139 lines
6.1 KiB
Markdown
139 lines
6.1 KiB
Markdown
# tiny-skia
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[](https://crates.io/crates/tiny-skia)
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[](https://docs.rs/tiny-skia)
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[](https://www.rust-lang.org)
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`tiny-skia` is a tiny [Skia] subset ported to Rust.
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The goal is to provide an absolute minimal, CPU only, 2D rendering library for the Rust ecosystem,
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with a focus on a rendering quality, speed and binary size.
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And while `tiny-skia` is definitely tiny, it support all the common 2D operations
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like: filling and stroking a shape with a solid color, gradient or pattern;
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stroke dashing; clipping; images blending; PNG load/save.
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The main missing feature is text rendering
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(see [#1](https://github.com/RazrFalcon/tiny-skia/issues/1)).
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**Note:** this is not a Skia replacement and never will be. It's more of a research project.
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## Motivation
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The main motivation behind this library is to have a small, high-quality 2D rendering
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library that can be used by [resvg]. And the choice is rather limited.
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You basically have to choose between [cairo], Qt and Skia. And all of them are
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relatively bloated, hard to compile and distribute. Not to mention that none of them
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are written in Rust.
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But if we ignore those problems and focus only on quality and speed alone,
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Skia is by far the best one.
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However, the main problem with Skia is that it's huge. Really huge.
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It supports CPU and GPU rendering, multiple input and output formats (including SVG and PDF),
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various filters, color spaces, color types and text rendering.
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It consists of 370 KLOC without dependencies (around 7 MLOC with dependencies)
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and requires around 4-8 GiB of disk space to be built from sources.
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And the final binary is 3-8 MiB big, depending on enabled features.
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Not to mention that it requires `clang` and no other compiler
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and uses an obscure build system (`gn`) which was using Python2 until recently.
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`tiny-skia` tries to be small, simple and easy to build.
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Currently, it has around 14 KLOC, compiles in less than 5s on a modern CPU
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and adds around 200KiB to your binary.
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## Performance
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Currently, `tiny-skia` is 20-100% slower than Skia on x86-64 and about 100-300% slower on ARM.
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Which is still faster than [cairo] and [raqote] in many cases.
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See benchmark results [here](https://razrfalcon.github.io/tiny-skia/x86_64.html).
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The heart of Skia's CPU rendering is
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[SkRasterPipeline](https://github.com/google/skia/blob/master/src/opts/SkRasterPipeline_opts.h).
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And this is an extremely optimized piece of code.
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But to be a bit pedantic, it's not really a C++ code. It relies on clang's
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non-standard vector extensions, which means that it works only with clang.
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You can actually build it with gcc/msvc, but it will simply ignore all the optimizations
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and become 15-30 *times* slower! Which makes it kinda useless.
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Also note, that neither Skia or `tiny-skia` are supporting dynamic CPU detection,
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so by enabling newer instructions you're making the resulting binary non-portable.
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Essentially, you will get a decent performance on x86 targets by default.
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But if you are looking for an even better performance, you should compile your application
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with `RUSTFLAGS="-Ctarget-cpu=haswell"` environment variable to enable AVX instructions.
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We support ARM AArch64 NEON as well and there is no need to pass any additional flags.
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You can find more information in [benches/README.md](./benches/README.md).
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## Rendering quality
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Unless there is a bug, `tiny-skia` must produce exactly the same results as Skia.
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## Safety
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While a quick search would shown tons of `unsafe`, the library is actually fully safe.
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All pixels access is bound-checked. And all memory-related operations are safe.
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We must use `unsafe` to call SIMD intrinsics, which is perfectly safe,
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but Rust's std still marks them as `unsafe` because they may be missing on the target CPU.
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We do check for that.
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We also have to mark some types (to cast `[u32; 1]` to `[u8; 4]` and vise-versa) as
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[bytemuck::Pod](https://docs.rs/bytemuck/1.4.1/bytemuck/trait.Pod.html),
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which is an `unsafe` trait, but still is perfectly safe.
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## Out of scope
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Skia is a huge library and we support only a tiny part of.
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And more importantly, we do not plan to support many feature at all.
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- GPU rendering.
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- Text rendering (maybe someday).
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- PDF generation.
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- Non-RGBA8888 images.
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- Non-PNG image formats.
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- Advanced Bézier path operations.
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- Conic path segments.
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- Path effects (except dashing).
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- Any kind of resource caching.
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- ICC profiles.
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## Notable changes
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Despite being a port, we still have a lot of changes even in the supported subset.
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- No global alpha.<br/>
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Unlike Skia, only `Pattern` is allowed to have opacity.
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In all other cases you should adjust colors opacity manually.
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- No bilinear + mipmap down-scaling support.
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- `tiny-skia` uses just a simple alpha mask for clipping, while Skia has a very complicated,
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but way faster algorithm.
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## Notes about the port
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`tiny-skia` should be viewed as a Rust 2D rendering library that uses Skia algorithms internally.
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We have a completely different public API. The internals are also extremely simplified.
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But all the core logic and math is borrowed from Skia. Hence the name.
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As for the porting process itself, Skia uses goto, inheritance, virtual methods, linked lists,
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const generics and templates specialization a lot, and all of this features are unavailable in Rust.
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There are also a lot of pointers magic, implicit mutations and caches.
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Therefore we have to compromise or even rewrite some parts from scratch.
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## Alternatives
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Right now, the only pure Rust alternative is [raqote].
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- It doesn't support high-quality antialiasing (hairline stroking in particular).
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- It's very slow (see [benchmarks](./benches/README.md)).
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- There are some rendering issues (like gradient transparency).
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- Raqote has a very rudimentary text rendering support, while tiny-skia has none.
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## License
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The same as used by [Skia]: [New BSD License](./LICENSE)
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[Skia]: https://skia.org/
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[cairo]: https://www.cairographics.org/
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[raqote]: https://github.com/jrmuizel/raqote
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[resvg]: https://github.com/RazrFalcon/resvg
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