[vert.glsl]

#version 120

varying vec2 pass_TexCoord;

void main()
{
    gl_Position = gl_Vertex;
    pass_TexCoord = vec2(0.5, 0.5) + 0.5 * gl_Vertex.xy;
}

[frag.glsl]

#version 120

varying vec2 pass_TexCoord;

uniform sampler2D source;
uniform sampler2D buffer;
uniform vec2 mix;

void main(void)
{
    vec4 old_color = texture2D(buffer, pass_TexCoord);
    vec4 new_color = texture2D(source, pass_TexCoord);

    /* The old way */
    //gl_FragColor = new_color * mix.x + old_color * mix.y;

    /* The new way: if new_color > old_color we want faster updates */
    gl_FragColor = max(new_color, new_color * mix.x + old_color * mix.y);
}