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GLSL fragment shader, prevent depth writing


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#1 Yours3!f   Members   -  Reputation: 1344

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Posted 17 May 2013 - 02:52 AM

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


Edited by Yours3!f, 17 May 2013 - 02:55 AM.


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#2 tanzanite7   Members   -  Reputation: 1304

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Posted 17 May 2013 - 01:03 PM

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.



#3 samoth   Crossbones+   -  Reputation: 4789

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Posted 17 May 2013 - 02:39 PM

That won't work. You have only 3 legitimate options:

  • disable depth writes (using the GL API, not in GLSL) -- this will not write depth for any pixel
  • don't write to gl_FragDepth in your shader (not at all) -- this will use the interpolated depth instead
  • discard -- this will kill the fragment, including depth (but also color)

 

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.


Edited by samoth, 17 May 2013 - 02:42 PM.


#4 Yours3!f   Members   -  Reputation: 1344

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Posted 18 May 2013 - 01:19 AM

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?

Attached Thumbnails

  • early_depth_test.png

Edited by Yours3!f, 18 May 2013 - 05:40 AM.


#5 tanzanite7   Members   -  Reputation: 1304

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Posted 18 May 2013 - 05:11 PM

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 :/.

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

Edited by tanzanite7, 18 May 2013 - 05:15 PM.


#6 Yours3!f   Members   -  Reputation: 1344

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Posted 19 May 2013 - 12:30 AM

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 :/.

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:
It is not a performance concern now, it will be if there are lots of overlapping thing in the scene, as I'm using a simple lightweight deferred system for lighting. And since early depth testing is usually used in such systems I thought I'd give it a try.

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.






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