'

[Eval @ 0x7f500f502d20] 0.000000
[Eval @ 0x7f4ff784b420] 0.707107
[Eval @ 0x7f4ff784ba20] 1.000000
[Eval @ 0x7f4ff784c020] 0.707107
[Eval @ 0x7f4ff784c620] 0.000000
[Eval @ 0x7f4ff784cc20] -0.707107
[Eval @ 0x7f4ff784d220] -1.000000
[Eval @ 0x7f4ff784d820] -0.707107
@end verbatim
}

@anchor{Command print}
@subsection Command @code{print}

The command @vgscmd{print} accepts an arbitrary number of arguments, and
for each one it writes:

@itemize
@item
The source location (line and column).
@item
The source code of the expression.
@item
The result of evaluating the expression.
@end itemize

When there are multiple expressions, they are separated by the @code{|}
character.

@codeexample{
The next script prints the position of the @ref{current point} after the
@vgscmd{l} command:

@example
M 10 20
l 100 100
print cx cy
stroke
@end example

For each frame, it produces this output:

@verbatim
[3:7] cx = 110.000000 | [3:10] cy = 120.000000
@end verbatim

The next example prints the values of @code{random(0)}:

@verbatim
$ ffmpeg \
    -f lavfi \
    -i 'color=r=8:d=1, drawvg=print (random(0))' \
    -f null /dev/null \
  |& grep 'drawvg @'

[drawvg @ 0x50a000000180] [1:7] (random(0)) = 0.229731
[drawvg @ 0x50a000000180] [1:7] (random(0)) = 0.959813
[drawvg @ 0x50a000000180] [1:7] (random(0)) = 0.071676
[drawvg @ 0x50a000000180] [1:7] (random(0)) = 0.044600
[drawvg @ 0x50a000000180] [1:7] (random(0)) = 0.134127
[drawvg @ 0x50a000000180] [1:7] (random(0)) = 0.320513
[drawvg @ 0x50a000000180] [1:7] (random(0)) = 0.857675
[drawvg @ 0x50a000000180] [1:7] (random(0)) = 0.562456
@end verbatim
}

@chapter Commands

@macro signature {sig}
@b{@code{\sig\}}
@end macro

@macro signatureimpl {sig}
@signature{\sig\} @ @ @ --- @ref{implicit commands,@i{Can be implicit}}
@end macro

@anchor{cmd_arc}
@section @code{arc}

@signatureimpl{arc @var{xc} @var{yc} @var{radius} @var{angle1} @var{angle2}}

Adds a circular arc of the given @var{radius} to the current path. The
arc is centered at @var{xc, yc}, begins at @var{angle1} and proceeds
in the direction of increasing angles to end at @var{angle2}.

If there is a @ref{current point}, a line is added from it to the beginning of
the arc. If this is not desired, use @vgscmd{newpath} before @vgscmd{arc} to clear
the @ref{current point}.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-Paths.html#cairo-arc,@code{cairo_arc}}
function for more details.

@codeexample{
@example
arc 120 120 60 0 (3 * PI / 2)
stroke
@end example
}

@anchor{cmd_arcn}
@section @code{arcn}

@signatureimpl{arcn @var{xc} @var{yc} @var{radius} @var{angle1} @var{angle2}}

Similar to @vgscmd{arc}, but it differs in the direction of the arc
between the two angles.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-Paths.html#cairo-arc-negative,@code{cairo_arc_negative}}
function for more details.

@codeexample{
In this example, both @vgscmd{arc} and @vgscmd{arcn} have the same angles,
but they render different arcs:

@example
arc  120  90 60 (PI / 2) 0

newpath
arcn 120 150 60 (PI / 2) 0

stroke
@end example
}

@vgscmd{newpath} is needed to prevent a line between the two arcs.

@anchor{cmd_break}
@section @code{break}

@signature{break}

@vgscmd{break} terminates the execution of the innermost block, either a
@vgscmd{repeat} loop or a procedure.

If it is used outside of a @vgscmd{repeat} / @vgscmd{proc} block, it
terminates the script for the current frame.

@anchor{cmd_call}
@section @code{call}

@signature{call @var{name} @var{args}*}

Invokes a procedure defined by @vgscmd{proc}.

See the @ref{Procedures} section above for more details.

