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I have a 3D scene I use to generate some images which I then process in Matlab. I noticed that when global illumination (GI) is not enabled, rendering always generates exactly the same pixel-for-pixel image (as far as I can tell). However, when I enable GI, the image is not exactly the same. I have been told (I originally accidentally posted here) that GI is stochastic and so this is to be somewhat expected. However, I am curious if there's any way to avoid it (while still using GI).

Thanks

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Yes, computation of global illumination is usually done in a stochastic way and as such it relies on randomly generated numbers.

Typically pseudo random generators are employed, which generate deterministic sequences, and usually have so called "seed" parameter. The seed basically says where to start in the sequence, which in turn affects the noise pattern of your rendered image. If you can use the same seed for each run, there is a high chance of getting identical results.

The question is whether the software that you use allows you to do that. In the rendering settings, I would suggest looking for for words like seed, random or deterministic.

Edit: Since you said you use Vray, it seems to me that using "Deterministic Monte Carlo" engine should be the way to go.

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  • $\begingroup$ Yes! Thanks! This solved the problem. As an additional note, "Deterministic Monte Carlo" is (now) known as "Brute Force" in Vray. I thought I tried this already, but suspect I may have failed to set it in both my primary and secondary GI engines $\endgroup$ Apr 29, 2016 at 16:38
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If you can make your samples deterministic (the same from run to run - don't base random sample points on time or any other non deterministic value), it will be a stable rendering that you get every time.

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