I am trying to create a script, that would allow me to merge multiple images, rendered with different seeds and different sample amounts into one image with a higher sample amount.

The script is using blender compositor nodes. At start it seemed to work, but upon closer inspection, some brighter parts of the rendered image are noisier than one image rendered with the same sample amount, as the combined image.

Below i'm attaching two renders from the Blender classroom scene, the first one is rendered with 1000 samples, and the second is a merge of 300, 300, 200, 100 and 100 samples.



At first glance, the images are similar, but the difference can be spotted if you zoom in at the top of the board, where it is lit up.

Also, below i'm attaching the node setup, that my script generated. The number in the name of the files shows with how many samples it was rendered.


My question is, is there some "magic" that blender does to make those large samples renders look better, and if yes, can i mimic it somehow in compositing? Or do i have to tweak some render settings when rendering those images for merging.

The images were rendered with direct clamping set to 0, and indirect to 10.

Also it was rendered in blender 3.0.0, with Cycles X.

  • $\begingroup$ I don't know, it seems logical to me that rendering higher samples yields better results. Otherwise I bet this kind of optimization would already be done internally and it wouldn't make sense to hack the compositor in that way. Are you adapting this workflow from another pipeline where this yields better results ? $\endgroup$
    – Gorgious
    Nov 29, 2021 at 11:09
  • $\begingroup$ I'm not really trying to get better results than the natural workflow, my goal is to render a single image on multiple pc-s. I did base my algorithm on this posts: blender.stackexchange.com/a/21809/101111 blender.stackexchange.com/questions/3720/… And mostly i'm curious if it's possible to reach the same level of quality with this method. $\endgroup$ Nov 29, 2021 at 11:28
  • $\begingroup$ And if not possible to reach the same level, then if they are some ways to improve it at least a little bit. $\endgroup$ Nov 29, 2021 at 11:30
  • $\begingroup$ Oh, alright ! Makes sense. :) Maybe you should see the comment chain on this other Q&A blender.stackexchange.com/q/243558/86891 as a matter of fact you may be pleased to see the feature is planned for Blender 3.1 which you can already download and test developer.blender.org/… $\endgroup$
    – Gorgious
    Nov 29, 2021 at 11:38
  • $\begingroup$ Wow, thank you so much. I just tested that feature, the merged render is identical to that rendered on just one machine, down to one pixel. Seems that rendering with different seeds messes with areas of high noise, as the noise has a different pattern each time. $\endgroup$ Nov 29, 2021 at 11:59


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