@anchor{cmd_circle}
@section @code{circle}

@signatureimpl{circle @var{xc} @var{yc} @var{radius}}

Adds a circle of the given @var{radius} to the current path. The circle
is centered at @var{xc, yc}. The @ref{current point} is cleared before and
after adding the circle.

This is a convenience wrapper for @vgscmd{arc}. A call to @vgscmd{circle} is
equivalent to:

@example
newpath
arc xc yc radius (0) (2 * PI)
newpath
@end example

@anchor{cmd_clip}
@anchor{cmd_eoclip}
@section @code{clip}, @code{eoclip}

@signature{clip, eoclip}

Establishes a new clip region by intersecting the current clip region
with the current path as it would be filled by @vgscmd{fill} or
@vgscmd{eofill}.

@vgscmd{eoclip} uses the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#CAIRO-FILL-RULE-EVEN-ODD:CAPS,even--odd
rule}. See @ref{fill rules} for more details.

The path is cleared after updating the clip region, unless the
@vgscmd{preserve} command is used before @vgscmd{clip} or @vgscmd{eoclip}.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-clip,@code{cairo_clip}}
function for more details.

@anchor{cmd_Z}
@anchor{cmd_z}
@anchor{cmd_closepath}
@section @code{Z}, @code{z}, @code{closepath}

@signature{Z, z, closepath}

Adds a line segment to the path from the @ref{current point} to the beginning
of the current sub-path, and closes this sub-path. The beginning is set by any
of the @emph{move} commands (@vgscmd{M}, @vgscmd{m}, @vgscmd{moveto},
@vgscmd{rmoveto}).

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-Paths.html#cairo-close-path,@code{cairo_close_path}}
function for more details.

@anchor{cmd_colorstop}
@section @code{colorstop}

@signatureimpl{colorstop @var{offset} @var{color}}

Adds a color stop to a gradient pattern.

@var{offset} is a value between @code{0} and @code{1}, and it specifies
the location along the gradient's control vector.

This command must be executed after @vgscmd{lineargrad} or
@vgscmd{radialgrad}.

Color stops can be added in any number of calls to @vgscmd{colorstop}. In
the next example, the 3 blocks define the same gradient:

@example
// 1
colorstop 0.0 red
colorstop 0.5 green
colorstop 1.0 blue

// 2
colorstop 0 red 0.5 green
colorstop 1 blue

// 3
colorstop 0 red 0.5 green 1 blue
@end example

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-cairo-pattern-t.html#cairo-pattern-add-color-stop-rgba,@code{cairo_pattern_add_color_stop_rgba}}
function for more details.

@codeexample{
In this example, color stops are added in a @vgscmd{repeat} loop.

@example
lineargrad 0 0 w h

repeat 6 @{
    defhsla s (i * 60) 0.8 0.5 1
    colorstop (i / 5) s
@}

rect 0 0 w h
fill
@end example

It is possible to avoid transitions between color stops by repeating the
same color in two stops:

@example
lineargrad 0 0 w h

repeat 6 @{
    defhsla s (i * 60) 0.8 0.5 1
    colorstop (i / 5) s
    colorstop ((i + 1) / 5) s
@}

rect 0 0 w h
fill
@end example
}

@anchor{cmd_C}
@anchor{cmd_curveto}
@section @code{C}, @code{curveto}

@signatureimpl{C, curveto @var{x1} @var{y1} @var{x2} @var{y2} @var{x} @var{y}}

Draw a cubic Bézier curve from the @ref{current point} to the @emph{end point}
specified by @var{x, y}. The @emph{start control point} is specified by
@var{x1, y1} and the @emph{end control point} is specified by @var{x2, y2}.

@macro mdncubicbeziercurve
@uref{https://developer.mozilla.org/en-US/docs/Web/SVG/Reference/Attribute/d#cubic_b%C3%A9zier_curve,Cubic Bézier Curve on MDN}
@end macro

@macro mdntutorialcurve
@uref{https://developer.mozilla.org/en-US/docs/Web/SVG/Tutorials/SVG_from_scratch/Paths#curve_commands,Curve Commands section of the Paths tutorial on MDN}
@end macro

The behaviour is identical to the @vgscmd{C} command in @svgpathlink{}. For more
details, see @mdncubicbeziercurve{}, and the @mdntutorialcurve{}.

