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I think that section 2.3 would benefit from bit-level illustrations and descriptions for computing offsets from coordinates. I think this would be a good complement to the C implementations which are great.

In the first approach (linear tiles?), the bits are combined as follows (modulo lineStride), from lsb to msb.

• least significant "twb" bits of x
• least significant "twh" bits of y
• remaining bits of x
• remaining bits of y

The Morton order effectively merges (similarly to MergeSort) the bits from the x and y coordinates.

• least significant bit of x
• least significant bit of y
• next-to-least significant bit of x
• next-to-least significant bit of y
• ...

In the pdf and html, it says:

RED1 = 
  red1/127.0, red1 > -128
  -1.0, **red0** = -128

Probably it should be

RED1 = 
  red1/127.0, red1 > -128
  -1.0, **red1** = -128

Doubled checked with https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_compression_rgtc.txt
And it seems it should be red1 instead of red0

@MarkCallow @fluppeteer and other interested parties: heads-up that I propose to rename the default branch of this repository to 'main' on or after 2021-08-09, following emerging Khronos & github practice. This will have very little impact, although if you have a checked-out repository clone, you may want to follow the simple instructions github will pop up on the repository webpage describing how to do the same locally.

If there is a concern about this, please raise it here by 2021-08-06.