I have an application that uses OpenGL to draw output images. For testing purposes I'm trying to create reference images and then use precision hash to compare them to program output. While it works flawlessly within a context of a single computer I've encountered strange problems when using the same approach with computers running different GPUs. While the images generated on different GPUs appear absolutely identical to human eye they cannot pass prescision hash test when compared to one another and per-pixel comparison reveals that there are several pixels that are "off". I've been trying to find problems in my code for several days to no avail and this behaviour manifests itself on all the shaders that I use. Could this possibly be due to differences in OpenGL implementation from different hardware manufacturers? Is it a valid approach to compare images generated on different GPUs with phash for testing purposes?
1 Answers
Could this possibly be due to differences in OpenGL implementation from different hardware manufacturers? Is it a valid approach to compare images generated on different GPUs with phash for testing purposes?
No, it is not. Quoting the OpenGL 4.6 core profile specification, Appendix A "Invariance" (emphaisis mine):
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. The purpose of this appendix is to identify and provide justification for those cases that require exact matches.
The guarantees for exact matches are made only within the same implementation, under very strict limits, and these are usueful for example for multi-pass approaches where you need to get exactly the same fragments in different passes.
Note that the other 3D rendering API are not pixel-exact either. The actual hardware implementations do vary between individual GPUs, and the specifications typically only specify broader rules that every implementation must fulfill, and you can rely on.
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