MetaCrate/crates/libremetaverse-imaging-skia/README.md

MetaCrate Skia adapter

This crate implements the pinned SkiaTextureCodec behavior at a bounded, project-owned image boundary. The core libremetaverse-imaging crate never exposes a skia-safe type and never discovers or links Skia.

Features and formats

The default feature set enables the pure-Rust rust-skia backend and has no native Google Skia dependency. SkiaTextureCodec::decode and RustSkiaTextureCodec::decode return a typed InvalidOperation error when their respective feature is disabled, while conversion from the checked project-owned backend::SKBitmap remains available for tests and callers that already own decoded pixels.

For a fully Rust implementation, enable:

cargo test -p libremetaverse-imaging-skia

rust-skia pins skia-rs-codec 0.3.0 with default features disabled and only its pure-Rust PNG, JPEG, and GIF codecs selected. Its webp feature is deliberately disabled because that feature selects libwebp-sys; WebP is instead decoded by the separately pinned, safe-Rust image-webp 0.2.4 crate. BMP, ICO, and WBMP are built into skia-rs-codec. The resulting normal/build graph has no codec -sys crate, C/C++ compilation, CMake, system-library lookup, or binary download.

The locked transitive codec lines are png 0.17.16, jpeg-decoder 0.3.2, jpeg-encoder 0.6.1 (the upstream JPEG feature couples decode and encode), and gif 0.13.3. skia-rs-codec also brings its ordinary Rust core, paint, and path crates; their build helpers only select Rust cfgs and compile no native code. A source audit of the locked skia-rs-* crates found no unsafe blocks, and image-webp declares forbid(unsafe_code). MetaCrate itself continues to compile with workspace unsafe_code = "forbid".

RustSkiaTextureCodec implements the same ITextureCodec abstraction and decodes BMP, GIF, ICO, baseline/progressive JPEG, interlaced PNG, WBMP, and lossless/lossy/animated WebP. GIF and WebP return frame one composited on the logical canvas; ICO chooses its largest valid entry. Both Skia features may be enabled together, and each codec remains independently constructible. The graph builds on the workspace Rust 1.96 MSRV; both selected direct codecs declare older compatible Rust floors.

Enable native decoding with:

cargo test -p libremetaverse-imaging-skia --features skia

The native skia feature decodes the CPU codec formats supported by the pinned rust-skia release: BMP, GIF, ICO, JPEG, PNG, WBMP, and WebP. Input is buffered to at most 64 MiB. Dimensions are rejected before native pixel allocation when they exceed the core 16,777,216-pixel limit, and decoded storage uses checked strides and owned Rust buffers.

The mapped C# adapter exposes decoding and SKBitmap-to-ManagedImage conversion only; it has no encoding or resize method. Encoding selection stays with the format-specific imaging APIs, and callers use ManagedImage's checked resize methods after decoding. Premultiplied pixels are converted to straight alpha at this boundary. RGB565, BGRA8888, RGBA8888, RGBA/BGRA1010102, Gray8, Alpha8, row padding, and the reference byte-width fallback retain the pinned C# channel and rounding rules.

Both paths accept any ReadWrite + Send stream and stop buffering after 64 MiB plus one sentinel byte. The pure-Rust path probes encoded dimensions before codec pixel allocation and enforces the shared 16,777,216-pixel ceiling; WebP also receives an explicit decoder memory limit. Checked dimensions, strides, buffer sizes, and fallible project-owned allocations turn malformed, truncated, oversized, and allocation failures into typed errors. Decoded rows have a top-left origin and premultiplied native samples are normalized to straight alpha at the shared ManagedImage boundary.

The pure-Rust graph has identical code on Linux, Windows, macOS, and WASM and does not call platform APIs. Checked static fixtures cover top-down/bottom-up BMP, palette/offset GIF, largest-entry ICO, baseline/progressive JPEG, interlaced palette PNG, multibyte WBMP, lossless-alpha/lossy WebP, malformed and oversized inputs, and concurrent decode/drop. Gitea runs the rust-skia suite in a dedicated ubuntu-latest job that installs no native codec prerequisites. image-webp 0.2's animated-canvas blend fast path rounds fully opaque nonzero samples one low; the bounded adapter corrects that single upstream edge so the checked first-frame planes remain byte-identical to native Skia.

Binary cache and source builds

skia-safe 0.99.0 downloads an official prebuilt Skia archive when the target and Cargo feature set match a published archive. MetaCrate deliberately selects the published WebP-capable feature sets:

• Linux and Windows: JPEG, PDF, SVG, text layout, Vulkan, and WebP;
• macOS: JPEG, PDF, SVG, text layout, and WebP.

Vulkan is selected on Linux and Windows only to match the published CPU/WebP archive; this adapter does not create a GPU context or call a platform graphics API. The same feature is therefore usable on macOS, Linux, and Windows without leaking platform-specific behavior.

Build prerequisites for the supported desktop targets are:

• Linux: a C++ linker/runtime, curl, pkg-config, FreeType, and Fontconfig;
• macOS: Xcode command-line tools and curl;
• Windows MSVC: the Rust MSVC toolchain, Visual Studio C++ build tools, and curl.

The build script downloads cache archives from the rust-skia/skia-binaries GitHub releases. Pin or mirror SKIA_BINARIES_URL in offline/reproducible build environments. FORCE_SKIA_BINARIES_DOWNLOAD=1 makes a missing archive fail instead of compiling Skia. If no archive matches and that variable is absent, rust-skia falls back to a source build, which additionally requires Python 3, Ninja, and an LLVM/Clang toolchain. FORCE_SKIA_BUILD=1 selects that path explicitly.

Licensing and redistribution

skia-safe and rust-skia's bindings are MIT licensed; the linked Skia library is BSD-3-Clause licensed. Official binary-cache archives contain compiled Skia. Products that redistribute the resulting native artifacts must preserve the applicable MIT and BSD notices and audit the exact archive they ship.

skia-rs-codec is MIT OR Apache-2.0 and image-webp is MIT OR Apache-2.0. They are ordinary Rust source dependencies recorded in Cargo.lock; generated dependency notices and the distribution manifest bind their crates.io checksums and license texts for source and binary redistribution.