[vert.glsl]

#version 120

void main()
{
    gl_Position = gl_Vertex;
    gl_TexCoord[0] = gl_MultiTexCoord0;
}

[frag.glsl]

#version 120

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

void main(void)
{
    vec4 old_color = texture2D(buffer, gl_TexCoord[0].xy);
    vec4 new_color = texture2D(source, gl_TexCoord[0].xy);

    /* 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);
}