Need metadata extension or some other solution for NV corner-sampled images

[elasota]

NVIDIA has an extension for corner-sampled images, which changes the rounding behavior (round-up instead of round-down) and minimum extents requirements (2 instead of 1) for image mipmaps. Since this isn't a new format or supercompression scheme, it would require some other kind of metadata signaling.

I'm guessing this needs a vendor extension, otherwise there is no way to store a mipmapped KTX for this type of image unless it has PO2 dimensions and the mip chain ends at whatever dimension reaches extents 2 first.

[MarkCallow]

I don't know about corner-sampled images. Please provide a reference. What are "extents" in this context?

Note that the KTX specification does not require that a file contain a full mip chain.

NVIDIA can certainly create a vendor extension. Just use NV_ as the prefix for the metadata name. You can submit a PR to add it to the spec.

[elasota]

I'm not from NVIDIA so not sure if I can do that, maybe this entire issue is pointless.

The problem is the VK_NV_corner_sampled_image Vulkan extension. Its effects on mipmap sizes is described in the "corner-sampled images" section here:
https://registry.khronos.org/vulkan/specs/latest/html/vkspec.html#resources-image-mip-level-sizing

Typically mip size behavior acts as described in section 2 of the KTX spec: Round-down, minimum size 1.

With the corner-sampled images, mip dimensions are round-up, minimum size 2.

[lexaknyazev]

The different mipmap size computation, specifically using round-up instead of round-down, would most certainly break multiple spec and implementation assumptions so a metadata entry can't enable that.

In a case of a strong demand, a spec version upgrade could enable such a feature.