Pixel-Composer/shaders/sh_override_channel/sh_override_channel.fsh

varying vec2 v_vTexcoord;
varying vec4 v_vColour;

uniform int mode;
uniform sampler2D samplerR;
uniform sampler2D samplerG;
uniform sampler2D samplerB;
uniform sampler2D samplerA;

uniform int useR;
uniform int useG;
uniform int useB;
uniform int useA;

float sample(vec4 col, int ch) {
	if(mode == 0) return (col[0] + col[1] + col[2]) / 3. * col[3];
	return col[ch];
}

void main() {
	vec4 base = texture2D( gm_BaseTexture, v_vTexcoord );
	
	float r = useR == 1? sample(texture2D( samplerR, v_vTexcoord ), 0) : base.r;
	float g = useG == 1? sample(texture2D( samplerG, v_vTexcoord ), 1) : base.g;
	float b = useB == 1? sample(texture2D( samplerB, v_vTexcoord ), 2) : base.b;
	float a = useA == 1? sample(texture2D( samplerA, v_vTexcoord ), 3) : base.a;
	
	gl_FragColor = vec4(r, g, b, a);
}