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I'm currently programming a scientific imaging application using OpenGL.

I would like to know if OpenGL rendering (in term of retrieved pixel from FBO) is supposed to be fully deterministic when my code (C++ / OpenGL and simple GLSL) is executed on different hardware (ATI vs NVidia, various NVidia generations and various OS)?

More precisely, I'd need the exact same pixels buffer everytime I run my code on any hardware (that can runs basic GLSL and OpenGL 3.0)...

Is that possible? Is there some advice I should consider?

If it's not possible, is there a specific brand of video card (perhaps Quadro?) that could do it while varying the host OS?

Nicol Bolas
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softmllx
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3 Answers3

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From the OpenGL spec (version 2.1 appendix A):

The OpenGL specification is not pixel exact. It therefore does not guarantee an exact match between images produced by different GL implementations. However, the specification does specify exact matches, in some cases, for images produced by the same implementation.

If you disable all anti-aliasing and texturing, you stand a good chance of getting consistent results across platforms. However, if you need antialiasing or texturing or a 100% pixel-perfect guarantee, use software rendering only: http://www.mesa3d.org/

user57368
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  • Thank you very much! Do you think I could get pixel-perfect results using only Quadro card (and using the same settings) ? – softmllx Oct 28 '11 at 14:18
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    Even with identical hardware, rendering can change across driver versions. Don't expect identical rendering between NVidia's Windows drivers and NVidia's Linux drivers, or even between two different versions of the same driver. And keep in mind that the driver settings can override whatever the application requests in terms of antialiasing or texture filtering quality. Odds are that you don't need pixel-perfect rendering at 60FPS, so you can probably get by with hardware rendering as a preview, and software rendering for your final, exact render that gets saved to disk. – user57368 Oct 28 '11 at 19:25
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By "Deterministic", I'm going to assume you mean what you said (rather than what the word actually means): that you can get pixel identical results cross-platform.

No. Not a chance.

You can change the pixel results you get from rendering just by playing with settings in your graphics driver's application. Driver revisions from the same hardware can change what you get.

The OpenGL specification has never required pixel-perfect results. Antialiasing and texture filtering especially are nebulous parts.

datenwolf
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Nicol Bolas
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If you read through the OpenGL specification, there are a number of deterministic conditions that must be met in order for the implementation to comply with the standard, but there are also a significant number of implementation details that are left entirely up to the hardware vendor / driver developer. Unless you render with incredibly basic techniques that fall under the deterministic / invariant categories (which I believe will keep you from using filtered texturing, antialiasing, lighting, shaders, etc), the standard allows for pretty significant differences between different hardware and even different drivers on the same hardware.

PfhorSlayer
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