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Recently I tried blender+cycles with denoiser turned on, and it's just awesome. I can use much fewer samples to get a better image. I became curious how does a denoiser work internally. I guess it's definitely not a simple image blur.

From my observation, a tile gets denoised only when the surrounding 3x3 tiles all gets rendered. This seems a little bit weird to me though. How can a neighbor tile provides any useful information to denoise the current tile?

I'm sure different denoisers have various implementations. It would be good though if someone can tell me how does a denoiser work in general, or point me to relevant literature.

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I am a beginner at blender but found your question amusing enough to want to find out. According to this source, Blender has just adopted Intel's Open Image Denoiser.

From what I understand (experienced users correct me if I am wrong): It analyses the samples at the pixel level in relation to one another defined by the radius setting. Then using a monte carlo trained deep learning library it identifies noise problem areas and reduces the number of samples to simplify the path traced samples that have acted unrealistically with their random bounces.

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  • $\begingroup$ Intel's denoiser is being tested by Blender developers, it's not implemented yet in the software. The way it works is that it's basically a neural network: you should google the, that is an awesome technology. The current denoiser detects high contrast area and gradual changes in the image to separate detail, which needs to be preserved, and noise, which is blurred. Of course this is not a perfect science and there are cases where the denoiser can't separate them, and that's when you get those horrible denoising artifacts. $\endgroup$ – Tareyes Oct 4 '19 at 10:33
  • $\begingroup$ Since it analyses patterns, it doesn't work well near edges, that's why the denoiser is started only when all tiles around are fully rendered $\endgroup$ – Tareyes Oct 4 '19 at 10:34
  • $\begingroup$ @quanke0801 You can use the new denoiser in cycles if you download the alpha version 2.81 [link] (builder.blender.org/download/…). $\endgroup$ – Junkflex Oct 4 '19 at 10:50

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