HDR skybox colors are too dark/bright with compatibility renderer

[discapes]

Godot version

v4.2.beta2.official [f8818f8], v4.1.2.stable.fedora [399c9dc]

System information

Fedora 40 (Rawhide) - OpenGL API 4.6 (Core Profile) Mesa 23.2.1 - Compatibility - Using Device: Intel - Mesa Intel(R) Xe Graphics (TGL GT2)

Issue description

When I add a skybox with a .hdr or .exr texture, it looks good on the Forward+ and Mobile renderers, but not with the Compatibility renderer. See images:

Expected (rendered with Forward+):

Actual (rendered with Compatibility):

Steps to reproduce

1. Create project
2. Add camera
3. Add WorldEnvironment
4. Set the background to Sky, and set the Sky Material to a PanoramicSky, and set the Panorama to a HDR image

• This texture was used in the above images

6. Observe how it looks good in Forward+ and Mobile, but not in Compatibility

Minimal reproduction project

N/A

[discapes]

Btw, this issue isn't present in v3.5.2.stable.fedora [170ba33], so I assume the regression tag is approprate

[AThousandShips]

We don't normally add regression tag between 3.x and 4.x, it mainly matters between 4.x version to track things that have done wrong

[jsjtxietian]

From https://docs.godotengine.org/en/stable/contributing/development/core_and_modules/internal_rendering_architecture.html,
"Unlike the mobile renderer, colors are tonemapped and stored in sRGB format so there is no HDR support."

[discapes]

Is it planned? I would be really nice since skyboxes are a big part of many games.

[clayjohn]

In the compatibility renderer it looks the same as the GLES2 renderer in 3.x which is kind of expected

Screenshot from 3.5.1 GLES2 renderer

The underlying issue here is that the sky shader assumes that it is going to receive a texture encoding in sRGB space (0-1 range). But instead it receives a linear texture. So the color space is all wonky.

We can fix this, but any fix comes with tradeoffs.

Option 1:

Force the Compatibility renderer to always use sRGB textures. This would result in us compressing the image to a lower precision format and compressing to the 0-1 range. This would make the HDRI look closer to the other renderers and also slightly improve performance. But it would restrict users from reading accurate color data out of HDRIs.

This issue will also affect 3D shaders if users use HDRIs as input. This option would fix the problem for both sky shaders and 3D shaders.

Option 2:

Detect if an HDRI is being used and provide a slightly different shader path for it. This would be worse for performance than option 1, but would look better. It would also allow users to continue user HDR textures in 3D shaders which would continue to be broken.

Edit: the tentative fix (option 1) looks like this:

Note how the bright parts of the sky wash out towards white

[ParsleighScumble]

There's got to be a good way to do this... Maybe if we internally convert the hdri into RGBE and make the sky shader expect that format instead it'll be ok for bandwidth? Just speculating, I'm not too sure about this.

[jane00]

I also got the dark one when I run the project on Android mobile.

[vaunakiller]

To anyone looking for a quick dirty hack:

1. Convert HDR texture to PNG or JPEG. (there are plenty of converters online - just google it)
2. This PNG/JPEG still will be too dark. Throw it into image editor of choice. Play with gamma values (around 2.0 worked fine for me). If either sky or ground are blown out you can try adjusting levels first.
   Ofcourse you will loose quality. But hey - that worked for me in "lower fidelity" graphics style 😊

P.S. Bonus points for getting compression artifacts - JPEG gets some muddy feeling, while PNG's remind me of dithering and banding.