SDL

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#include <metal_stdlib>
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using namespace metal;
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struct VertexToFragment {
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    float2 tex;
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    float4 pos [[position]];
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};
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struct SourceRegion {
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    float2 UVLeftTop;
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    float2 UVDimensions;
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    uint MipLevel;
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    float LayerOrDepth;
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};
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#if COMPILE_FullscreenVert
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vertex VertexToFragment FullscreenVert(uint vI [[vertex_id]]) {
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   float2 inTex = float2((vI << 1) & 2, vI & 2);
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   VertexToFragment out;
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   out.tex = inTex;
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   out.pos = float4(inTex * float2(2.0f, -2.0f) + float2(-1.0f, 1.0f), 0.0f, 1.0f);
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   return out;
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}
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#endif
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#if COMPILE_BlitFrom2D
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fragment float4 BlitFrom2D(
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    VertexToFragment input [[stage_in]],
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    constant SourceRegion &sourceRegion [[buffer(0)]],
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    texture2d<float> sourceTexture [[texture(0)]],
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    sampler sourceSampler [[sampler(0)]])
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{
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    float2 newCoord = sourceRegion.UVLeftTop + sourceRegion.UVDimensions * input.tex;
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    return sourceTexture.sample(sourceSampler, newCoord, level(sourceRegion.MipLevel));
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}
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#endif
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#if COMPILE_BlitFrom2DArray
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fragment float4 BlitFrom2DArray(
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    VertexToFragment input [[stage_in]],
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    constant SourceRegion &sourceRegion [[buffer(0)]],
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    texture2d_array<float> sourceTexture [[texture(0)]],
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    sampler sourceSampler [[sampler(0)]])
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{
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    float2 newCoord = sourceRegion.UVLeftTop + sourceRegion.UVDimensions * input.tex;
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    return sourceTexture.sample(sourceSampler, newCoord, (uint)sourceRegion.LayerOrDepth, level(sourceRegion.MipLevel));
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}
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#endif
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#if COMPILE_BlitFrom3D
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fragment float4 BlitFrom3D(
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    VertexToFragment input [[stage_in]],
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    constant SourceRegion &sourceRegion [[buffer(0)]],
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    texture3d<float> sourceTexture [[texture(0)]],
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    sampler sourceSampler [[sampler(0)]])
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{
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    float2 newCoord = sourceRegion.UVLeftTop + sourceRegion.UVDimensions * input.tex;
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    return sourceTexture.sample(sourceSampler, float3(newCoord, sourceRegion.LayerOrDepth), level(sourceRegion.MipLevel));
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}
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#endif
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#if COMPILE_BlitFromCube
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fragment float4 BlitFromCube(
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    VertexToFragment input [[stage_in]],
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    constant SourceRegion &sourceRegion [[buffer(0)]],
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    texturecube<float> sourceTexture [[texture(0)]],
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    sampler sourceSampler [[sampler(0)]])
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{
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    // Thanks, Wikipedia! https://en.wikipedia.org/wiki/Cube_mapping
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    float2 scaledUV = sourceRegion.UVLeftTop + sourceRegion.UVDimensions * input.tex;
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    float u = 2.0 * scaledUV.x - 1.0;
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    float v = 2.0 * scaledUV.y - 1.0;
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    float3 newCoord;
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    switch ((uint)sourceRegion.LayerOrDepth) {
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        case 0: newCoord = float3(1.0, -v, -u); break; // POSITIVE X
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        case 1: newCoord = float3(-1.0, -v, u); break; // NEGATIVE X
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        case 2: newCoord = float3(u, -1.0, -v); break; // POSITIVE Y
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        case 3: newCoord = float3(u, 1.0, v); break; // NEGATIVE Y
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        case 4: newCoord = float3(u, -v, 1.0); break; // POSITIVE Z
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        case 5: newCoord = float3(-u, -v, -1.0); break; // NEGATIVE Z
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        default: newCoord = float3(0, 0, 0); break; // silences warning
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    }
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    return sourceTexture.sample(sourceSampler, newCoord, level(sourceRegion.MipLevel));
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}
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#endif
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