Strongly typed vector math with glam

This crate uses bytemuck to implement a zero-cost[^zero_cost] strongly typed interface on top of glam.

The API is similar to euclid, but more ergonomic (although YMMV).

One of the API design goals of glam is to avoid complexity by not going bananas with traits and generics. This crate is the opposite. But it does allow you to easily drop down to plain glam when needed.

See the docs module for detailed documentation.

[^zero_cost]: Zero-cost at runtime, in release builds. This crate may increase compile times and make debug builds slower due to increased code size.

Step-By-Step Quickstart Guide

1. Declare your units by defining a "unit" type (can be empty, doesn't need any traits to be derived).
2. Implement Unit for that struct. Unit::Scalar determines the primitive type used in vector components.
3. The scalar must be f32, f64, i32, u32, i64, u64, i16, or u16.
4. The basic primitive scalars are also units in their own right ("untyped").

Example

use glamour::prelude::*;

struct MyUnit;
impl Unit for MyUnit {
    type Scalar = f32;
}

// Start using your new unit:
let vector: Vector4<MyUnit> = Vector4 { x: 1.0, y: 2.0, z: 3.0, w: 4.0 };
let size: Size2<MyUnit> = Size2 { width: 100.0, height: 200.0 };

// Use untyped units when needed:
let vector_untyped: &Vector4<f32> = vector.as_untyped();

// Use glam when needed:
let vector_raw: &glam::Vec4 = vector.as_raw();

See the documentation module for more examples.

Feature gates

• std - enables the glam/std feature. Enabled by default.
• libm - required to compile with no_std (transitively enables glam/no_std).
• mint - enables conversion to/from mint types.
• encase: Enables implementations of encase::ShaderType for vector and matrix types, which enables them for use in GPU shaders.
• scalar-math: Don't use SIMD vector instructions, even if they are supported by the target architecture. Note that this flag is required to run tests under Miri, due to vector instructions not being supported. Transitively enables the glam/scalar-math feature.

Advantages

• Structural type construction is sometimes better because it doesn't rely on positional arguments. It also allows us to give more meaningful names to things - for example, the members of Size2 are called width and height, rather than x and y.
• The user is able to easily drop down to plain glam types when needed.

Over plain glam

• Lifts some correctness checks to the type system. This can prevent certain common bugs, such as using a vector from one coordinate space in a context that logically expects a different coordinate space.
• Improves API comprehension and code readability by annotating expectations as part of function signatures.
• Distinguishing between points, vectors, and sizes can also prevent certain classes of bugs. For example, the "transform" operation in 3D is different for points and vectors.

Over euclid

• Type names are more concise (single generic parameter instead of two).
• Support for bytemuck.

Disadvantages

• The API is heavily reliant on metaprogramming tricks. A complex maze of traits is required to support the goals. The trade-off can be summed up as: simplicity, ergonomics, type-safety - pick two. This crate picks ergonomics and type-safety.
• Generic struct definitions have trait bounds. This is usually considered an antipattern in Rust, but we need to encode two things with one type parameter to support structural construction of vector types, so it is unavoidable.

Compared to glam

• Due to its simplicity, glam is a very approachable API.
• glam is able to support a wide range of transformation primitives (e.g. glam::Affine3A, glam::Quat, etc.), and the user has a lot of flexibility to choose the most performant kind for their use case. These are simply unimplemented in glamour.

Compared to euclid

• The same unit tag cannot be used with different scalars.
• Any type cannot be used as the unit tag - it must implement Unit.

Goals

• Strongly typed linear algebra primitives.
• Bitwise compatibility with glam.
• First-class field struct expression support in vector types.
• Support direct memory mapping (e.g. upload to GPU buffers).
• Support no_std.
• Adhere to glam API conventions - "principle of least surprise".
• Add only a few additional geometric primitives, like rects, transforms, and axis-aligned boxes.
• Impose no runtime overhead at all (compared to using glam directly). Comprehensive benchmarks pending.
• 100% test coverage.

Non-goals

• Complex linear algebra. Use nalgebra or Euclid instead.
• Vector sizes beyond 4 dimensions (the maximum supported by glam).
• Type parameterization on vector/matrix size.
• Non-square matrices.
• Wrapping all of the glam API. Instead, we make it really easy (and performant) to drop down to glam types when needed.
• Hiding the glam API. It's OK to use glam types in public APIs.
• The "AoSoA" pattern ("extra wide" vector types). Use ultraviolet instead[^use_uv].

[^use_uv]: Ultraviolet supports bytemuck as well, and the types in this library are actually compatible with the non-wide vector types in Ultraviolet, so it may actually just work (using bytemuck::cast() and friends), but no guarantees.

Performance

All operations should perform exactly the same as their glam counterparts. There is a zero-tolerance policy for overhead in release builds.

However, debug build performance is also important in some cases. For example, for a video game it can make the difference between being playable or not in debug mode.

This crate should be expected to incur an overhead of about a factor 2 compared to glam in debug builds. This may be alleviated in the future, but it seems that even glam itself does not go out of its way to perform well in debug builds.