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Cycles tiling artifacts with path guiding and caustics #113799

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opened 2023-10-17 04:02:27 +02:00 by Masud-Al-Mahdi · 8 comments
Masud-Al-Mahdi commented 2023-10-17 04:02:27 +02:00
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System Information
Operating system: Windows-10-10.0.22621-SP0 64 Bits
Graphics card: AMD Radeon(TM) Graphics ATI Technologies Inc. 4.6.0 Core Profile Context 23.8.1.230810

Blender Version
Broken: version: 4.0.0 Beta, branch: blender-v4.0-release, commit date: 2023-10-02 01:10, hash: 883033a0969f
Worked: (newest version of Blender that worked as expected)

Short description of error
Rendering artifacts on cycles. As soon as increasing rendering resolution. (probably when tiled rendering takes place).

Exact steps for others to reproduce the error

Render the .blend file without any change. resolution 3840x2160. (causes artifact) Render the .blend file at 1920x1080. resolution 1920x1080. (no artifacts)
artifacted-image non-artifact-image

Where I think problems are

  1. Probably on tilted drawing.
  2. Could also be at denoising. (Denoising was turned on in both).
  3. Otherwise I don't know.
**System Information** Operating system: Windows-10-10.0.22621-SP0 64 Bits Graphics card: AMD Radeon(TM) Graphics ATI Technologies Inc. 4.6.0 Core Profile Context 23.8.1.230810 **Blender Version** Broken: version: 4.0.0 Beta, branch: blender-v4.0-release, commit date: 2023-10-02 01:10, hash: `883033a0969f` Worked: (newest version of Blender that worked as expected) **Short description of error** Rendering artifacts on cycles. As soon as increasing rendering resolution. (probably when tiled rendering takes place). **Exact steps for others to reproduce the error** | Render the .blend file without any change. resolution 3840x2160. (causes artifact) | Render the .blend file at 1920x1080. resolution 1920x1080. (no artifacts) | | -- | -- | | ![artifacted-image](https://i.ibb.co/RDQj89Z/artifact-3840.png) | ![non-artifact-image](https://i.ibb.co/BV2fXS6/no-artifact-1920.png) | **Where I think problems are** 1. Probably on tilted drawing. 2. Could also be at denoising. (Denoising was turned on in both). 3. Otherwise I don't know.
4kres_cycles_artifact.blend
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Jesse Yurkovich commented 2023-10-17 04:21:20 +02:00
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Those images look like Path Guiding was turned on, a CPU only feature, however the .blend attached is set to use the GPU for render. Can you confirm whether you were rending on CPU w/Path Guiding, or just on the GPU?

Does this occur for you if you shrink the final render resolution and also shrink the Tile size in Render properties -> Performance -> Memory -> Tile size. I tried a smaller image with correspondingly smaller tiles and did not observe the artifacts here at least.

As always, please try with a more recent version of the 4.0 beta in case problems have been fixed in the past 2 weeks.

Those images look like Path Guiding was turned on, a CPU only feature, however the .blend attached is set to use the GPU for render. Can you confirm whether you were rending on CPU w/Path Guiding, or just on the GPU? Does this occur for you if you shrink the final render resolution and also shrink the Tile size in `Render properties` -> `Performance` -> `Memory` -> `Tile size`. I tried a smaller image with correspondingly smaller tiles and did not observe the artifacts here at least. As always, please try with a more recent version of the 4.0 beta in case problems have been fixed in the past 2 weeks.
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Masud-Al-Mahdi commented 2023-10-17 07:30:16 +02:00
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The issue still Appears on the latest build.

Following your answer, I pulled the latest commit on blender4.0 & built blender again (today 8:00 AM at GMT+6). Also set the cycles device to "cpu" and path guiding "on".

More details:

  1. Artifact appears in cycles, before denoising
  2. Artifact appears when tiling-size is 0.5x width of the resolution (ex: if resolution is 1920x1080 tilesize is 1024).
  3. Artifact on smaller resolutions artifact is still small. on 4k artifact is almost half the image.

