Completely unrelated to the issue above, but as another ~"small thing that I'll probably fix soon":

There currently is a constant KtxPackAstcBlockDimension.D6x4. I think that this should be called D6x5. I'll rename that, unless someone disagrees...

Please fix these soon. We'll be making a release in early February, hopefully.

@MarkCallow OK, given that this is only a small fix, I created a PR at #972 .

(Otherwise, I would have added this to #968 , but it's hard to give an exact timeline for that. It might be that in that process, further "small fixes" might be necessary, but we can address them when they come up).

Regarding the inputSwizzle: I already did have a look at this, in the context of #876 . It can be a bit tricky to "follow the path of the swizzle" here: It is taken from the command line as input-swizzle, stored as inputSwizzle (which is supposedly always a char[4], with 0-elements by default), passed on as swizzle (which then is a string), makes its way through a swizzle_array (as a vector<astcenc_swz*>), and ends up as swz (a astcenc_swizzle). For the "basis" swizzle, the path is different. And at each point, different checks and conversions are performed, handling the case of a "non-default swizzle" in different ways...

tl;dr: The main source of confusion here was probably that

1. the swizzle parameter is initialized to a char[4] array with all 0 elements by default
2. this is converted into a string at some point, resulting in an empty string for the default, which means "no swizzle"

So, strictly speaking, the documentation at https://github.khronos.org/KTX-Software/libktx/structktxAstcParams.html#a0d67004efa49e08a7f39f71c93286b0a that says

char inputSwizzle[4]

A swizzle to provide as input to astcenc. It must match the regular expression /^[rgba01]{4}$/.

should/could include the hint that the default value is a char[4] (0,0,0,0), and this is also an accepted value (similarly for ktxBasisParams at https://github.khronos.org/KTX-Software/libktx/structktxBasisParams.html#a0d67004efa49e08a7f39f71c93286b0a )