Module swash::scale

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Scaling, hinting and rasterization of visual glyph representations.

Scaling is the process of generating an appropriately sized visual representation of a glyph. The scaler can produce rendered glyph images from outlines, layered color outlines and embedded bitmaps. Alternatively, you can request raw, optionally hinted outlines that can then be further processed by zeno or fed into other crates like lyon or pathfinder for tessellation and GPU rendering.

§Building the scaler

All scaling in this crate takes place within the purview of a ScaleContext. This opaque struct manages internal LRU caches and scratch buffers that are necessary for the scaling process. Generally, you’ll want to keep an instance with your glyph cache, or if doing multithreaded glyph rasterization, one instance per thread.

The only method available on the context is builder which takes a type that can be converted into a FontRef as an argument and produces a ScalerBuilder that provides options for configuring and building a Scaler.

Here, we’ll create a context and build a scaler for a size of 14px with hinting enabled:

// let font = ...;
let mut context = ScaleContext::new();
let mut scaler = context.builder(font)
    .size(14.)
    .hint(true)
    .build();

You can specify variation settings by calling the variations method with an iterator that yields a sequence of values that are convertible to Setting<f32>. Tuples of (&str, f32) will work in a pinch. For example, you can request a variation of the weight axis like this:

// let font = ...;
let mut context = ScaleContext::new();
let mut scaler = context.builder(font)
    .size(14.)
    .hint(true)
    .variations(&[("wght", 520.5)])
    .build();

Alternatively, you can specify variations using the normalized_coords method which takes an iterator that yields NormalizedCoords (a type alias for i16 which is a fixed point value in 2.14 format). This method is faster than specifying variations by tag and value, but the difference is likely negligible outside of microbenchmarks. The real advantage is that a sequence of i16 is more compact and easier to fold into a key in a glyph cache. You can compute these normalized coordinates by using the Variation::normalize method for each available axis in the font. The best strategy, however, is to simply capture these during shaping with the Shaper::normalized_coords method which will have already computed them for you.

See ScalerBuilder for available options and default values.

§Outlines and bitmaps

The Scaler struct essentially provides direct access to the outlines and embedded bitmaps that are available in the font. In the case of outlines, it can produce the raw outline in font units or an optionally hinted, scaled outline. For example, to extract the raw outline for the letter ‘Q’:

// let font = ...;
let mut context = ScaleContext::new();
let mut scaler = context.builder(font).build();
let glyph_id = font.charmap().map('Q');
let outline = scaler.scale_outline(glyph_id);

For the same, but hinted at 12px:

// let font = ...;
let mut context = ScaleContext::new();
let mut scaler = context.builder(font)
    .hint(true)
    .size(12.)
    .build();
let glyph_id = font.charmap().map('Q');
let outline = scaler.scale_outline(glyph_id);

The scale_outline method returns an Outline wrapped in an option. It will return None if an outline was not available or if there was an error during the scaling process. Note that scale_color_outline can be used to access layered color outlines such as those included in the Microsoft Segoe UI Emoji font. Finally, the _into variants of these methods (scale_outline_into and scale_color_outline_into) will return their results in a previously allocated outline avoiding the extra allocations.

Similar to outlines, bitmaps can be retrieved with the scale_bitmap and scale_color_bitmap for alpha and color bitmaps, respectively. These methods return an Image wrapped in an option. The associated _into variants are also available.

Unlike outlines, bitmaps are available in strikes of various sizes. When requesting a bitmap, you specify the strategy for strike selection using the StrikeWith enum.

For example, if we want the largest available unscaled image for the fire emoji:

// let font = ...;
let mut context = ScaleContext::new();
let mut scaler = context.builder(font).build();
let glyph_id = font.charmap().map('🔥');
let image = scaler.scale_color_bitmap(glyph_id, StrikeWith::LargestSize);

Or, to produce a scaled image for a size of 18px:

// let font = ...;
let mut context = ScaleContext::new();
let mut scaler = context.builder(font)
    .size(18.)
    .build();
let glyph_id = font.charmap().map('🔥');
let image = scaler.scale_color_bitmap(glyph_id, StrikeWith::BestFit);

This will select the best strike for the requested size and return a bitmap that is scaled appropriately for an 18px run of text.

Alpha bitmaps should generally be avoided unless you’re rendering small East Asian text where these are sometimes still preferred over scalable outlines. In this case, you should only use StrikeWith::ExactSize to select the strike, falling back to an outline if a bitmap is unavailable.

§Rendering

In the general case of text rendering, you’ll likely not care about the specific details of outlines or bitmaps and will simply want an appropriately sized image that represents your glyph. For this purpose, you’ll want to use the Render struct which is a builder that provides options for rendering an image. This struct is constructed with a slice of Sources in priority order and will iterate through them until it finds one that satisfies the request. Typically, you’ll want to use the following order:

Render::new(&[
    // Color outline with the first palette
    Source::ColorOutline(0),
    // Color bitmap with best fit selection mode
    Source::ColorBitmap(StrikeWith::BestFit),
    // Standard scalable outline
    Source::Outline,
]);

The Render struct offers several options that control rasterization of outlines such as format for selecting a subpixel rendering mode, offset for applying fractional positioning, and others. See the struct documentation for detail.

After selecting your options, call the render method, passing your configured Scaler and the requested glyph identifier to produce an Image. Let’s put it all together by writing a simple function that will render subpixel glyphs with fractional positioning:

fn render_glyph(
    context: &mut ScaleContext,
    font: &FontRef,
    size: f32,
    hint: bool,
    glyph_id: GlyphId,
    x: f32,
    y: f32,
) -> Option<Image> {
    use zeno::{Format, Vector};
    // Build the scaler
    let mut scaler = context.builder(*font).size(size).hint(hint).build();
    // Compute the fractional offset-- you'll likely want to quantize this
    // in a real renderer
    let offset = Vector::new(x.fract(), y.fract());
    // Select our source order
    Render::new(&[
        Source::ColorOutline(0),
        Source::ColorBitmap(StrikeWith::BestFit),
        Source::Outline,
    ])
    // Select a subpixel format
    .format(Format::Subpixel)
    // Apply the fractional offset
    .offset(offset)
    // Render the image
    .render(&mut scaler, glyph_id)
}

Note that rendering also takes care of correctly scaling, rasterizing and compositing layered color outlines for us.

There are other options available for emboldening, transforming with an affine matrix, and applying path effects. See the methods on Render for more detail.

Modules§

Structs§

  • Render
    Builder type for rendering a glyph into an image.
  • ScaleContext
    Context that manages caches and scratch buffers for scaling.
  • Scaler
    Scales outline and bitmap glyphs.
  • ScalerBuilder
    Builder for configuring a scaler.

Enums§

Type Aliases§