Artifact on (960x540) resolution:

artifact on smaller resolutions

Artifact on 4k resolution:

artifact on bigger resolutions
tilesize:2048

Note: increasing tiling size should not be a workaround to fix rendering artifacts. Because obviously there will be a point where we users can not increase tiling any more (tiling was introduced for exact same reason i believe).

**The issue still Appears on the latest build.** Following your answer, I pulled the latest commit on blender4.0 & built blender again (today 8:00 AM at GMT+6). Also set the cycles device to "cpu" and path guiding "on". ### More details: 1. Artifact appears in cycles, before denoising 2. Artifact appears when tiling-size is 0.5x width of the resolution (ex: if resolution is 1920x1080 tilesize is 1024). 3. Artifact on smaller resolutions artifact is still small. on 4k artifact is almost half the image. ### **Artifact on (960x540) resolution:** ![artifact on smaller resolutions](https://i.ibb.co/Qbgvzvr/960x540.png) ### **Artifact on 4k resolution:** ![artifact on bigger resolutions](https://i.ibb.co/RDQj89Z/artifact-3840.png) **tilesize:2048** Note: increasing tiling size should not be a workaround to fix rendering artifacts. Because obviously there will be a point where we users can not increase tiling any more (tiling was introduced for exact same reason i believe).
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Alaska commented 2023-10-17 15:07:26 +02:00
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I can confirm this issue with CPU rendering with Path Guiding in 3.6 and 4.0.

I have an idea on what might be causing this.

Path Guiding generates a structure to store information about how rays should be guided. The structure is filled with useful information as samples are taken in different parts of the scene/structure.

What I believe is happening is that with tiling enabled, the "left" side the structure is filled out with information while the tile is on the left, while the "right" side of the structure gets very little information due to rays rarely going to that side. Then when the tile goes to the right side of the image, Path Guiding training has stopped (because the 128 training samples have been used up), and the rays on the right tile have to rely on the poorly formed Path Guiding structure, resulting in noticeable artifacts.

If tiling is off, then both the left and right side of the structure are filled out with information fairly evenly throughout the render, meaning there's no issues of "poorly formed" structures inside the camera view.

Here's some "simple" ideas from the code side on how to fix it:

  • When the render finishes a tile and starts a new one, re-enable Path Guiding training/reset the Path Guiding Structure.
    • This should remove artifacts, but will probably introduce discontinuities near the tile borders in some scenes.
  • If tiling is active, turn off the Path Guiding training limit, meaning training is happening all the time.
    • Testing reveled that this doesn't help. Which makes sense because the samples from the right tile are weighted lower than the initial samples in the Path Guiding Structure, meaning the artifacts can be seen through the enhanced Path Guiding structure that forms as samples are taken.
  • If tiling is active, render a low resolution version of the scene (without tiling) to fill out the Path Guiding structure, then render the full resolution image (with tiling), using the generated Path Guiding structure from earlier with no further training.
    • This should resolve the issue, but will increase render times, and using the low resolution render to generate the path guiding structure may accidentally introduce other issues (E.G. Reduced Path Guiding quality?).
  • If tiling is active, don't allow people to use Path Guiding.
    • Not really a fix. Just a way of letting people know this combo is not supported and not letting them use it.
  • If Path Guiding is active, don't allow people to use tiling.
    • Not really a fix. Just a way of letting people know this combo is not supported and not letting them use it.
  • Something else I haven't thought of.

CC @brecht and/or @Sebastian-Herholz to weigh in on what should be done.

