Thank you andres. It works! Image may be NSFW.
Clik here to view.
I also see now where the error was in my test code. I called pg.filter(shader) outside beginDraw-endDraw.
Now that you have shown how to apply it correctly, it works fine.
Clik here to view.

I also see now where the error was in my test code. I called pg.filter(shader) outside beginDraw-endDraw.
Now that you have shown how to apply it correctly, it works fine.