Lossless WebP import does not make use of grayscale (L8/LA8) formats when possible

Calinou commented 2 years ago

Godot version

4.0.dev (1032c2c43), 3.x (08ce24a3c), 3.4.2.stable

System information

Fedora 34, GeForce GTX 1080 (NVIDIA 495.46)

Issue description

Unlike PNG compression, lossless WebP compression (enabled by default since Godot 3.4) will not use the L8/LA8 formats (referred to as Lum8 and LumAlpha8 in the inspector preview). Instead, RGB8 or RGBA8 will always be used, which wastes memory.

This can be worked around by enabling Force Png in the Project Settings then reimporting all textures that are imported with lossless compression.

See my complete investigation here: https://github.com/godotengine/godot-proposals/issues/2107#issuecomment-1005752219

cc @mortarroad

Steps to reproduce

Create an image in GIMP with grayscale colors (use Image > Mode > Grayscale before exporting it).
Export the image as lossless WebP using GIMP.
Import the texture in a Godot project.
Double-click the texture in the FileSystem dock and look at the inspector preview on the right.
Notice the texture is using RGB8 format instead of the expected Lum8.
Enable Force Png in the Project Settings. Select the texture in the FileSystem dock and click Reimport.
Double-click the texture in the FileSystem dock again and look at the inspector preview on the right.
Notice the texture is using the Lum8 format.

Minimal reproduction project

master: test_image_formats.zip

3.x: test_image_formats_3.x.zip

mortarroad commented 2 years ago

WebP only supports a limited set of formats. The only ones that are of interest to us are RGB and RGBA. (The others are YUV and other exotic things.)

I see 2 options:

Store a hint for which channels are needed into the .stex file. This is already done on master (not on 3.x, doing that without breaking compat could be tricky), but the importers don't use the field. This hint can be used generically in the StreamTexture::_load_data() function, or passed into the specific loaders (which might be able to do the conversion more efficiently)
Check which channels are used right after loading, and throw them away accordingly. For WebP, we already use that for Alpha, as it provides us with that information.

If we want to go the route with storing a hint, we should also look into checking which channels are used during import, to make sure a grayscale texture always gets loaded as grayscale, even if the PNG file is RGB.

Calinou commented 2 years ago

If we want to go the route with storing a hint, we should also look into checking which channels are used during import, to make sure a grayscale texture always gets loaded as grayscale, even if the PNG file is RGB.

I think this would be the ideal solution, as not everyone saves their grayscale images with a grayscale format. It would slow down importing a little as every pixel's color needs to be checked, but I think it's worth it.

godotengine / godot