Gray scale window fragment GLSL shader works with Picom compositor

grayscale.glsl

/* To get this shader working, Run picom like the following
   picom --glx-fshader-win "$(cat ~/.config/picom/gs.glsl)"
   
   NOTE: You need to set the backend to "glx" to get this working.
*/
uniform float opacity;
uniform bool invert_color;
uniform sampler2D tex;

void main() {
	vec4 c = texture2D(tex, gl_TexCoord[0].xy);
	float g = (c.r + c.g + c.b) / 3.0;	// EDIT1: Average.
	c = vec4(vec3(g), c.a);			// EDIT2: Color.
 	if (invert_color)
		c = vec4(vec3(c.a, c.a, c.a) - vec3(c), c.a);
	c *= opacity;
	gl_FragColor = c;
}