Texture compression results in bigger file

[SaTae66]

Hi!

When using only the -tc flag with a 253Mb big glb-file, the result is somehow 281Mb. Any ideas on why or what to do different?

[zeux]

I believe that's possible if the source file contains JPEG textures. You can try using lower -tq settings (default is 8, so eg try -tq 5), but overall Basis Universal doesn't guarantee that the file size is going to be smaller compared to lossy alternatives like JPEG - however, your file will consume dramatically less video memory when loaded, so it's a tradeoff.

[SaTae66]

Thank you, that makes sense! However, I don't understand what you mean with

your file will consume dramatically less video memory when loaded

Even though the file size is bigger, it will consume less video memory? Would you mind explaining that briefly?

[zeux]

Briefly, JPG/PNG are compressing image formats on disk, but the GPUs can't directly decode these for display purposes, so the browser loads the file from disk/network and then decompresses it into ~4 bytes per pixel (+30% for mipmaps). When using -tc, gltfpack uses BasisU super-compression - see https://www.khronos.org/news/press/khronos-ktx-2-0-textures-enable-compact-visually-rich-gltf-3d-assets - which is still compressed on disk/network, and still is decompressed for display purposes, but it's decompressed into a special block-compressed format that GPUs can read directly, which takes ~0.5-1 bytes per pixel (+30% for mipmaps). https://www.khronos.org/ktx/ has an image with an example of the difference between file size & in-memory size between different formats for one model, but of course the actual comparison is going to depend on the formats and image data used.