Celestia

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uniform mat4 m;
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uniform mat3 viewMat;
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uniform float size;
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uniform float brightness;
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uniform float minimumFeatureSize;
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in Vertex
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{
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    vec3  color;
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    float size;
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    float brightness;
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} vertex[];
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out vec4 v_Color;
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out vec2 v_TexCoord;
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void main()
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{
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    float s = size * vertex[0].size;
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    if (s >= minimumFeatureSize)
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    {
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        vec4 p = m * gl_in[0].gl_Position;
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        float screenFrac = s / length(p);
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        if (screenFrac < 0.1)
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        {
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            /*
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             * This shader assumes that vertices are rendered in CCW order.
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             */
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            vec4 v0 = vec4(viewMat * vec3(-1.0,  1.0, 0.0) * s, 0.0);
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            vec4 v1 = vec4(viewMat * vec3(-1.0, -1.0, 0.0) * s, 0.0);
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            vec4 v2 = vec4(viewMat * vec3( 1.0,  1.0, 0.0) * s, 0.0);
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            vec4 v3 = vec4(viewMat * vec3( 1.0, -1.0, 0.0) * s, 0.0);
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            float alpha = (0.1 - screenFrac) * vertex[0].brightness * brightness;
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            vec4 color = vec4(vertex[0].color, alpha);
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            set_vp(p + v0);
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            v_TexCoord  = vec2(0.0, 1.0);
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            v_Color     = color;
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            EmitVertex();
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            set_vp(p + v1);
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            v_TexCoord  = vec2(0.0, 0.0);
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            v_Color     = color;
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            EmitVertex();
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            set_vp(p + v2);
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            v_TexCoord  = vec2(1.0, 1.0);
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            v_Color     = color;
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            EmitVertex();
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            set_vp(p + v3);
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            v_TexCoord  = vec2(1.0, 0.0);
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            v_Color     = color;
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            EmitVertex();
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        }
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    }
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    EndPrimitive();
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}
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