@codeexample{
@example
moveto 20 20

curveto
    0 (h / 2)           // start control point
    w (h / 2)           // end control point
    (w - 20) (h - 20)   // end point

stroke
@end example
}

@anchor{cmd_c}
@anchor{cmd_rcurveto}
@section @code{c}, @code{rcurveto}

@signatureimpl{c, rcurveto @var{dx1} @var{dy1} @var{dx2} @var{dy2} @var{dx} @var{dy}}

Like @vgscmd{curveto}, but the coordinates are relative to the @ref{current
point}.

@anchor{cmd_defhsla}
@section @code{defhsla}

@signature{defhsla varname @var{h} @var{s} @var{l} @var{a}}

Similar to @vgscmd{sethsla}, but instead of establishing the color for
stroke and fill operations, the computed color is assigned to the
variable @var{varname}.

@var{varname} can then be used as a color for @vgscmd{setcolor} and
@vgscmd{colorstop}.

See @vgscmd{sethsla} for more details on how the color is computed.

@anchor{cmd_defrgba}
@section @code{defrgba}

@signature{defrgba varname @var{r} @var{g} @var{b} @var{a}}

Computes a color from the @emph{red}, @emph{green}, @emph{blue}, and
@emph{alpha} components, and assigns it to the variable @var{varname}.

All components are values between @code{0} and @code{1}. Values outside
that range are clamped to it.

@anchor{cmd_ellipse}
@section @code{ellipse}

@signatureimpl{ellipse @var{cx} @var{cy} @var{rx} @var{ry}}

Adds an ellipse to the current path. Similar to @vgscmd{circle}, but it is
possible to use different radius for both axes.

@codeexample{
@verbatim
ellipse 120 120 75 50
stroke
@end verbatim
}

@anchor{cmd_fill}
@anchor{cmd_eofill}
@section @code{fill}, @code{eofill}

@signature{fill, eofill}

Fill the current path, using the @ref{current pattern} (either
a solid color or a gradient).

@vgscmd{eofill} uses the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#CAIRO-FILL-RULE-EVEN-ODD:CAPS,even--odd
rule}. See @ref{fill rules} for more details.

The path is cleared after the operation, unless the @vgscmd{preserve}
command is used before @vgscmd{fill} or @vgscmd{eofill}.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-fill,@code{cairo_fill}}
function for more details.

@anchor{cmd_getmetadata}
@section @code{getmetadata}

@signature{getmetadata @var{varname} @var{key}}

Get the value of a metadata entry created by another filter, and assign
it to the variable @var{varname}.

If there is no metadata entry for @var{key}, or its value is not a
number, @var{varname} is set to
@uref{https://en.wikipedia.org/wiki/NaN,@code{NaN}}.

See the @ref{Frame Metadata} section above for an
example.

@anchor{cmd_H}
@anchor{cmd_h}
@section @code{H}, @code{h}

@signatureimpl{H, h @var{x}}

Draw a horizontal line from the @ref{current point} to x.

The coordinate for @vgscmd{H} is absolute, and for @vgscmd{h} it is relative
to the @ref{current point}.

@anchor{cmd_if}
@section @code{if}

@signature{if @var{condition} @{ @var{block} @}}

Executes a block if the value of @var{condition} is not zero, and a
finite number (unlike
@uref{https://en.wikipedia.org/wiki/NaN,@code{NaN}}).

See the @ref{comp-operators,Comparison and Logical Operators} section
above for more details on how to write conditional expressions.

@anchor{cmd_lineargrad}
@section @code{lineargrad}

@signature{lineargrad @var{x0} @var{y0} @var{x1} @var{y1}}

Set the @ref{current pattern} to a new linear gradient, along
the line from the coordinates @var{x0, y0} to @var{x1, y1}.

This gradient can be used for stroke and fill operations.

Use @vgscmd{colorstop} to set the color for each position in the gradient.

@anchor{cmd_L}
@anchor{cmd_lineto}
@section @code{L}, @code{lineto}

@signatureimpl{L, lineto @var{x} @var{y}}

Draw a line from the @ref{current point} to the coordinates at @var{x, y}.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-Paths.html#cairo-line-to,@code{cairo_line_to}}
function for more details.

@anchor{cmd_l}
@anchor{cmd_rlineto}
@section @code{l}, @code{rlineto}

@signatureimpl{l, rlineto @var{dx} @var{dy}}

Like @vgscmd{lineto}, but the coordinates are relative to the @ref{current
point}.

