Reduction shader

uniform sampler2D texture;
uniform float width; //x resolution of the image

void main()
{
    float pixeltopixeldistance = 1.0f / width;

    //odd pixel?
    bool thisrow = fract(gl_TexCoord[0].x * width / 2.0f) < 0.5f;

    vec2 otherpixel;
    if(!thisrow)    otherpixel = texture2D(texture, gl_TexCoord[0].xy - vec2(pixeltopixeldistance,0)).rg; //color to the left
            else    otherpixel = texture2D(texture, gl_TexCoord[0].xy + vec2(pixeltopixeldistance,0)).rg; //color to the right

    vec2 color  = texture2D(texture, gl_TexCoord[0].xy);
    vec2 result = vec2(min(color, otherpixel));  //, min(min(color.g, colorL.g) , colorR.g));
    gl_FragColor = vec4(min(color, otherpixel),0,1);
}