# 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 has no 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:

```sh
cargo test -p libremetaverse-imaging-skia --no-default-features --features rust-skia
```

`rust-skia` pins [`skia-rs-codec` 0.3.0](https://github.com/rust-skia/skia-rs)
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](https://github.com/image-rs/image-webp) 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:

```sh
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.