@anchor{cmd_M}
@anchor{cmd_moveto}
@section @code{M}, @code{moveto}

@signatureimpl{M, moveto @var{x} @var{y}}

Begin a new sub-path, and set the @ref{current point} to @var{x, y}.

@anchor{cmd_m}
@anchor{cmd_rmoveto}
@section @code{m}, @code{rmoveto}

@signatureimpl{m, rmoveto @var{dx} @var{dy}}

Like @vgscmd{moveto}, but the coordinates are relative to the @ref{current
point}.

@anchor{cmd_newpath}
@section @code{newpath}

@signature{newpath}

Begin a new sub-path. Like @vgscmd{moveto}, but there is no
@ref{current point} after it.

@codeexample{
In the next example, @vgscmd{newpath} is used in the path on the right to
prevent the line connecting both arcs.

@verbatim
setlinewidth 3

setcolor skyblue
arcn 70 90 20 0 (PI)
arc 70 150 20 0 (PI)
stroke

setcolor seagreen
arcn 170 90 20 0 (PI)
newpath
arc 170 150 20 0 (PI)
stroke
@end verbatim
}

@anchor{cmd_preserve}
@section @code{preserve}

@signature{preserve}

Indicates that the next operation to fill, stroke, or clip, must
preserve the path, so the same path can be used in multiple operations.

It has effect on these commands:

@itemize
@item @vgscmd{clip}
@item @vgscmd{eoclip}
@item @vgscmd{eofill}
@item @vgscmd{fill}
@item @vgscmd{stroke}
@end itemize

The script can contain any command between @vgscmd{preserve} and the
associated operation. This allows modifying other properties, like the
current color.

@codeexample{
In this example, the same path is used for both @vgscmd{fill} and
@vgscmd{stroke}, but with different colors.

@verbatim
circle (w / 2) (h / 2) (w / 3)

setcolor skyblue
preserve fill

setlinewidth 10
setcolor seagreen
stroke
@end verbatim
}

@vgscmd{preserve} can be called multiple times, if the same path has to be
used in 3 or more operations.

@codeexample{
In this example, the path created by @vgscmd{circle} is used by
@vgscmd{fill}, @vgscmd{stroke}, and @vgscmd{clip}. After @vgscmd{clip}, the
path is cleared.

@verbatim
circle 100 100 50

preserve fill
preserve stroke
clip
@end verbatim
}

@anchor{cmd_print}
@section @code{print}

@signatureimpl{print @var{expr}}

Print its arguments to the FFmpeg log.

See the @ref{Command print} section above for more details.

@anchor{cmd_proc}
@section @code{proc}

@signature{proc @var{name} @var{params}* @{ @var{block} @}}

Assign the block and the parameters for the procedure @var{name}. The
procedure can be called multiple times with the @vgscmd{call} command.

See the @ref{Procedures} section above for more details.

@anchor{cmd_Q}
@section @code{Q}

@signature{Q x1 y1 @var{x} @var{y}}

Draw a quadratic Bézier curve from the @ref{current point} to the @emph{end
point} specified by @var{x, y}. The @emph{control point} is specified by
@var{x1, y1}.

@macro mdnquadbeziercurve
@uref{https://developer.mozilla.org/en-US/docs/Web/SVG/Reference/Attribute/d#quadratic_b%C3%A9zier_curve,Quadratic Bézier curve on MDN}
@end macro

The behaviour is identical to the @code{Q} command in @svgpathlink{}. For more
details, see @mdnquadbeziercurve{}, and the @mdntutorialcurve{}.

@codeexample{
@verbatim
moveto 20 20

Q
    0 h                 // control point
    (w - 20) (h - 20)   // end point

stroke
@end verbatim
}

@anchor{cmd_q}
@section @code{q}

@signature{q @var{dx1} @var{dy1} @var{dx} @var{dy}}

Like @vgscmd{Q}, but the coordinates are relative to the @ref{current point}.

@anchor{cmd_radialgrad}
@section @code{radialgrad}

@signature{radialgrad @var{cx0} @var{cy0} @var{radius0} @var{cx1} @var{cy1} @var{radius1}}

Creates a new radial gradient between the two circles defined by
@var{cx0 cy0 radius0} and @var{cx1 cy1 radius1}. Each set of arguments
is the coordinates of the center and the radius.

This gradient can be used for stroke and fill operations.

Use @vgscmd{colorstop} to set the color for each position in the gradient.

