Basis feature support

Here's what we support right now:

• .basis universal format, which is transcodable to BC1-5, ETC1, PVRTC1 4bpp (currently opaque only), and BC7 (currently opaque only). Alpha support for PVRTC1/BC7 is coming, and enhanced quality for BC7 and ETC2 are on the way. This is a universal solution with two quality modes (baseline and BC7), so by its very nature it trade offs max achievable quality for GPU format support.
• For really small images (think icon-size), .basis can switch to using fixed selector codebooks to cut down on selector codebook overhead
• Format supports arbitrary resolution texture arrays, all referring to a single set of compressed codebooks.
• RDO BC1-5 - Creates more compressible files than crunch's (RDO in crunch was an afterthought and was pretty dumb/low quality), but slower compression. We put the most effort into optimizing BC1's output for LZ coding. Supports up to 32K entry codebooks (vs. crunch's 8K).
• RDO ETC1 - Supports all ETC1 features, and very high quality levels (up to 32K entry codebooks), usable even on complex normal maps.
• ETC1 intermediate format (supports all features of the ETC1 format, i.e. flips and both differential and individual colors). 10-20% smaller files at same SSIM vs. Unity crunch's the last I checked.

All of these codecs have been utilized by customers for different purposes.

We don't support an intermediate file format exclusively for BC1-5, only ETC1. Instead, we're focusing on universal solutions first, and then we'll focus on an intermediate format solution for BC7, BC6H, and ASTC.

I get asked all the time how these solutions compare to crunch's. I'll be working on extensive benchmarks soon. I've learned a lot since I designed and wrote crunch in 2009.