src/shaders/luminosityHighPass.glsl

uniform sampler2D tDiffuse;
uniform vec3 defaultColor;
uniform float defaultOpacity;
uniform float luminosityThreshold;
uniform float smoothWidth;

varying vec2 vUv;

void main() {
  vec4 texel = texture2D( tDiffuse, vUv );
  vec3 luma = vec3( 0.299, 0.587, 0.114 );

  float v = dot( texel.xyz, luma );

  vec4 outputColor = vec4( defaultColor.rgb, defaultOpacity );

  float alpha = smoothstep( luminosityThreshold, luminosityThreshold + smoothWidth, v );

  gl_FragColor = mix( outputColor, texel, alpha );
}