KTX2Loader: Fix ETC1S/UASTC prioritization #31871
Merged
+17
−6
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
zeux
force-pushed
the
ktx2-transcode-priority
branch
from
42c34fb to
1b6fbfe
Compare
|
WebGL report for posterity; ETC2 and ASTC are emulated but you can only tell by observing a significant performance degradation when using these: 
|
donmccurdy
reviewed
Sep 15, 2025
zeux
force-pushed
the
ktx2-transcode-priority
branch
from
1b6fbfe to
fd77d42
Compare
Due to a historical accident, the list of formats for ETC1S was sorted using UASTC priority. This change fixes that. However, this exposes a problem that already exists for UASTC to ETC1S as well: on Linux, Mesa drivers for various Intel and AMD GPUs expose support for ETC2 and ASTC extensions even if the hardware does not support them, as part of GLES compatibility. When a texture with an emulated format is uploaded, the driver runs a very expensive CPU side decompression; this runs on the main thread and causes performance and memory issues. When using Chrome based browsers, ANGLE filters out ASTC and ETC extensions for us; we now detect Gecko based browsers like Firefox that don't use ANGLE and do this filtering ourselves. In principle, it is possible for GPUs to support all formats - notably, Safari exposes all formats on macOS when using Apple Silicon hardware, as it genuinely supports all possible formats. In this case we still should prefer native (ASTC/ETC2) format targets as they are faster to transcode to. A corner case is a combination of Firefox / Asahi Linux on Apple Silicon hardware; in the future it might be possible to detect the unmasked vendor to disambiguate, but even that combination will simply use BC7 for UASTC or dual-slice ETC1S which is probably reasonable.
zeux
force-pushed
the
ktx2-transcode-priority
branch
from
fd77d42 to
2a7f67f
Compare
donmccurdy
approved these changes
Sep 17, 2025
There was a problem hiding this comment.
Made a quick test at https://jsfiddle.net/donmccurdy/bkr3q8mc/ — prints navigatior.platform and whether Firefox is detected (true/false) — and as expected the Linux+Firefox checks return false on devices I have available. That's as expected, I don't have a Linux device currently.
Thank you @zeux!
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Due to a historical accident, the list of formats for ETC1S was sorted using UASTC priority. This change fixes that.
However, this exposes a problem that already exists for UASTC to ETC1S as well: on Linux, Mesa drivers for various Intel and AMD GPUs expose support for ETC2 and ASTC extensions even if the hardware does not support them, as part of GLES compatibility. When a texture with an emulated format is uploaded, the driver runs a very expensive CPU side decompression; this runs on the main thread and causes performance and memory issues.
When using Chrome based browsers, ANGLE filters out ASTC and ETC extensions for us; we now detect Gecko based browsers like Firefox that don't use ANGLE and do this filtering ourselves.
In principle, it is possible for GPUs to support all formats - notably, Safari exposes all formats on macOS when using Apple Silicon hardware, as it genuinely supports all possible formats. In this case we still should prefer native (ASTC/ETC2) format targets as they are faster to transcode to. A corner case is a combination of Firefox / Asahi Linux on Apple Silicon hardware; in the future it might be possible to detect the unmasked vendor to disambiguate, but even that combination will simply use BC7 for UASTC or dual-slice ETC1S which is probably reasonable.
See #29730 (comment) for motivating profiles.
Fixes #29745