I can confirm this issue with CPU rendering with Path Guiding in 3.6 and 4.0. --- I have an idea on what might be causing this. Path Guiding generates a structure to store information about how rays should be guided. The structure is filled with useful information as samples are taken in different parts of the scene/structure. What I believe is happening is that with tiling enabled, the "left" side the structure is filled out with information while the tile is on the left, while the "right" side of the structure gets very little information due to rays rarely going to that side. Then when the tile goes to the right side of the image, Path Guiding training has stopped (because the 128 training samples have been used up), and the rays on the right tile have to rely on the poorly formed Path Guiding structure, resulting in noticeable artifacts. If tiling is off, then both the left and right side of the structure are filled out with information fairly evenly throughout the render, meaning there's no issues of "poorly formed" structures inside the camera view. --- Here's some "simple" ideas from the code side on how to fix it: - When the render finishes a tile and starts a new one, re-enable Path Guiding training/reset the Path Guiding Structure. - This should remove artifacts, but will probably introduce discontinuities near the tile borders in some scenes. - ~~If tiling is active, turn off the Path Guiding training limit, meaning training is happening all the time.~~ - Testing reveled that this doesn't help. Which makes sense because the samples from the right tile are weighted lower than the initial samples in the Path Guiding Structure, meaning the artifacts can be seen through the enhanced Path Guiding structure that forms as samples are taken. - If tiling is active, render a low resolution version of the scene (without tiling) to fill out the Path Guiding structure, then render the full resolution image (with tiling), using the generated Path Guiding structure from earlier with no further training. - This should resolve the issue, but will increase render times, and using the low resolution render to generate the path guiding structure may accidentally introduce other issues (E.G. Reduced Path Guiding quality?). - If tiling is active, don't allow people to use Path Guiding. - Not really a fix. Just a way of letting people know this combo is not supported and not letting them use it. - If Path Guiding is active, don't allow people to use tiling. - Not really a fix. Just a way of letting people know this combo is not supported and not letting them use it. - Something else I haven't thought of. CC @brecht and/or @Sebastian-Herholz to weigh in on what should be done.
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Brecht Van Lommel changed title from Cycles rendering artifact when rendering at 3840x2160 render resolution to Cycles tiling artifacts with path guiding and caustics 2023-10-17 16:43:55 +02:00
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Brecht Van Lommel commented 2023-10-17 16:49:02 +02:00
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This is a known limitation of the path guiding implementation currently. First is that it's not really designed to render caustics like this, it can find them sometimes but the probably is low and so it will not do so consistently across tiles or different frames in an animation.

If tiling is active, render a low resolution version of the scene (without tiling) to fill out the Path Guiding structure, then render the full resolution image (with tiling), using the generated Path Guiding structure from earlier with no further training.

Something along these lines is probably the right solution, but it of course has downsides.

More generally, the issue is that we don't have an efficient way to render these caustics, and if we had a dedicated mechanism for that it would sidestep this problem somewhat.

This is a known limitation of the path guiding implementation currently. First is that it's not really designed to render caustics like this, it can find them sometimes but the probably is low and so it will not do so consistently across tiles or different frames in an animation. > If tiling is active, render a low resolution version of the scene (without tiling) to fill out the Path Guiding structure, then render the full resolution image (with tiling), using the generated Path Guiding structure from earlier with no further training. Something along these lines is probably the right solution, but it of course has downsides. More generally, the issue is that we don't have an efficient way to render these caustics, and if we had a dedicated mechanism for that it would sidestep this problem somewhat.
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Sebastian Herholz commented 2023-11-07 19:48:10 +01:00
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The observation of @Alaska is right. Path guiding does not work well together with tiled rendering.
The training of the guiding structures works best when we have samples from rendering the full image.
This way, we gather information about the whole light transport of the scene at once, and information from paths originating from multiple image regions can be shared.

At the moment, there exists no guiding method, which is based on online learning, that works well with tiled rendering.

The easiest (short-term) solution would be to disable path guiding when the user needs to use tiled rendering or vice-versa (i.e., disable tiled rendering when path guiding is enabled).

The best (long-term) solution, in my opinion, is the one also proposed by @Alaska, is to have a pre-rendering path using a lower resolution to learn the guiding structure and then switch to full-resolution tiled rendering when the training is done.
One could even think about jittering the samples in the training phase in such a way that the samples are splatted in a high-resolution buffer (e.g., rendering every odd pixel in the first iteration and every even in the second).

@brecht is there an easy way to achieve the first solution for now?

