truc.glsl

uniform sampler2D texture;
uniform sampler2D index;
uniform float clicked;
varying vec2 vUv;

void main() {
  vec4 currentTexture = texture2D(texture, vUv);
  vec4 currentIndex = texture2D(index, vUv);
  float fader = 0.0;
  if (clicked/currentIndex.r == 1.0) fader = 1.0;
  gl_FragColor = mix(currentTexture, currentIndex*255.0, fader);
}