invert.frag.log69.cpp

#version 140
uniform sampler2D sampler;
uniform vec4 modulation;
uniform float saturation;

in vec2 texcoord0;

out vec4 fragColor;

void main() {
  vec4 tex = texture(sampler, texcoord0);
  if(tex.a > 0.0) { 
    tex.rgb /= tex.a; 
  }
  float lightness = (tex.r + tex.g + tex.b) / 3.0; 
  float delta = (1.0 - lightness) - lightness; 

  tex.rgb = delta + tex.rgb;
  tex *= modulation;
  tex.rgb *= tex.a; 
 
  fragColor = tex;
}