ETC1 is a well thought out, elegant little GPU format. In my experience a few years ago writing a production quality block ETC1 encoder, I found it to be far less fiddly than DXT1. Both use 64-bits to represent a 4x4 texel block, or 4-bits per texel.
I've been very curious how hard it would be to add ETC1/2 support to crunch. Also, many people have asked about ETC1 support, which is guaranteed to be available on OpenGL ES 2.0 compatible Android devices. crunch currently only supports the DXT1/5/N (3DC) texture formats. crunch's higher level classes are highly specific to the DXT formats, so adding a new format is not trivial.
One of the trickier (and key) problems in adding a new GPU format to crunch is figuring out how to group blocks (using some form of cluster analysis) so they can share the same endpoints. GPU formats like DXT1 and ETC1 are riddled with block artifacts, and bad groupings can greatly amplify them. crunch for DXT has a endpoint clusterization algorithm that was refined over many tens of thousands of real-life game textures and satellite photography. I've just begun experimenting with ETC1, and so far I'm very impressed with how well behaved and versatile it is.
Note this experiment was conducted in a new data compression codebase I've been building, which is much larger than crunch's.
ETC1 Texture Compression
Unlike DXT1, which only supports 3 or 4 unique block colors, the ETC1 format supports up to 8 unique block colors. It divides up the block into either two 4x2 or 2x4 pixel "subblocks". A single "flip" bit controls whether or not the subblocks are oriented horizontally or vertically. Each subblock has 4 colors, for 8 total.
The 4 subblock colors are created by taking the subblock's base color and adding to it 4 grayscale colors from an intensity table. Each subblock has 3 bits which selects which intensity table to apply. The intensity tables are constant and part of the spec.
To encode the two block colors, ETC1 supports two modes: an "individual" mode, where each color is encoded to 4:4:4, or a "differential" mode, where the first color is 5:5:5 and the second color is a two's complement encoded 3:3:3 delta relative to the base color. The delta is applied before the base color is scaled to 8-bits.
From an encoding perspective, individual mode is most useful when the two subblocks have wildly different colors (favoring color diversity vs. encoding precision), and delta mode is most useful when encoding precision is more useful than diversity.
Each pixel is represented using 2-bit selectors, just like DXT1. Except in ETC1, the color selected depends on which subblock the pixel is within.
So that's ETC1 in a nutshell. In practice, from what I remember its quality is a little lower than DXT1, but not by much. Its artifacts look more pleasant to me than DXT1's (obviously subjective). Each ETC1 block is represented by 2 colorspace lines that are always parallel to the grayscale axis. By comparison, with DXT1, there's only a single line, but it can be in any direction, and perhaps that gives it a slight advantage.
ETC1 Endpoint Clusterization
ETC1 (subset) with 64 unique endpoints:
ETC1 (subset) 256 unique endpoints:
ETC1 (subset) 512 unique endpoints:
ETC1 (subset) 1024 unique endpoints:
ETC1 (subset) 4096 unique endpoints:
- ETC2 support
(Currently, I'm conducting these experiments in my spare time, in between VR and optimization contracts. If you're really interested in accelerating development of crunch for a specific GPU format please contact email@example.com.)