Specular Aliasing

[harrycollin]

I'm encountering some issues with what I believe is called 'Specular Aliasing'. After many failed attempts to tweak SMAA & MSAA parameters I'm a bit stuck on how to solve this. The image attached showcases the issue quite well, the white edges on the front of the car appear even after using MSAA 8X with Ultra settings.

A similar problem has been talked about over here but doesn't seem to have a concrete solution. I'm open to any suggestions :)

[vanruesc]

This doesn't look like it's related to postprocessing. Does the problem also occur without using postprocessing?

[harrycollin]

Apologies for the lack of information & structure in my original issue. It was intended to be more of an exploratory issue/discussion rather than something directly inheritably wrong with postprocessing. Perhaps I should have posted over here.

The aliasing does indeed appear without using postprocessing. I'm not sure what causes this to happen, only that it's more obvious on ultra smooth surfaces in scenes that use HDRs with high contrast.

I would have gladly filled out the original issue with more concrete cause/effect information, hoverer I'm not that knowledgeable on this subject.

[vanruesc]

No problem 👍

The aliasing artifacts you're seeing are likely caused by the lighting setup in your scene, including the HDR background which is probably being used as a scene.environment.

I think the problem is that MSAA, and any other antialiasing algorithm, doesn't work with very bright pixels.

Lights with high intensities cause pixel values to exceed the LDR range [0, 1]. Antialiasing is a convolution/blurring technique that combines neighboring pixels into one to create smooth linear gradients. This works well if the pixels are in the LDR range because then the resulting gradients can be displayed on screen. However, if any one of the pixels involved in the blurring exceeds the value 1, the result will be more likely to lie outside of the renderable LDR range. And since you have a lot of very bright highlights along geometric edges in your scene, almost all of those pixels probably exceed the LDR range.

You can tune the lighting and the material settings of your objects to combat this. The issue should also go away if you use three's inline tone mapping (e.g. renderer.toneMapping = AgXToneMapping), but this approach is incompatible with postprocessing in general.

[vanruesc]

See also https://mynameismjp.wordpress.com/2012/10/24/msaa-overview/ for more infos on MSAA. The section "Working with HDR and Tone Mapping" covers this topic as well.

[harrycollin]

Thank you for the comprehensive breakdown. I've read through and have a better understand of the problem. The inline toneMapping does indeed work, but isn't good for postprocessing, I'm guessing this is because the pixels fed to postprocessing is already in LDR by that point? I was under the impression I could use the ToneMappingEffect before using SMAAEffect, but it seems my understand of how the effects pass data to each other is wrong. I wrongly assumed that the pixel data after ToneMappingEffect would be converted to LDR.

[vanruesc]

I'm guessing this is because the pixels fed to postprocessing is already in LDR by that point?

Correct.

I was under the impression I could use the ToneMappingEffect before using SMAAEffect, but it seems my understand of how the effects pass data to each other is wrong.

That's a fair assumption and it's actually how it should be done. However, while it's technically possible to merge AA with tone mapping in one pass, it's not guaranteed that all jagged lines will be properly smoothed this way. A better solution is to have one EffectPass that performs tone mapping and a separate final EffectPass that applies antialiasing based on the LDR output of the previous pass.

Still, this won't solve the original issue of broken lines because doing AA in post is image-based while MSAA is geometry based. MSAA actually renders and blends additional pixels around geometric edges and that's something postprocesing can't do. SSAA is the closest thing to MSAA, but it's prohibitively expensive. Other AA such as FXAA and SMAA won't be able to reconstruct the broken lines.

I wrongly assumed that the pixel data after ToneMappingEffect would be converted to LDR.

When tone mapping and AA are merged into one pass, the AA algorithm will still operate on the input image of the pass which won't be affected by the tone mapping. The AA effect will pull in pixels that are not tone mapped and it will blend them with the pixel that is currently going through the combined effect shader.