power-of-two flag in texture compression cli commands

[LoJunKai]

Description
Hi, I would just like to clarify the --power-of-two flag in the texture compression cli commands. It is stated in the docs that it:

Resizes any non-power-of-two textures to the closest power-of-two dimensions, not exceeding 2048x2048px. Required for compatibility on some older devices and APIs, particularly WebGL 1.0.

I was just wondering why is there a 2048x2048px restriction for this closest power-of-two resizing.

To Reproduce

1. Model used: https://drive.google.com/file/d/1gULSZ7VRz06Aq9-7oG7T3soDCCN7GxuU/view?usp=sharing
2. Run the following command: gltf-transform etc1s ./MosquitoInAmber/MosquitoInAmber.gltf ./MosquitoInAmber-resize/MosquitoInAmber.gltf --power-of-two -v
3. Observe that MosquitoInAmber0.jpg is being resized by --resize 4096x2048 in the debug messages

*model taken from: https://sketchfab.com/3d-models/mosquito-in-amber-53a61d58c09b4d2ab30e269aa3e22078
*manually resized MosquitoInAmber0.jpg to 4100x4000px

Question
In the source code it is written that

    if (hi > max) return lo;
    if (hi - value > value - lo) return lo;
    return hi;

Why is the first line needed? Wouldn't it be better to return the closest power-of-two all regardless of input dimension?

Thank you for your time! :)

[donmccurdy]

Hm, 4100x4000 → 4096x2048 is admittedly an odd-feeling result here. Some background on the thing I'm trying to avoid: My general impression is that...

1. Textures >4K do not render correctly on a pretty significant number of mobile devices.
2. Textures >2K do not render correctly on a small but non-trivial number of mobile devices.

That being the case, I didn't really want to upscale an image silently in a way that's going to reduce device support. At the time I added KTX2/Basis support, this library didn't have any other way to resize images... it does now, so maybe this could be addressed with a simple warning about larger images, advising that the user resize them first if needed. I'm assuming 4096x4096 is the result you would want in this case?

EDIT: Fixed in c1c66fb, this now warns rather than limiting to 4K.

[LoJunKai]

Hey Don, really appreciate the explanation! Yep, I was looking for it to be resized to 4096x4096 instead, the warning would indeed be helpful!

Thank you and have a great day :)

[donmccurdy]

Sounds good! This should be fixed shortly by c1c66fb.