[Impeller] Support compressed texture formats (BC, ETC2, ASTC) with a format capability query #187072

Use case

Impeller has no compressed texture support today. There are no compressed PixelFormat values, no compressed upload path in any backend, and the descriptor sizing math (BytesPerPixelForPixelFormat, GetByteSizeOfBaseMipLevel, GetBytesPerRow) is per-pixel, which is invalid for block-compressed data.

Compressed textures reduce GPU memory and bandwidth by roughly 4x to 8x and are required for realistic 3D content. Flutter GPU and Flutter Scene cannot expose them (for example glTF textures) until Impeller supports them.

No single compressed format is supported across all platforms, so a runtime capability query is a prerequisite for exposing any of these formats safely.

This is distinct from #170597, which covers runtime lossy compression of regular images. This issue is about block-compressed asset formats (BC, ETC2, ASTC).

Proposal

Add compressed texture support to Impeller, grouped into the same three hardware families that WebGPU exposes as optional features:

• BC (S3TC, RGTC, BPTC): BC1 to BC7. Desktop GPUs.
• ETC2 and EAC. Mobile, and any OpenGL ES 3.0 or WebGL2 context.
• ASTC LDR. Modern mobile and some desktop.

Guided by WebGPU and WebGL, skip these during the initial implementation: PVRTC (deprecated, Apple only, being removed from WebGL), ETC1 (legacy, superseded by ETC2), ASTC HDR, and BC6H HDR. They can be added later behind the same query.

Work items:

1. Add the compressed PixelFormat values for the three families above.
2. Make descriptor math block-aware. Add bytes-per-block and block dimensions to formats.h, and update GetByteSizeOfBaseMipLevel and GetBytesPerRow to round to block extents.
3. Add a compressed upload path: glCompressedTexSubImage2D (GLES), vkCmdCopyBufferToImage with block extents (Vulkan), and replaceRegion with compressed formats (Metal).
4. Enforce usage restrictions: sample plus copy-destination only. No render target, no storage, no transient storage mode, and no runtime mip generation.
5. Map the formats in each backend, gated by capability detection: Vulkan feature flags (textureCompressionBC, textureCompressionETC2, textureCompressionASTC_LDR), GLES extension strings, and Metal GPU family checks.

Capability query API

Add to impeller::Capabilities:

enum class TextureCompressionType {
  kBC,    // Desktop. BC1 to BC7.
  kETC2,  // Mobile, GLES3, WebGL2.
  kASTC,  // Modern mobile, some desktop.
};

// True if the device supports a whole compressed family.
virtual bool SupportsTextureCompression(TextureCompressionType type) const = 0;

// Per-format, per-usage query. Covers compressed and uncompressed formats.
virtual bool SupportsTextureFormat(PixelFormat format,
                                   TextureUsage usage) const = 0;

SupportsTextureFormat is the general primitive. SupportsTextureCompression is a convenience for the common "can I use this family at all" check. This mirrors WebGPU, where compression is exposed as three optional device features.

Formats supported almost universally at landing

No single compressed format works everywhere, which is why the query exists. Coverage is near-universal per tier:

• ETC2 and EAC: guaranteed on OpenGL ES 3.0 and WebGL2, present on all mobile GPUs and Apple Silicon.
• BC: effectively guaranteed on desktop (Windows, Linux, macOS discrete GPUs) through Vulkan, desktop GL, and Metal.
• ASTC: modern mobile (Mali, Adreno, Apple A8 and newer).

Together the three families cover essentially every target. A single asset reaches all of them through KTX2 or Basis Universal transcoding at the asset layer, with uncompressed RGBA as the final fallback. See the dependent Flutter GPU issue.