OpenGl Blending

Question

i want to blend two rects, but i want to draw only blended area (area where rects are intersecting), How to do it

Make yourself registered user. You will have more answers :-) — Yuriy Vikulov, May 15 '11 at 02:10

score 1 · Accepted Answer · answered May 03 '11 at 22:51

1

If you don't want to compute the intersection you can probably use the stencil buffer to achieve that. read about it here:
http://bluevoid.com/opengl/sig00/advanced00/notes/node118.html

You can draw the two rects and with increment on the stencil buffer and then mask only the pixels that have value > 2, i.e. the pixels where 2 or more rects were drawn.

answered May 03 '11 at 22:51

shoosh

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I dont think thats possible, or maybe i dont fully understand how stencil buffer works, i have one big vertex array with all rects inside, i cannot draw them one by one because there are lot of them and its very slow. I need to go deeper with scencil :) – ZZZ May 05 '11 at 11:56

score 0 · Answer 2 · answered May 03 '11 at 22:48

0

The intersection of two convex rects is always a rect. so why not just compute the intersection and draw only the that?

answered May 03 '11 at 22:48

shoosh

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Only if they are axis aligned ;) – ChrisF May 03 '11 at 22:50
Well, they are axix aligned but calculating those intersections may be slow, i have about 400 rects and 1ghz hummingbird. – ZZZ May 05 '11 at 11:58

score 0 · Answer 3 · answered May 06 '11 at 04:58

GLES20.glEnable( GLES20.GL_BLEND );
GLES20.glBlendFunc( GLES20.GL_SRC_ALPHA, GLES20.GL_ONE_MINUS_SRC_ALPHA );

But you should set behavior of your blend function yourown. And in the shader I set alpha channel. You can see the result:

blending post. the source of android project

OpenGl Blending

3 Answers3