Hi,
is there any way to prevent OGL to write the depth buffer from the fragment shader? I'm thinking of something like discard.
I'd like to throw away pixels whose alpha is not big enough. This would help me to perform early depth testing to reduce overdraw.
Node that this is a 2D engine, so all of this are basically sprites.
Best regards,
Yours3lf
Early depth testing is very efficient (nothing even remotely as efficient can be done in shader) - mucking with the depth in fragment shader will turn early depth test off (because there is no depth early available to do anything with ... early).
You can not disable depth writes in fragment shader - just discard the fragment completely (the "discard" you mentioned) omitting the depth write also.
Newer OGL versions might provide something that could be of use tho (but i doubt it) - i rarely use anything above OGL3.3.
That won't work. You have only 3 legitimate options:
Writing to gl_FragDepth for some fragments and not doing it for some others is theoretically possible, but a shader doing so is not well-formed. The outcome is undefined.
To throw away pixels that you don't want, I'd just discard, that's what it's for. It discards the color value, nothing is written out.
thanks both of you. I tried it out, and the results I got was:
-when early depth testing is enabled: layout(early_fragment_tests) in;
the whole sprite is overwritten the by topmost (despite the black areas being discarded, see pic)
-if disabled it works fine.
so I'd like to overcome this. Any ideas?
Weird :/.thanks both of you. I tried it out, and the results I got was:
-when early depth testing is enabled: layout(early_fragment_tests) in;
the whole sprite is overwritten the by topmost (despite the black areas being discarded, see pic)
-if disabled it works fine.
Weird :/.thanks both of you. I tried it out, and the results I got was:
-when early depth testing is enabled: layout(early_fragment_tests) in;
the whole sprite is overwritten the by topmost (despite the black areas being discarded, see pic)
-if disabled it works fine.
Generally, if early depth test is possible then it will be done - there is no reason to ask for it (funky exceptions, which i am not aware of, excepted). When explicitly requiring it causes "discard" to be ignored then ... that is a strong suggestion that early depth test is impossible given the shader.
Discard is similar to alpha testing - it sounds like your hardware just does not support early depth test with alpha testing. When i think about it then it rings true - i vaguely remember that alpha test indeed is not compatible with early depth test.
Logically it is also true - depth test and write happen at the same time, it can not do that early if the alpha test/discard result is only known after the fragment program ran.
In short: cannot use early depth test.
So, bac to: Why do you need it ... really? What platform are you on that it is an concern? How do you know the overdraw is a problem? Is discard without early depth test really too slow? Could you perhaps simplify your shader?
edit:
Wiki agrees with me: http://www.opengl.org/wiki/Early_Fragment_Test
thanks for clarifying. Seems like I can't use early depth testing like this :/
Well I thought that I could eliminate overdraw by doing it. I plan to create a 2D game and if lots of effects will be going on at the same place it would kill the frame rate.
something like this:
For now only draw calls are a problem, but I'll solve it by instancing.
I thought about a depth pre-pass pass :) but that would require evaluating the fragment shader, which is not good.
