Crate zeno

This crate provides a high performance, low level 2D rasterization library with support for rendering paths of various styles into alpha or subpixel masks.

Broadly speaking, support is provided for the following:

• 256x anti-aliased rasterization (8-bit alpha or 32-bit RGBA subpixel alpha)
• Pixel perfect hit testing with customizable coverage threshold
• Non-zero and even-odd fills
• Stroking with the standard set of joins and caps (separate start and end caps are possible)
• Numerically stable dashing for smooth dash offset animation
• Vertex traversal for marker placement
• Stepped distance traversal for animation or text-on-path support
• Abstract representation of path data that imposes no policy on storage

While this crate is general purpose, in the interest of interoperability and familiarity, the feature set was chosen specifically to accommodate the requirements of the SVG path specification.

Furthermore, the rasterized masks are nearly identical to those generated by Skia (sans slight AA differences) and as such, should yield images that are equivalent to those produced by modern web browsers.

Rendering

Due to the large configuration space for styling and rendering paths, the builder pattern is used pervasively. The Mask struct is the builder used for rasterization. For example, to render a simple triangle into a 64x64 8-bit alpha mask:

use zeno::{Mask, PathData};

// The target buffer that will contain the mask
let mut mask = [0u8; 64 * 64];

// Create a new mask with some path data
Mask::new("M 8,56 32,8 56,56 Z")
    // Choose an explicit size for the target
    .size(64, 64)
    // Finally, render the path into the target
    .render_into(&mut mask, None);

Note that, in this case, the path itself is supplied as a string in SVG path data format. This crate provides several different kinds of path data by default along with support for custom implementations. See the PathData trait for more detail.

The previous example did not provide a style, so a non-zero Fill was chosen by default. Let’s render the same path with a 4 pixel wide stroke and a round line join:

use zeno::{Join, Mask, PathData, Stroke};

let mut mask = [0u8; 64 * 64];

Mask::new("M 8,56 32,8 56,56 Z")
    .size(64, 64)
    .style(Stroke::new(4.0).join(Join::Round))
    .render_into(&mut mask, None);

Or to make it a bit more dashing:

use zeno::{Cap, Join, Mask, PathData, Stroke};

let mut mask = [0u8; 64 * 64];

Mask::new("M 8,56 32,8 56,56 Z")
    .style(
        Stroke::new(4.0)
            .join(Join::Round)
            .cap(Cap::Round)
            // dash accepts a slice of dash lengths and an initial dash offset
            .dash(&[10.0, 12.0, 0.0], 0.0),
    )
    .size(64, 64)
    .render_into(&mut mask, None);

See the Stroke builder struct for all available options.

So far, we’ve generated our masks into fixed buffers with explicit sizes. It is often the case that it is preferred to ignore all empty space and render a path into a tightly bound mask of dynamic size. This can be done by eliding the call for the size method:

use zeno::{Mask, PathData};

// Dynamic buffer that will contain the mask
let mut mask = Vec::new();

let placement = Mask::new("M 8,56 32,8 56,56 Z")
    // Insert an inspect call here to access the computed dimensions
    .inspect(|format, width, height| {
        // Make sure our buffer is the correct size
        mask.resize(format.buffer_size(width, height), 0);
    })
    .render_into(&mut mask, None);

The call to size has been replaced with a call to inspect which injects a closure into the call chain giving us the opportunity to extend our buffer to the appropriate size. Note also that the render method has a return value that has been captured here. This Placement struct describes the dimensions of the resulting mask along with an offset that should be applied during composition to compensate for the removal of any empty space.

Finally, it is possible to render without a target buffer, in which case the rasterizer will allocate and return a new Vec<u8> containing the mask:

use zeno::{Mask, PathData};

// mask is a Vec<u8>
let (mask, placement) = Mask::new("M 8,56 32,8 56,56 Z")
    // Calling render() instead of render_into() will allocate a buffer
    // for you that is returned along with the placement
    .render();

Both Mask and Stroke offer large sets of options for fine-grained control of styling and rasterization including offsets, scaling, transformations, formats, coordinate spaces and more. See their respective documentation for more detail.

Hit testing

Hit testing is the process of determining if a point is within the region that would be painted by the path. A typical use case is to determine if a user’s cursor is hovering over a particular path. The process generally follows the same form as rendering:

use zeno::{HitTest, PathData};

// A 20x10 region with the right half covered by the path
let hit_test = HitTest::new("M10,0 10,10 20,10 20,0 Z");

assert_eq!(hit_test.test([15, 5]), true);
assert_eq!(hit_test.test([5, 5]), false);

Due to the fact that paths are anti-aliased, the hit test builder offers a threshold option that determines how much “coverage” is required for a hit test to pass at a particular point.

use zeno::{HitTest, PathData};

let mut hit_test = HitTest::new("M2.5,0 2.5,2 5,2 5,0 Z");

// Require full coverage for a successful hit test
hit_test.threshold(255);
assert_eq!(hit_test.test([2, 0]), false);

// Succeed for any non-zero coverage
hit_test.threshold(0);
assert_eq!(hit_test.test([2, 0]), true);

See the HitTest type for more detail.

