CODEWITHSUNDEEP
opengldepth-bufferdepth-testing
Viktor Sehr
Viktor Sehr

Reputation: 13099

How can I write a different value to the depth buffer than the value used for depth comparisson?

In Open GL, is it possible to render a polygon with a regular depth test enabled, but when the depth buffer value is actually written to the depth buffer, I want to write a custom value?

(The reason is I'm rendering a particle system, which should be depth-tested against the geometry in the scene, but I want to write a very large depth value where the particle system is located, thus utilizing the depth-blur post-processing step to further blur the particle system)

Update To further refine the question, is it possible without rendering in multiple passes?

Upvotes: 2

Views: 3961

Answers (3)

matthias_buehlmann
matthias_buehlmann

Reputation: 5081

One way to do it in a single pass is by doing the depth-test "manually".

Set glDepthFunc to GL_ALWAYS

Then in the fragment shader, sample the current value of the depth buffer and depending on it discard the fragment using discard;

To sample the current value of the depth buffer, you either need ARM_shader_framebuffer_fetch_depth_stencil (usually on mobile platforms) or NV_texture_barrier. The later however will yield undefined results if multiple particles of the same drawcall render on top of each other, while the former in this case will use the written depth value of the last particle rendered at the same location.

You can also do it without any extension by copying the current depth buffer into a depth texture before you render the particles and then use that depth texture for your manual depth test. That also avoids that particles which render on top of each other would interfere, as they'll all use the old depth value for the manual test.

Upvotes: 1

Nicol Bolas
Nicol Bolas

Reputation: 474136

You don't.

OpenGL does not permit you to lie. The depth test tests the value in the depth buffer against the incoming value to be written in the depth buffer. Once that test passes, the tested depth value will be written to the depth buffer. Period.

You cannot (ab)use the depth test to do something other than testing depth values.

ARB_conservative_depth/GL 4.2 does allow you a very limited form of this. Basically, you promise the implementation that you will only change the depth in a way that makes it smaller or larger than the original value. But even then, it would only work in specific cases, and then only so long as you stay within the limits you specified.

Enforcing early fragment tests will similarly not allow you to do this. The specification explicitly states that the depth will be written before your shader executes. So anything you write to gl_FragDepth will be ignored.

Upvotes: 3

Alex Sokolowski
Alex Sokolowski

Reputation: 222

You can use gl_FragDepth in fragment shader to write your custom values.

gl_FragDepth = 0.3;

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/gl_FragDepth.xhtml

Upvotes: 0

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