use different blending technique

shaders/matcap_frag.glsl

@@ -3,27 +3,25 @@ uniform sampler2D tMatcap;
 uniform vec3 color;
 varying vec2 vNormal;
 
-// color blending from https://www.w3.org/TR/compositing-1/#blendingcolor
-float lum(vec3 c) {
-  return c.r * .3 + c.g * .59 + c.b * .11;
+vec4 screen(vec4 b, vec4 s) {
+  return b + s - (b * s);
 }
 
-vec3 clipColor(vec3 c) {
-  float l = lum(c);
-  float n = min(min(c.r, c.g), c.b);
-  float x = max(max(c.r, c.g), c.b);
-  if (n < 0.) c = l + (((c - l) * l) / (l - n));
-  if (x > 1.) c = l + (((c - l) * (1. - l)) / (x - l));
-  return c;
+vec4 multiply(vec4 b, vec4 s) {
+  return b * s;
 }
 
-vec3 setLum(vec3 c, float l) {
-  float d = l - lum(c);
-  return clipColor(c + d);
+// source https://www.w3.org/TR/compositing-1/#blendingdarken
+vec4 hardLight(vec4 b, vec4 s) {
+  if (length(s) < .5) {
+    return multiply(b, 2. * s);
+  } else {
+    return screen(b, 2. * s - 1.);
+  }
 }
 
 void main() {
   vec4 matcap = texture2D(tMatcap, vNormal);
-  vec3 coloredMatcap = setLum(color, lum(matcap.rgb));
-  gl_FragColor = vec4(coloredMatcap, opacity);
+  vec4 coloredMatcap = hardLight(matcap, vec4(color, 1.));
+  gl_FragColor = vec4(coloredMatcap.rgb, opacity);
 }