Display artifacts with ambient occlusion on Mac

[GRSnyder]

I'm seeing problems with shadow rendering on my Mac Mini while working in the Design workspace. Visual style is set to Shaded with Visible Edges Only. The environment is Photo Booth. All rendering effects are turned off except Ambient Occlusion and Anti-Aliasing.

I believe the problem has to do specifically with the rendering for ambient occlusion, the subtle shading that pops in a moment after you stop rotating or zooming the workpiece. In the past this shading has displayed flawlessly. But now I often see a variety of grainy patterns or repeated striations. The severity of the artifacts varies widely depending on the view angle and zoom level. Often, there are no artifacts at all. Also commonly, things look pretty good but you can see repeated linear patterns in the shadows.

The photo below is about as bad as I could get it to reproduce. (View at full resolution to get the full effect.) Note the variety of patterns on the roof of the vault. There are regions with crisp artifact patterns and regions with similar patterns but heavily blurred. Note also the graininess in the sharp notches of the legs.

Here's a close-up of one of the notches. Hard to say if this is just a rendering artifact or if there's actually something wrong with the underlying shadowing: there's a well-delineated mustache at the edge that seems to drop off abruptly into subtle shading.   

If there's a configuration dump I can do to more precisely describe the rendering and display settings, please let me know - I'd be happy to post it.

 

[Phil.E]

Thanks for the report.

It sounds like your computer is struggling to display the two effects. If you notice shadowing goes away when you orbit or zoom, it means the frame rate has dropped below the level set in Preferences, below which effects are temporarily turned off.

Just so you know, Ambient Occlusion and Anti-aliasing are the two most computationally intense effects to render. In my testing on a mac mini I do not see such drastic mustaches as you describe them. 

It also looks like the metallic appearance material is showing the artifacts, but not the purple.

Do you find this effect goes away if you turn off anti-aliasing?

Does the affect only appear on metals with some kind of texture?

Does this happen on all designs, or just designs with more occluded geometry?

Thanks,

[GRSnyder]

---

@Phil.E wrote: It sounds like your computer is struggling to display the two effects. If you notice shadowing goes away when you orbit or zoom, it means the frame rate has dropped below the level set in Preferences, below which effects are temporarily turned off.

Well, "struggling" is a bit strong. Certainly, the Mac Mini has a puny little on-chip hamster-wheel GPU. But you'd be surprised how well it runs Fusion 360, a fact that I very much appreciate! Motion animation is fluid and responsive; it just doesn't include these effects.

The Mini has never been able to display antialiasing or ambient occlusion during motion, so I'm accustomed to those effects popping in after the motion stops. But in the past, the rendering quality has been very good once it appears.

It also looks like the metallic appearance material is showing the artifacts, but not the purple. Does the effect only appear on metals with some kind of texture?

I don't normally have much need for materials or appearances, so I usually work in Component Color mode. That's what you're seeing in the images above: the purple comes from the component color, and the single material applied to the whole model is Steel, which seems to be the default on my setup. The light areas are just at a certain angle to the lighting; they're purple when viewed from a different angle.

As a test, I applied the material "Plastic: Laminate, Red, Matte" to the model and turned off component colors, but the display artifacts are very similar.

Do you find this effect goes away if you turn off anti-aliasing?

No, turning off antialiasing has no effect.

Does this happen on all designs, or just designs with more occluded geometry?

The effect does require some shadowing to be rendered, so I don't see it on, e.g., a cube or a sphere. But the model does not have to be complex to demonstrate the artifacts. For example:

 

Again, view at full resolution to see the artifacts: vertical stripes on the left and horizontal stripes on the right.

And again, keep in mind that the rendering is often perfectly fine. For example, here's that same L-bracket from a slightly different angle and view distance:

I see nothing to complain about there. 

[GRSnyder]

Hmm, here's an interesting case I don't think I've seen before. Here, the blue part is the active component. Everything else should be dimmed out and rendered in wireframe. And yet, there seem to be ambient-occlusion-like artifacts in a variety of places in the background components: 

This is also a good example of "grainy" artifacts in the active component.

 

[Phil.E]

Thanks for the details.

The problem is not related to Mac or M1 or any specific hardware or OS. I can reproduce the issue on Windows.

Whether this is new or not, I'm doubtful it's new. Ambient Occlusion has always been a bit messy. Also, the active component view does not provide "wireframe" for inactive components, it provides a lower opacity setting. Again, I don't think this is new to see ground shadows and object shadows/occlusion on semi transparent objects. I could be wrong, but it's a bit moot anyway.

The real question here is what is the impact? Please describe how this impacts your workflow negatively so I can share your feedback with developers. 

[GRSnyder]

---

@Phil.E wrote: ...I'm doubtful it's new. Ambient Occlusion has always been a bit messy.

It's definitely new for me. I've noticed oddities in ambient occlusion shading over the years, but they've all been in the category of "reasonable renderings of implausible lighting." The striations and graininess are new.

That doesn't necessarily mean that the change is something within Fusion 360. This Mac is relatively new, and it has a slightly newer GPU implementation than the one I used to use. The system software changes all the time. And it's always possible this is something configuration-related, although as far as I'm aware, I haven't touched any graphics settings recently.

The real question here is what is the impact? Please describe how this impacts your workflow negatively so I can share your feedback with developers. 

It's just a display issue. However, the "grains of sand" appearance is not a typical GPU artifact. I know nothing about F360's rendering architecture, but my guess would be that there's some kind of Monte Carlo point sampling going on which is then supposed to be blurred into the appearance of a smooth shadow. But as can be seen in the images above, there seem to be cases where no blurring is occurring at all. That mechanism could potentially explain the other artifacts (striations and mustaches) as well.

As long as I'm Dunning-Kreugerizing based on no actual knowledge (😁), my guess would be that because blurring is expensive, the pipeline tries to guess the amount of blurring needed in each area based on the actual amount of screen real estate it will occupy. And that there's an error in this assessment that causes some large and quite visible areas to be misrated as "no need to blur this texture since it's going to be rendered at such a small size anyway."

So, my pitch to the developers is that this appears to be evidence of a specific and identifiable rendering problem. It's not a general complaint that "ambient occlusion doesn't look right."

[Phil.E · Accepted solution]

Thanks for the details and thorough explanation.

I logged this as a bug for graphics. (Internal note: FUS-85110)