'maybe I'm using it wrong? when I make any shape sprite it fills up the entire sprite box and you can't see anything behind the sprite at all. Opacity doesn't work, and all the transparency of the base sprite seems to be gone..
I'd like to use it as fog-like effect over the background image.'
I can't upload any examples to Dropbox at the moment, so I can't post you an example...you only need to add,change the following in the fx and xlm files-
1, Add a param named 'mixamount' to the fx file
uniform float mixamount; //parameters 0(no mix) to 100 (full mix),
2, change the last line to-
gl_FragColor = mix(front,mixcolor,((mixamount/10.)*0.2))*front.a;
This will create a moving fog effect on top of your sprite.
I will post an example as soon as I can.