Support custom key/value pairs in KTX2 compression

[cfnptr]

It would be great to have the ability to set custom key/value pairs for KTX2 files. For example, this could be used to store additional information about the target transcoding format, texture content, or channels.

The KTX2 format already includes a mechanism to handle this. We just need a way to pass basist::ktx2_transcoder::key_value_vec ktx2_key_values during compression. As a solution, this could be added to the basis_compress2() arguments:

void* basis_compress2(
    basist::basis_tex_format mode,
    const basisu::vector<image>& source_images,
    uint32_t flags_and_quality, int quality_level, int effort_level,
    size_t* pSize,
    image_stats* pStats = nullptr,
    const key_value_vec* pKeyValueVec = nullptr);

The KTX2 specification supports custom, non "KTX..." keys and describes how they should be handled.

[richgel999]

I'll add this feature to the C API. In the mean time you can always use the C++ API directly, which supports custom .KTX2 key value fields. See basis_compressor_params::m_key_values.