Texture compression

The engine uses zstd+bc7 compressed cubemap textures for faster processing in shaders and lower file sizes.

Alternatively, you can skip compression altogether and use toktx instead.

You may wish to do so when:

  • Banding may appear if the image has a lots of smooth gradients.
  • GPU codecs operate on 2x2 blocks. It may turn out your image has an unlucky alignment resulting in massive loss of precision.
  • Compression codec may misinterpret data in semi-transparent images.
  • Using basisu results in a crash or something likewise bad and you won’t be bothered with that.

Using KTX format is important however and you get mipmaps for free.

Since KTX is merely a metadata format, it contains raw bitmap data and requires an extra file compression step to prevent files from growing huge. The engine can deal with any zstd-compressed files transparently to other codecs so you can have best of both worlds.

The next version, KTX-2 allows for “supercompression” of its bitmap data, but the engine is not ready. Consider adding v2 support to ktx-codec and keid-core.

1. Apply GPU compression

Use patched Basis Universal tool: https://gitlab.com/dpwiz/basis_universal

Due to https://github.com/BinomialLLC/basis_universal/issues/227

You can skip this and use a recent mainstream basisu binary and just delete extra files after transcoding.

For flat textures:

basisu \
  -mipmap \
  -uastc -uastc_level 4 \
  flat.png

For cubemaps:

Use online converter if you need to get PNG input images: https://jaxry.github.io/panorama-to-cubemap/ 

basisu \
  -mipmap \
  -tex_type cubemap \
  -uastc -uastc_level 4 \
  right.png left.png top.png bottom.png front.png back.png # the order is important

2. Transcode to KTX format

basisu -unpack -ktx-only -format-only 6 right.basis

3. Apply file compression

zstd -T16 -19 *.ktx

Some clean up… 

rm right_*.png *.ktx

Ready, ship your .ktx.zst file! 

mv \
  right_transcoded_cubemap_BC7_RGBA_0.ktx.zst \
  ../../../resources/cubemaps/env.ktx.zst
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