@codeexample{
The animation in the next example shows how the two circles defined in
the @vgscmd{radialgrad} arguments interact with each other.

The red circle represent the circle for the @var{cx0 cy0 radius0}
arguments, and the yellow circle is the one for the
@var{cx1 cy1 radius1} arguments.

@verbatim
setvar cx0 (mod(t * 30, w))
setvar cy0 120
setvar radius0 20

setvar cx1 120
setvar cy1 120
setvar radius1 70

radialgrad
    cx0 cy0 radius0
    cx1 cy1 radius1

colorstop
    0 lightblue
    1 darkblue

// Fill the frame with the gradient.
rect 0 0 w h
fill

// Draw inner circle.
circle cx0 cy0 radius0
setcolor red
stroke

// Draw outer circle.
circle cx1 cy1 radius1
setcolor yellow
stroke
@end verbatim
}

@anchor{cmd_rect}
@section @code{rect}

@signature{rect @var{x} @var{y} @var{width} @var{height}}

Adds a rectangle of the given size (@var{width} × @var{height}), at
position @var{x, y}, to the current path. The @ref{current point} is cleared
before and after adding the rectangle.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-Paths.html#cairo-rectangle,@code{cairo_rectangle}}
function for more details.

@anchor{cmd_repeat}
@section @code{repeat}

@signature{repeat @var{count} @{ @var{block} @}}

Executes a block the number of times indicated by @var{count}.

In each iteration, the variable @var{i} is used as a
@uref{https://en.wikipedia.org/wiki/For_loop#Loop_counters,loop
counter}. It takes the values from @code{0} to @code{count - 1}. When
the loop is terminated, the variable is restored to the value before
starting the loop.

If @var{count} is less than @code{1}, or it is not a finite number
(like @uref{https://en.wikipedia.org/wiki/NaN,@code{NaN}}), the block is
not executed.

@anchor{cmd_resetclip}
@section @code{resetclip}

@signature{resetclip}

Reset the current clip region to its original state, covering the whole
frame.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-reset-clip,@code{cairo_reset_clip}}
function for more details.

@anchor{cmd_resetdash}
@section @code{resetdash}

@signature{resetdash}

Disable the dash pattern to be used by @vgscmd{stroke}. This reverts any
change made by @vgscmd{setdash} and @vgscmd{setdashoffset}.

It calls
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-set-dash,@code{cairo_set_dash}}
with @code{num_dashes} set to @code{0}.

@anchor{cmd_resetmatrix}
@section @code{resetmatrix}

@signature{resetmatrix}

Resets the current @ref{transformation matrix}.

@anchor{cmd_restore}
@section @code{restore}

@signature{restore}

Restores the state saved by a preceding call to @vgscmd{save}.

For more details, see the @ref{State Stack} section above, and the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-restore,@code{cairo_restore}}
function.

@anchor{cmd_rotate}
@section @code{rotate}

@signature{rotate @var{angle}}

Modifies the current @ref{transformation matrix} by rotating the user-space
axes by @var{angle} radians.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-Transformations.html#cairo-rotate,@code{cairo_rotate}}
function for more details.

@codeexample{
In this example:

@itemize
@item @vgscmd{scalexy} maps the coordinates to a 1x1 frame.
@item @vgscmd{translate} put @code{0, 0} at the center of the frame.
@item @vgscmd{rotate} rotates 45°.
@item
@vgscmd{resetmatrix} reverts the transformations before @vgscmd{stroke}, so the
line width is not affected by the scale.
@end itemize

@verbatim
scalexy w h
translate 0.5 0.5
rotate (PI / 4)
rect -0.25 -0.25 0.5 0.5
resetmatrix
stroke
@end verbatim
}

@anchor{cmd_roundedrect}
@section @code{roundedrect}

@signatureimpl{roundedrect @var{x} @var{y} @var{width} @var{height} @var{radius}}

Like @vgscmd{rect}, but a circular arc is used for the corners.

@codeexample{
The next example shows the same rectangle, with different values for the
corner radius.

The radius is computed by multiplying @var{i} (the
@uref{https://en.wikipedia.org/wiki/For_loop#Loop_counters,loop counter}) by
@code{4.5}. This number is chosen to make the last shape a perfect circle.

