AvoidXferMode to replace a color on canvas

Question

I'm trying to replace a color for something that is drawn on a Canvas using AvoidXferMode. From the android docs it looks like it's exactly what I need:

AvoidXfermode xfermode will draw the src everywhere except on top of the opColor or, depending on the Mode, draw only on top of the opColor.

What I'm trying is something like this:

Paint paint = new Paint();
paint.setColor(Color.RED);
canvas.drawPaint(paint); // actually drawing a bitmap here
paint.setXferMode(new AvoidXferMode(Color.RED, 0, TARGET);
paint.setColor(Color.GREEN);
canvas.drawPaint(paint);

However, this just gives a red screen, not green as I would expect (replacing the red with green). I guess I'm missing the point some where...Any suggestions?

Answer 1

I'm currently having the same problem but I got it working by specifying 255 as tolerance instead. According to the API documentation this is wrong (It should draw the destination EVERYWHERE with this full-tolerance setting) but for some reason the value 255 does exactly what the value 0 should do.

Answer 2

I finally found out what is issue is, there are some clues here: http://stackoverflow.com/questions/2553694/avoidxfermode-tolerance but it really hit me when I read this post http://stuffthathappens.com/blog/2010/06/04/android-color-banding/ by Eric Burke. The tolerance is failing because the view canvas is not in 8888 mode.

That means that when you draw a color or bitmap on that canvas the colors get converted to the target pixel format and the color might slightly change. To fix this you can either switch the entire window pixel format as seen in Eric's post or you can draw onto a 8888 back buffer.