dithering on grad overlay

assets/diff.frag

@@ -0,0 +1,17 @@
+#version 120
+#ifdef GL_ES
+    precision mediump float;
+#endif
+#extension GL_EXT_gpu_shader4 : enable
+
+
+varying vec4 v_color;
+varying vec2 v_texCoords;
+uniform sampler2D u_texture;
+
+void main(void) {
+    vec4 inColor = texture2D(u_texture, v_texCoords);
+    ivec4 bytes = ivec4(255.0 * inColor);
+    ivec4 mask = ivec4(0x55);
+    gl_FragColor = (bytes ^ mask) / 255.0;
+}