@example
repeat 9 @{
    roundedrect
        (mod(i, 3) * 80 + 5)     // x
        (floor(i / 3) * 80 + 5)  // y
        70 70                    // size
        (i * 4.5)                // radius
@}

stroke
@end example
}

@anchor{cmd_save}
@section @code{save}

@signature{save}

Saves a copy of the current state on an internal stack. This copy can be
restored later with @vgscmd{restore}.

For more details, see the @ref{State Stack} section above, and the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-save,@code{cairo_save}}
function.

@anchor{cmd_scale}
@section @code{scale}

@signature{scale @var{sxy}}

Similar to @vgscmd{scalexy}, but the same value is used for both axes. It
is equivalent to:

@signature{scalexy @var{sxy} @var{sxy}}

@anchor{cmd_scalexy}
@section @code{scalexy}

@signature{scalexy @var{sx} @var{sy}}

Modifies the current @ref{transformation matrix} by scaling the X and Y
user-space axes by @var{sx} and @var{sy} respectively.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-Transformations.html#cairo-scale,@code{cairo_scale}}
function for more details.

See @vgscmd{rotate} for an example on combining multiple transformations.

@anchor{cmd_setcolor}
@section @code{setcolor}

@signature{setcolor @var{color}}

Set a solid color as the @ref{current pattern} for stroke and fill operations

See the @ref{Colors} section above for more details.

@anchor{cmd_setdash}
@section @code{setdash}

@signatureimpl{setdash @var{length}}

Sets the dash pattern to be used by @vgscmd{stroke}.

Each call to @vgscmd{setdash} adds a length to the pattern, alternating
between @emph{on} and @emph{off} portions of the stroke.

After a call to @vgscmd{setdash}, @vgscmd{resetdash} is needed either to
create a new pattern, or to discard the current one.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-set-dash,@code{cairo_set_dash}}
function for more details.

@anchor{cmd_setdashoffset}
@section @code{setdashoffset}

@signature{setdashoffset @var{offset}}

Set the offset into the dash pattern at which the stroke should start.

@vgscmd{setdash} must be called @emph{before} @vgscmd{setdashoffset}.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-set-dash,@code{cairo_set_dash}}
function for more details.

@codeexample{
The next animation shows the effect of @vgscmd{setdashoffset} when its
argument changes over time.

@verbatim
scalexy w h
M 0.5 1
curveto 0 0.5, 1 0.5, 0.5 0
resetmatrix

setdash 20 5 // 20 on, 5 off
setdashoffset (t * 100)

setlinewidth 20
stroke
@end verbatim
}

@anchor{cmd_sethsla}
@section @code{sethsla}

@signature{sethsla @var{h} @var{s} @var{l} @var{a}}

Set the @ref{current pattern} to a solid color, given the @emph{hue},
@emph{saturation}, and @emph{lightness}, and @emph{alpha} components.

h is the @emph{hue}, a value between @code{0} and @code{359}. Negative
values are clamped to @code{0}, and values greater than @code{359} are
interpreted as modulo 360.

s (@emph{saturation}), l (@emph{lightness}), and a (@emph{alpha}), are
values between @code{0} and @code{1}.

The conversion to RGB is implemented according to the
@uref{https://en.wikipedia.org/wiki/HSL_and_HSV#HSL_to_RGB,formulae from
Wikipedia}.

@anchor{cmd_setlinecap}
@section @code{setlinecap}

@signature{setlinecap @var{cap}}

Set the current line cap style, which determines the shape used to draw
the end points of lines.

@var{cap} must be one of the following names:

@itemize
@item @code{butt}
@item @code{round}
@item @code{square}
@end itemize

It calls to
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-set-line-cap,@code{cairo_set_line_cap}}
to set the line cap style.

@codeexample{
This example draws 3 lines with the same length, each one with a
different line cap style:

@verbatim
setlinewidth 40

setlinecap butt
setcolor tomato
M 60 40 v 100 stroke

setlinecap round
setcolor seagreen
M 120 40 v 100 stroke

setlinecap square
setcolor skyblue
M 180 40 v 100 stroke

M 20 40 H 220 m 0 100 H 20
setcolor black@0.5
setlinewidth 2
stroke
@end verbatim
}

@anchor{cmd_setlinejoin}
@section @code{setlinejoin}

@signature{setlinejoin @var{join}}

Sets the current line join style, which determines the shape used to
join two line segments.

@var{join} must be one of the following names:

@itemize
@item @code{bevel}
@item @code{miter}
@item @code{round}
@end itemize

It calls to
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-set-line-join,@code{cairo_set_line_join}}
to set the line join style.

