Pixel-Composer/shaders/sh_channel_R/sh_channel_R.fsh

//
// Simple passthrough fragment shader
//
varying vec2 v_vTexcoord;
varying vec4 v_vColour;

uniform int keepAlpha;

void main() {
	vec4 col = texture2D( gm_BaseTexture, v_vTexcoord);
    gl_FragColor = vec4(col.r, 0., 0., keepAlpha == 1? col.a : 1.);
}