UASTC KTX2 with a single (R) channel fails to load: "Failed to transcode mip level 0 from UASTC to BC4: TranscodeFailed"

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Valutazione

Difficoltà
3/5
Tempo stimato
1-2 giorni
Idoneità per principianti
72/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
rust
Ambito
backend

Direzione di ricerca

Start in crates/bevy_image/src/ktx2.rs at the TranscodeFormat::Uastc branch and trace how source mip and layer slices are sized for each target format. The report shows BC4 fails because its 8-byte blocks are used to size 16-byte UASTC source blocks; compare the BC target fix with the separate uncompressed fallback, which has its own reported failure. Done means single-channel UASTC KTX2 loads on the affected compressed targets without regressing RGBA UASTC.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

A-Assets A-Rendering C-Bug D-Straightforward S-Ready-For-Implementation X-Uncontroversial

Bevy version

0.20.0 (bevy_image 0.20.0). The code in question is unchanged on main as of 2026-10-24; 0.19.1 has it too.

What you did

Loaded a single-channel UASTC KTX2 (a height map) on a GPU with BC support:

ktx create --format R8_UNORM --encode uastc --generate-mipmap --assign-tf linear h.png h.ktx2   # KTX-Software 4.4.2

Image::from_buffer(&bytes, ImageType::Extension("ktx2"), CompressedImageFormats::BC,
                   false, ImageSampler::Default, RenderAssetUsages::default())

What went wrong

failed to decompress an image: Failed to transcode mip level 0 from UASTC to BC4: TranscodeFailed

An RGBA UASTC file built the same way loads fine as Bc7RgbaUnorm.

Cause

In crates/bevy_image/src/ktx2.rs, the TranscodeFormat::Uastc branch works out how many bytes of source data each level and layer takes using the target format's block size:

let block_bytes = texture_format_info.block_copy_size(None).unwrap(); // target format: BC4 = 8
let level_bytes = (num_blocks_x * num_blocks_y * block_bytes) as usize;
transcoder.transcode_slice(&level_data[offset..(offset + level_bytes)], ...)

UASTC blocks are always 4×4 and 16 bytes. BC4 and EacR11Unorm blocks are 8 bytes, so the transcoder gets only half of each level. basisu_lowlevel_uastc_transcoder::transcode_slice checks image_data_size < total_expected_block_bytes and returns false. BC7, BC5 and ASTC 4×4 targets work only because their blocks happen to be 16 bytes too. This affects every UASTC texture with TextureChannelLayout::Rrr on BC- or ETC2-capable hardware.

Fix (tested for BC targets)

Size the source slice in UASTC blocks:

// The source is UASTC whatever the target: 4x4 blocks of 16 bytes.
let (block_width_pixels, block_height_pixels) = (4, 4);
let block_bytes = 16;

I patched a local copy of bevy_image 0.20.0 with this and loaded the following with CompressedImageFormats::BC:

file stock 0.20.0 patched
R8 UASTC 64×64, 7 mips TranscodeFailed (BC4) Bc4RUnorm, 7 mips
R8 UASTC 60×36, 6 mips TranscodeFailed (BC4) Bc4RUnorm, 6 mips
RGBA8 UASTC 64×64, 7 mips Bc7RgbaUnorm Bc7RgbaUnorm

Related: the uncompressed fallback

With CompressedImageFormats::NONE the same branch is also wrong, and the fix above does not repair it:

  • stock: R8 UASTC gives TranscodeFailed (RGBA32), and RGBA8 UASTC panics on the out-of-range slice;
  • patched: the process aborts with malloc(): corrupted top size inside transcode_slice (basis-universal 0.3.1).

So that path needs its own look: its output is RGBA32, while get_transcoded_formats labels it R8Unorm / Rg8Unorm. I haven't dug further. A fix for the compressed targets alone shouldn't change it, since it can't load either way today.

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