Vulkan: Glow on small bright highlights looks box or square shaped

[golddotasksquestions]

Godot version

v4.0.alpha1.official [31a7ddb]

System information

Win 64 bit, Nvidea765M, Vulcan

Issue description

Glow in Godot4 alpha1 always looks way more box shaped compared to Godot3 glow.

In Godot3 this also was notably the case (but much less so) when Bicubic Upscale property was disabled, but with Bicubic glow looked reasonably roundish. In Godot 4 it Glow looks completely square with or without Bicubic Upscale, which in my eyes is quite a massive downgrade in visual fidelity.

godot4_alpha1_glow.mp4

The box shape glow in Godot4 is especially apparent on small light sources reaching over the HDR Threshold.

I increased the intensity in the following examples to make it even more apparent:

Godot 4 with Upscale mode Liniar and HighQuality disabled:

Godot4, with Upscale mode Bicubic and High Quality enabled:

Godot3 with Bicubic Upscale and High Quality disabled:

Godot3 with Bicubic Upscale and High Quality enabled:

Youtube video showing Godot 4.0 stable square shaped glow in a regular scene:
https://www.youtube.com/watch?v=Q8bGOfXGqLU

Steps to reproduce

Enable Glow in a scene with small areas over the HDR threshold

Minimal reproduction project

No response

[Calinou]

Small bright highlights will always be difficult to manage for glow in an efficient way. I don't think much can be done to improve this, other than making your highlight larger and more diffuse (or playing with the glow levels in Environment).

I have a pull request that tweaks the glow defaults, could you try the values specified there?

Note that Godot 4's glow behaves much better than Godot 3's glow when you have small flickering highlights ("fireflies"), as is often the case with normal mapped non-rough materials: #54409

#57682 may affect this, though I don't know for certain.

[golddotasksquestions]

It's just most apparent with smaller HDR areas. This issue here has nothing to do with flickering highlights (fireflies) as far as I can tell. It's about the shape of the glow. I've noticed the boxyness of the glow the first time I saw glow in Godot4. It's really ugly and looks like a step down in visual quality compared to Godot3. Godot 3 already had a lot worse looking glow and bloom compared to Unity and Unreal, this is really a step further in the wrong direction.

I don't know why the Bicubic glow in Godot3 looks so much rounder. Maybe more blur (lower res mipmap) was used?

[clayjohn]

I took a look into this briefly as I worked on a related issue. To get a rounder appearing glow we need to use a smaller gaussian sigma (we need to give more strength to the central pixels and less strength to the pixels that are further away). Using a sigma of 1.5 calculated from this site gives very pleasing results in still scenes. The falloff appears perfectly circular.

That being said, the problem with a smaller sigma is that it gives more strength to the central pixels so you end up with more flickering and jumping around in motion. Especially in scenes with small bright spots.

Right now, the best tool to fight the square falloff is to lower the intensity a little bit. Both in 3.x and in 4.0 high brightness values expose the square shape, and lower intensities restore the circular shape.

In the future, I would like to make the sigma configurable so users can choose between a more pleasing circular falloff (with artifacts in motion) or a better experience in motion (with a more square shape).

#56452 is related as these stem from the same underlying issue (i.e. the gaussian blur gives a relatively high weight to all pixels in the blur)