Path building

While SVG paths are a reasonable choice for static storage, there sometimes arise cases where paths must be built dynamically at runtime:

use zeno::{Command, Mask, PathBuilder, PathData};

// Create a vector to store the path commands
let mut path: Vec<Command> = Vec::new();

// Construct the path with chained method calls
path.move_to([8, 56]).line_to([32, 8]).line_to([56, 56]).close();

// Ensure it is equal to the equivalent SVG path
assert!((&path).commands().eq("M 8,56 32,8 56,56 Z".commands()));

// &Vec<Command> is also valid path data
Mask::new(&path).render(); // ...

Here, a vector of Commands is used to store the path data and the PathBuilder trait provides the extension methods necessary for building a path.

Beyond the four basic path commands, the path builder trait also provides arcs (and position relative versions of all previous commands) along with rectangles, round rectangles, ellipses and circles:

use zeno::{Angle, ArcSize, ArcSweep, Command, PathBuilder, PathData};

let mut path: Vec<Command> = Vec::new();

path.move_to([1, 2]).rel_arc_to(
    8.0,
    4.0,
    Angle::from_degrees(30.0),
    ArcSize::Small,
    ArcSweep::Positive,
    [10, 4],
);

assert!((&path).commands().eq("M1,2 a8,4,30,0,1,10,4".commands()));

Along with incremental building of paths, path builder can also be used as a “sink” for capturing the result of the application of a style and transform to some path data. For example, it is possible to store the output of a stroke style to avoid the cost of stroke evaluation for future rendering or hit test operations with the use of the apply function:

use zeno::{apply, Cap, Command, PathBuilder, PathData, Stroke};

let mut stroke: Vec<Command> = Vec::new();

apply("L10,0", Stroke::new(4.0).cap(Cap::Round), None, &mut stroke);

PathBuilder is only implemented for Vec<Command> by default, but custom implementations are possible to support capturing and building paths into other data structures.

Traversal

Path traversal involves incremental evaluation of a path by some metric. This crate currently provides two methods of traversal.

The Vertices iterator yields a variant of the Vertex enum at the beginning and end of each subpath and between each path command. Each variant provides all the geometric information necessary to place SVG style markers.

The Walk type is an iterator-like type that allows for stepping along the path by arbitrary distances. Each step yields the position on the path at the next distance along with a vector describing the left-ward direction from the path at that point. This is useful for animating objects along a path, or for rendering text attached to a path.

Transient memory allocations

The algorithms in this crate make a concerted effort to avoid dynamic allocations where possible, but paths of significant size or complexity may cause spills into temporary heap memory. Specifically, stroke evaluation and rasterization may cause heap allocations.

To amortize the cost of these, the appropriately named Scratch struct is available. This type contains internal heap allocated storage and provides replacement methods for functions that may allocate. In addition, the Mask::with_scratch and HitTest::with_scratch constructors are provided which take a scratch instance as an argument and redirect all transient allocations to the reusable storage.

Structs

• Angle
  Represents an angle in degrees or radians.
• Bounds
  Axis-aligned bounding box.
• HitTest
  Builder for configuring and executing a hit test.
• Mask
  Builder for configuring and rendering a mask.
• Placement
  Describes the offset and dimensions of a rendered mask.
• Scratch
  Scratch memory for reusable heap allocations.
• Stroke
  Describes the visual style of a stroke.
• Transform
  Two dimensional transformation matrix.
• Vector
  Two dimensional vector.
• Vertices
  An iterator over the vertices of a path.
• Walk
  An iterator like type that walks along a path by arbitrary steps.

Enums

• ArcSize
  Describes the size of an arc.
• ArcSweep
  Describes the sweep direction for an arc.
• Cap
  Defines the shape to be drawn at the beginning or end of a stroke.
• Command
  Path command.
• Fill
  Describes the visual style of a fill.
• Format
  The desired output image format for rendering.
• Join
  Defines the connection between two segments of a stroke.
• Origin
  The origin of the coordinate system for rendering.
• Style
  Represents the style of a path for rendering or hit testing.
• Verb
  Action of a path command.
• Vertex
  A vertex of a path.

Traits

• PathBuilder
  Trait for types that accept path commands.
• PathData
  Trait for types that represent path data.

Functions

• apply
  Applies the style and transform to the path and emits the result to the specified sink.
• bounds
  Computes the bounding box of the path.
• length
  Computes the total length of the path.
• validate_svg
  Returns an error indicating the first position of invalid SVG path data.

Type Aliases

• Point
  Alias for vector to distinguish intended use.