Alpha hashed material produces shifting artifacts when animated in EEVEE #88331

New Issue

Xavier Cho · 2021-05-16T18:30:38+02:00

Xavier Cho commented

2021-05-16 18:30:38 +02:00

When an object with material that has alpha hashed transparency is animated in EEVEE, the noise pattern may look either unstable or shifting as shown below:
alpha-hashed-transparency-problem.mp4

You can observe that the artifacts on top staying in place rather than rotating with the cube while those on the sides keep flickering, thus producing shifting and unstable appearance.

The symptom is also noticeable when changing the camera position in rendered viewport mode in Blender. And it's particularly problematic in UPBGE (a fork of the old BGE based on the latest Blender source) because a game engine must animate objects in realtime with relatively small number of samples per frame.

Upon an examination from the developers of UPBGE, it is found that the problem is due to the fact that the current implementation of alpha hashed transparency uses the world coordinate instead of the object space, as it is often recommended (e.g. 2017 ACM SIGGRAPH Symposium on Interactive 3D Graphics and Games).

It appears that a simple change from the world to object coordinate fixes the problem as shown in this test commit (see line 80) , which produces the following result:
alpha-hashed-problem-solved.mp4

It's my understanding that the original author decided to use the world matrix to avoid the difficulties with certain corner cases, like when the object is scaled by an extreme factor. However, while such a problem may be circumvented relatively easily (e.g. discard the scale information from the matrix) such a problem described above may pose more substantial issues with making an animation or rendering in realtime.

So, I hope this problem and the suggested fix may be evaluated again since we have more people interested in using Blender for such purposes now, thanks to the gaining popularity of EEVEE.

When an object with material that has alpha hashed transparency is animated in EEVEE, the noise pattern may look either unstable or shifting as shown below: [alpha-hashed-transparency-problem.mp4](https://archive.blender.org/developer/F10153380/alpha-hashed-transparency-problem.mp4) You can observe that the artifacts on top staying in place rather than rotating with the cube while those on the sides keep flickering, thus producing shifting and unstable appearance. The symptom is also noticeable when changing the camera position in rendered viewport mode in Blender. And it's particularly problematic in UPBGE (a fork of the old BGE based on the latest Blender source) because a game engine must animate objects in realtime with relatively small number of samples per frame. Upon an examination from the developers of UPBGE, it is found that the problem is due to the fact that the current implementation of alpha hashed transparency uses the world coordinate instead of the object space, as it is often recommended (e.g. [2017 ACM SIGGRAPH Symposium on Interactive 3D Graphics and Games](https://youtu.be/p4TYf5DDpbQ)). It appears that a simple change from the world to object coordinate fixes the problem as shown in this [test commit (see line 80) ](https://github.com/UPBGE/upbge/commit/e0a8638d0b255f2fc6ce778d10dd0d12a467b4d3#diff-2b8bf4f9e10009510d02c14e3659ee032c73f829de5453d6abcdf6a3f884fdfdR80), which produces the following result: [alpha-hashed-problem-solved.mp4](https://archive.blender.org/developer/F10153381/alpha-hashed-problem-solved.mp4) It's my understanding that the [original author ](https://developer.blender.org/p/fclem/) decided to use the world matrix to avoid the difficulties with certain corner cases, like when the object is scaled by an extreme factor. However, while such a problem may be circumvented relatively easily (e.g. discard the scale information from the matrix) such a problem described above may pose more substantial issues with making an animation or rendering in realtime. So, I hope this problem and the suggested fix may be evaluated again since we have more people interested in using Blender for such purposes now, thanks to the gaining popularity of EEVEE.

Xavier Cho commented

2021-05-16 18:30:38 +02:00

Added subscriber: @mysticfall

Jacob Merrill commented

2021-05-16 19:50:31 +02:00

Added subscriber: @JacobMerrill-1

Jacob Merrill commented

2021-05-16 19:50:31 +02:00

I have also observed this phenomenon - it also effects alpha hash shadows as well

Dalai Felinto commented

2021-05-17 10:10:28 +02:00

Added subscriber: @dfelinto

Dalai Felinto commented

2021-05-17 10:10:28 +02:00

Marking the BF Blender tag so it shows in people's radar. The report is missing most of the required information to qualify as a report, and mostly likely belongs elsewhere. (bf-committers or devtalk) I will leave up to the triaging team to handle this.

Xavier Cho commented

2021-05-18 01:06:16 +02:00

Sorry, if I failed to follow a proper procedure somehow. It's my first time to report an issue to Blender project, so it's possible that I missed something. If that to be the case, I'd appreciate it if someone could either move it to a proper place or let me know where and how I should report such a problem. Thanks!

Philipp Oeser commented

2021-05-18 11:14:45 +02:00

Added subscribers: @fclem, @lichtwerk

Philipp Oeser commented

2021-05-18 11:14:45 +02:00

Changed status from 'Needs Triage' to: 'Needs Developer To Reproduce'

Philipp Oeser commented

2021-05-18 11:14:45 +02:00

Why not let @fclem share his wisdom here.

Evan Wilson commented

2021-06-01 23:12:21 +02:00

Added subscriber: @EAW

Evan Wilson commented

2021-06-01 23:12:21 +02:00

@mysticfall I've rewrapped your videos as .mp4 instead of .mkv so that they are playable in the browser.

alpha-hashed-transparency-problem.mp4

alpha-hashed-problem-solved.mp4

@mysticfall I've rewrapped your videos as `.mp4` instead of `.mkv` so that they are playable in the browser. [alpha-hashed-transparency-problem.mp4](https://archive.blender.org/developer/F10153380/alpha-hashed-transparency-problem.mp4) [alpha-hashed-problem-solved.mp4](https://archive.blender.org/developer/F10153381/alpha-hashed-problem-solved.mp4)

Jacob Merrill commented

2021-06-02 00:22:21 +02:00

thanks @EAW 😄

Philipp Oeser removed the

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Alpha hashed material produces shifting artifacts when animated in EEVEE #88331