@codeexample{
This example draws 3 lines with the same length, each one with a
different line join style:

@verbatim
setlinewidth 30

setlinejoin bevel
setcolor tomato
M 70 20 l 50 50 50 -50 stroke

setlinejoin miter
setcolor seagreen
M 70 90 l 50 50 50 -50 stroke

setlinejoin round
setcolor skyblue
M 70 160 l 50 50 50 -50 stroke
@end verbatim
}

@anchor{cmd_setlinewidth}
@section @code{setlinewidth}

@signature{setlinewidth @var{width}}

Set the line width for @vgscmd{stroke}.

@var{width} is affected by the @ref{transformation matrix}.

To specify a width that is not affected by other transformations,
@vgscmd{resetmatrix} can be used between @vgscmd{save} / @vgscmd{restore}:

@verbatim
save

resetmatrix
setlinewidth 1
stroke

// Restore matrix after stroke.
restore
@end verbatim

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-set-line-width,@code{cairo_set_line_width}}
function for more details.

@anchor{cmd_setrgba}
@section @code{setrgba}

@signature{setrgba @var{r} @var{g} @var{b} @var{a}}

Set the @ref{current pattern} to a solid color, given the
@emph{red}, @emph{green}, @emph{blue}, and @emph{alpha} components.

All components are values between @code{0} and @code{1}. Values outside
that range are clamped to it.

@anchor{cmd_setvar}
@section @code{setvar}

@signature{setvar @var{varname} @var{value}}

Set the variable @var{varname} to @var{value}.

See the @ref{User Variables} section above for more details.

@anchor{cmd_stroke}
@section @code{stroke}

@signature{stroke}

Strokes the current path according to the current line width, line join,
line cap, and dash settings.

The path is cleared after the operation, unless the @vgscmd{preserve}
command is used before @vgscmd{stroke}.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-stroke,@code{cairo_stroke}}
function for more details.

@anchor{cmd_S}
@anchor{cmd_s}
@section @code{S}, @code{s}

@signatureimpl{S, s @var{x2} @var{y2} @var{x} @var{y}}

Draw a smooth cubic Bézier curve from the @ref{current point} to the @emph{end
point} specified by @var{x, y}. The @emph{end control point} is specified by
@var{x2, y2}.

The @emph{start control point} is the reflection of the @emph{end
control point} of the previous curve command about the @emph{current
point}.

The behaviour is identical to the @code{S} command in @svgpathlink{}. For more
details, see @mdncubicbeziercurve{}, and the @mdntutorialcurve{}.

@vgscmd{s} is like @vgscmd{S}, but the coordinates are relative to the @ref{current
point}.

@codeexample{
@example
M 20 120

c 25 -50, 25 50, 50 0

repeat 3 @{
    s 20 50, 50 0
@}

stroke
@end example
}

@anchor{cmd_translate}
@section @code{translate}

@signature{translate @var{tx} @var{ty}}

Modifies the current @ref{transformation matrix} by
translating the user-space origin by @var{tx, ty}.

See the documentation of the
@uref{https://www.cairographics.org/manual/cairo-Transformations.html#cairo-translate,@code{cairo_translate}}
function for more details.

@anchor{cmd_T}
@anchor{cmd_t}
@section @code{T}, @code{t}

@signatureimpl{T, t @var{x} @var{y}}

Draw a smooth quadratic Bézier curve from the @ref{current point} to the
@emph{end point} specified by @var{x, y}.

The @emph{control point} is the reflection of the @emph{control point}
of the previous curve command about the @emph{current point}.

The behaviour is identical to the @code{T} command in @svgpathlink{}. For more
details, see @mdnquadbeziercurve{}, and the @mdntutorialcurve{}.

@vgscmd{t} is like @vgscmd{T}, but the coordinates are relative to the
@ref{current point}.

@codeexample{
@example
M 20 120

q 10 -20, 20 0

repeat 9 @{
    t 20 0
@}

stroke
@end example
}

@anchor{cmd_V}
@anchor{cmd_v}
@section @code{V}, @code{v}

@signatureimpl{V, v @var{y}}

Draw a vertical line from the @ref{current point} to y.

The coordinate for @vgscmd{V} is absolute, and for @vgscmd{v} it is relative
to the @ref{current point}.

@bye