Use Scratch's workaround for fully transparent pixels

Author: adazem009

src/shaders/sprite.frag

@@ -87,11 +87,18 @@ const vec2 kCenter = vec2(0.5, 0.5);
 
 void main()
 {
-    vec4 texColor = texture2D(u_skin, v_texCoord);
+    vec2 texcoord0 = v_texCoord;
+
+    gl_FragColor = texture2D(u_skin, texcoord0);
+
+    #if defined(ENABLE_color) || defined(ENABLE_brightness)
+    // Divide premultiplied alpha values for proper color processing
+    // Add epsilon to avoid dividing by 0 for fully transparent pixels
+    gl_FragColor.rgb = clamp(gl_FragColor.rgb / (gl_FragColor.a + epsilon), 0.0, 1.0);
 
     #ifdef ENABLE_color
     {
-        vec3 hsv = convertRGB2HSV(texColor.rgb);
+        vec3 hsv = convertRGB2HSV(gl_FragColor.rgb);
 
         // Force grayscale values to be slightly saturated
         const float minLightness = 0.11 / 2.0;
@@ -102,22 +109,20 @@ void main()
         hsv.x = mod(hsv.x + u_color, 1.0);
         if (hsv.x < 0.0) hsv.x += 1.0;
 
-        texColor.rgb = convertHSV2RGB(hsv);
+        gl_FragColor.rgb = convertHSV2RGB(hsv);
     }
     #endif // ENABLE_color
 
     #ifdef ENABLE_brightness
-    texColor.rgb = clamp(texColor.rgb + vec3(u_brightness), vec3(0), vec3(1));
+    gl_FragColor.rgb = clamp(gl_FragColor.rgb + vec3(u_brightness), vec3(0), vec3(1));
     #endif // ENABLE_brightness
 
+    // Re-multiply color values
+    gl_FragColor.rgb *= gl_FragColor.a + epsilon;
+
     #ifdef ENABLE_ghost
-    texColor *= u_ghost;
+    gl_FragColor *= u_ghost;
     #endif // ENABLE_ghost
 
-    // Set RGB components to zero if the color is fully transparent
-    // This is a workaround for rendering issues when alpha is zero
-    if(texColor.a == 0.0)
-        gl_FragColor = vec4(0.0, 0.0, 0.0, 0.0);
-    else
-        gl_FragColor = texColor;
+    #endif // defined(ENABLE_color) || defined(ENABLE_brightness)
 }