Possible "seam" bug with MSAA, accentuated by new mipmap code

[themanuel]

After updating to 5755, which nicely fixes the skybox issue on SMG2 and light rays in TLoZTP, I noticed something that apparently was there before but less prevalent.

This may be in other games as well but I just noticed it in SMG2 in World #1, where the surrounding sky is blue and cloudy.  If you look at the skybox or even the planet underneath, there are some obvious seam lines delineating the triangles that make up the background.

I have narrowed down the issue to MSAA and happens regardless of internal resolution being rendered.  This does not happen without AA or when using SSAA.

The problem was present on the previous version I was using (r5636) but was much less noticeable and I only realized it after looking very closely while trying to narrow down the cause of the issue.  I suspect the new mipmap code that fixes the skybox makes the issue more obvious.

You can see some comparisons below:

Skybox 3xIR 4xMSAA r5636
Issue present close to the light band in the middle but not very noticeable.

Skybox 3xIR 4xMSAA r5755
Issue present and much more noticeable.

Save Select 3xIR 4xMSAA r5755
Issue present but a bit hard to see in screenshot.

Save Select 3xIR NO AA r5755
Issue not present without AA.

Should I submit a but report or is this a known issue?

[JMC47]

My suggestion is to not use AA, or to use SSAA as MayImilae indicates it doesn't have an issue.

Adding anti-aliasing into games breaks things for reasons... maybe we should write a blog article about that... it sounds interesting the more I think of it. Why does adding AA break a game but not increasing the internal resolution?

[themanuel]

Yep. I'm OK with that. It's barely noticeable and so far only in that particular stage (i'm up to sixty-something stars).

[Shonumi]

(11-04-2017, 07:51 AM)JMC47 Wrote: it sounds interesting the more I think of it.  Why does adding AA break a game but not increasing the internal resolution?

Probably something to do with the fact that MSAA only operates on geometry and Dolphin doesn't like that in certain cases. SSAA and variants blow up everything​ then downscale the result, so Dolphin is happy with it, since it's basically like using a high IR but with better downsampling. FXAA shouldn't be a problem either, since it's done as post-processing.

[DaRkL3AD3R]

This is just the nature of the beast. It goes beyond just an emulator and graphics settings. Here is the same phenomenon in Max Payne 1 when you use anti-aliasing: https://i.imgur.com/UDidXcy.png Same exact line traces around polygon edges and is very noticeable in motion.

[degasus]

The point here is about using or not using centroid. Without centroid, you may call the pixel stage outside of the primitive, which leads to wrapping issues. With centroid, you interpolate at a different location than the pixel will be written, which leads to the shown effect. Pick your poison.