Add ability to set param of stretch

image.frag

@@ -24,7 +24,10 @@ void main(void)
         color = log(a*color + 1.0) / log(vec4(a+1));
         break;
     case 4:
-        color = asinh(color / a) / asinh(1 / a);
+    {
+        float l = color.r*0.2126f + color.g*0.7152f + color.b*0.7152f;
+        color *= asinh(l / a) / asinh(1 / a);
+    }
         break;
     }