Provide mip-mapping capabilities in libKTX

[thokra1]

Currently, there doesn't seem to be a way to let the BasisU encoder generate mip maps. There is a host of options one can pass to the latest BasisU CLI:

  "Mipmap generation options:\n"
  " -mipmap: Generate mipmaps for each source image\n"
  " -mip_srgb: Convert image to linear before filtering, then back to sRGB\n"
  " -mip_linear: Keep image in linear light during mipmap filtering (i.e. do not convert to/from sRGB for filtering purposes)\n"
  " -mip_scale X: Set mipmap filter kernel's scale, lower=sharper, higher=more blurry, default is 1.0\n"
  " -mip_filter X: Set mipmap filter kernel, default is kaiser, filters: box, tent, bell, blackman, catmullrom, mitchell, etc.\n"
  " -mip_renorm: Renormalize normal map to unit length vectors after filtering\n"
  " -mip_clamp: Use clamp addressing on borders, instead of wrapping\n"
  " -mip_fast: Use faster mipmap generation (resample from previous mip, not always first/largest mip level). The default (as of 1/2021)\n"
  " -mip_slow: Always resample each mipmap level starting from the largest mipmap. Higher quality, but slower. Opposite of -mip_fast. Was the prior default before 1/2021.\n"
  " -mip_smallest X: Set smallest pixel dimension for generated mipmaps, default is 1 pixel\n"

Currently, while using libKTX works well for most other encoding-related tasks, all of the above has to be implemented separately (in our case using STBI resize, which sadly lacks Kaiser window filtering, IIRC).

[MarkCallow]

Mipmap generation in toktx uses the same code as basisu. Use --genmipmap to invoke it. It automatically does -mip_srgb if the input images are srgb, as determined from the input image. Similarly -mip_linear is done if the input images are linear. You can override the image metadata with--{assign,convert}_oetf which will also change the mipmap generation. You can set the filter with --filter, the scale with --fscale and the clamp mode with --wmode. The only things missing are -mip_renormalize, -mip_fast and -mip_slow. I have a TODO for renormalize. toktx currently always does the same as -mip_slow. The resampling speed has never troubled me so I didn't have a TODO for fast & slow.

[MarkCallow]

Whoops! You want the mipmap generation functionality in libktx. If I add it, I'll make it work for any format not just when using the BasisU encoder. Like with toktx, mip_srgb and mip_linear will be chosen based on the texture format but it will be possible to override.

[thokra1]

This would be highly appreciated. Alternatively, I could spare some time and give you a pull request, if you're interested.

[MarkCallow]

A PR would be great. The PR can access the resampler in the BasisU code in a similar way to how toktx does it. The genmipmap function should give KTX_INVALID_OPERATION if the texture is already in a block-compressed format or if it already has mipmaps.