Metal texture management and image upload #116

Phase 12: Metal GPU Compositor (3/7)#

Goal: Manage GPU textures for images and glyph data.

Depends on: Metal render pipeline: 2D shaders and vertex format

Requirements#

• Implement MTLTexture creation and pixel data upload:

  • MTLTextureDescriptor configuration (2D, BGRA8Unorm or R8Unorm for glyphs)
  • replaceRegion:mipmapLevel:withBytes:bytesPerRow: for uploading pixel data
  • Support both RGBA8 image data and single-channel grayscale glyph data

• Implement MTLSamplerState creation:

  • Nearest-neighbor filtering (matching current software renderer behavior)
  • Clamp-to-edge addressing

• Add texture upload for decoded images:

  • Convert RGBA8 image data (from image crate decoders) to MTLTexture
  • Cache uploaded textures by node_id to avoid re-uploading unchanged images
  • Handle texture lifecycle (release when no longer needed)

• Extend the Metal fragment shader to sample from textures:

  • Bind texture and sampler to fragment shader
  • Sample texture at interpolated UV coordinates
  • Multiply sampled color by vertex color for tinting

Acceptance Criteria#

• Can create and upload RGBA8 textures to GPU
• Can create and upload R8 (grayscale) textures to GPU
• Texture cache avoids redundant uploads
• Fragment shader correctly samples textures with proper alpha blending
• Decoded PNG/JPEG images render correctly via Metal
• No external crate dependencies
• cargo clippy --workspace -- -D warnings passes
• cargo test --workspace passes