The observation of @Alaska is right. Path guiding does not work well together with tiled rendering. The training of the guiding structures works best when we have samples from rendering the full image. This way, we gather information about the whole light transport of the scene at once, and information from paths originating from multiple image regions can be shared. At the moment, there exists no guiding method, which is based on online learning, that works well with tiled rendering. The easiest (short-term) solution would be to disable path guiding when the user needs to use tiled rendering or vice-versa (i.e., disable tiled rendering when path guiding is enabled). The best (long-term) solution, in my opinion, is the one also proposed by @Alaska, is to have a pre-rendering path using a lower resolution to learn the guiding structure and then switch to full-resolution tiled rendering when the training is done. One could even think about jittering the samples in the training phase in such a way that the samples are splatted in a high-resolution buffer (e.g., rendering every odd pixel in the first iteration and every even in the second). @brecht is there an easy way to achieve the first solution for now?
Brecht Van Lommel commented 2023-11-08 18:55:15 +01:00
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It's certainly possible to disable path guiding or tiled rendering automatically, but I don't think doing either silently is a good user interface. Reporting and error with a suggestions to use disable tiling seems better to me (through device->set_error).

But, it may also be that case that path guiding works fine with tiling for less challenging scenes? The same kind of scenes where there is enough coherency for animation perhaps? There is also some amount of overlapping pixels between tiles. If that is common, it would be good to still allow users to use both together.

It's certainly possible to disable path guiding or tiled rendering automatically, but I don't think doing either silently is a good user interface. Reporting and error with a suggestions to use disable tiling seems better to me (through `device->set_error`). But, it may also be that case that path guiding works fine with tiling for less challenging scenes? The same kind of scenes where there is enough coherency for animation perhaps? There is also some amount of overlapping pixels between tiles. If that is common, it would be good to still allow users to use both together.
Masud-Al-Mahdi commented 2023-11-09 22:10:34 +01:00
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From what i think, If path guiding fills up PGS at the start of the rendering, how about:

  • if path guiding is on & tiles are smaller then render resolution:
    • for final render(which is most important):
      • switch tile size to full resolution and fill up the PGS (path guiding stucture) after that switch tile sizes back to original and render the rest of the image.
    • warn the user about it
From what i think, If path guiding fills up PGS at the start of the rendering, how about: - if path guiding is on & tiles are smaller then render resolution: - for final render(which is most important): - switch tile size to full resolution and fill up the PGS (path guiding stucture) after that switch tile sizes back to original and render the rest of the image. - warn the user about it
Jakub Jaszewski commented 2023-11-14 20:45:38 +01:00
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  • If tiling is active, render a low resolution version of the scene (without tiling) to fill out the Path Guiding structure, then render the full resolution image (with tiling), using the generated Path Guiding structure from earlier with no further training.
    • This should resolve the issue, but will increase render times, and using the low resolution render to generate the path guiding structure may accidentally introduce other issues (E.G. Reduced Path Guiding quality?).

@Alaska I'm doing a fair amount of large renders (between 10000 and 20000px) and often those scenes cannot fit into VRAM, so I'm left with rendering on a CPU with tiles.
Having the ability to bake PG on lower res image - lets say 50% of the final size - would be absolutely great to have.
PG paths could be also stored for faster re-renders as part of persistent data option if nothing in the scene have changed.

> - If tiling is active, render a low resolution version of the scene (without tiling) to fill out the Path Guiding structure, then render the full resolution image (with tiling), using the generated Path Guiding structure from earlier with no further training. > - This should resolve the issue, but will increase render times, and using the low resolution render to generate the path guiding structure may accidentally introduce other issues (E.G. Reduced Path Guiding quality?). @Alaska I'm doing a fair amount of large renders (between 10000 and 20000px) and often those scenes cannot fit into VRAM, so I'm left with rendering on a CPU with tiles. Having the ability to bake PG on lower res image - lets say 50% of the final size - would be absolutely great to have. PG paths could be also stored for faster re-renders as part of persistent data option if nothing in the scene have changed.
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