krilla

krilla is a high-level Rust crate that allows for the creation of PDF files. It builds on top of the pdf-writer crate, but abstracts away all complexities that are involved in creating a PDF file, instead providing an interface with high-level primitives, such as fills, strokes, gradient, glyphs and images which can be used and combined easily without having to worry about low-level details.

Features

krilla supports most features you would expect from a 2D graphics library, including:

• Filling and stroking arbitrary paths.
• Affine transformations.
• Alpha and luminosity masks.
• Clip paths.
• Blend modes and layer isolation.
• A high-level text API for rendering sequences of characters.
• A low-level text API for drawing sequences of positioned glyphs.
• Excellent OpenType font support, supporting all major font types, including color fonts.
• Linear, radial and sweep gradients, as well as patterns.
• Embedding bitmap and SVG images.

In addition to that, the library also supports the following PDF features:

• Great subsetting for both, CFF-flavored and TTF-flavored fonts, ensuring small file sizes.
• Creating document outlines.
• Setting page labels.
• Inserting links to webpages or intra-document positions.
• Adding document metadata.

Scope

This crate labels itself as a high-level crate, and this is what it is: It abstracts away most of the complexity of the PDF format and instead provides high-level primitives for creating PDF files. However from a document-creation perspective, this crate is still very low-level: It does not provide functionality like text layouting, creation of tables, page breaking, inserting headers/footers, etc. This kind of functionality is strictly out of scope for krilla.

krilla's main "target group" is libraries that have some kind of intermediate representation of layouted content (whether it be from HTML or other input sources), and want to easily translate this representation into a PDF file. If this is your use case, then krilla is probably a very suitable, if not the most suitable choice for you.

If not, depending on what exactly you want to do, there are other Rust crates you can use:

• Creating PDF files with very low-level access to the resulting file: pdf-writer.
• Creating documents requiring high-level functionality like automatic text layouting, page breaking, inserting headers and footers: typst.
• Reading existing PDF documents and manipulating them in a certain way: pdf-rs.

In addition to that, the PDF specification is huge and supports tons of features with a lot of customization, including complex color spaces and shadings. The goal of krilla is not to expose high-level bindings for all functionality, but instead expose only a relevant subset of it.

Testing

Testing is a major pain point for most PDF-creation libraries. The reason is that it is very hard to do: It is very easy to accidentally create invalid PDF files, and just testing PDF files in one PDF viewer is not enough to be confident about its correctness. The reason for this is that PDF viewers are often tolerant in what they accept, meaning that it is possible that a PDF just happens to show up fine in one viewer you tested, but fails in all other ones.

Because of this, ensuring proper testing has been one of my main priorities when building this crate, and is probably one of the main distinguishing features from other crates. krilla has two approaches for testing:

Snapshot-based tests

We currently have 50+ snapshot tests, which basically contain an ASCII representation of various "PDF snippets" and have been manually checked to ensure they look as expected. This allows us to detect regressions in the actual output of our PDFs.

Visual-based tests

Unit tests

As mentioned above, checking one PDF viewer for correct output is not enough. Because of this, our visual regression tests are run against 7 distinct PDF viewers (although only 6 are run in CI) to ensure that basic krilla features are displayed correctly in all major viewers. The current selection of viewers includes:

• ghostscript
• mupdf
• poppler
• pdfbox
• pdfium (used in Google Chrome)
• pdf.js (used in Firefox)
• Quartz (used in Safari, this one is not checked in CI, but will run locally on any Mac machine)

We currently have a combined 210+ tests that run against those viewers, which check that basic aspects such as text and gradients are encoded correctly and supported on all (or most) viewers.

This selection unfortunately does not include Adobe Acrobat, which is arguably the most important viewer. The reason for this is simply that it is pretty much impossible to conveniently render PDFs with it. However, all tests have been opened at least once with Acrobat to ensure that no errors appear when opening it.

Integration tests

Finally, we also have visual integration tests to test distinct features as well as combinations of them (like for example gradients with transforms). We use the resvg test suite for that, which conveniently also allows us to automatically test the accuracy of the SVG conversion of krilla. Those tests are only run against one viewer (in most cases pdfium), as it would be pretty wasteful to save reference images for all of them.

Currently, we have over 1500 such tests, and although they mostly focus on testing adherence to the SVG specification, they indirectly also test various interactions of krilla-specific features.

Summary

While krilla does have a very extensive test suite, there is still a lot that is untested, and krilla also has just been released and hasn't been used on a wide scale yet, so there might still be issues. However, I think the current test setup makes it very easy to track future bugs and puts krilla in a very good spot to ensure that no regressions occur in the future.

Future work

For the immediate future, I plan to at least add support for:

• Adding document metadata.
• Support for tagged PDFs for accessibility.
• Support for validated PDF export, like for example PDF/UA.

Although it will probably take some time until I get to it.

Example

The following example shows some of the features of krilla in action.

The example creates a PDF file with two pages. On the first page, we add two small pieces of text, and on the second page we embed a full-page SVG. Consult the documentation to see all features that are available in krilla.

For more examples, feel free to take a look at the the examples directory of the GitHub repository.

// Create a new document.
let mut document = Document::new();
// Load a font.
let mut font = {
    let path =
        PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("assets/fonts/NotoSans-Regular.ttf");
    let data = std::fs::read(&path).unwrap();
    Font::new(Arc::new(data), 0, vec![], None).unwrap()
};

// Add a new page with dimensions 200x200.
let mut page = document.start_page_with(PageSettings::new(200.0, 200.0));
// Get the surface of the page.
let mut surface = page.surface();
// Draw some text.
surface.fill_text(
    Point::from_xy(0.0, 25.0),
    Fill::default(),
    font.clone(),
    14.0,
    &[],
    "This text has font size 14!",
    false,
    None
);
// Draw some more text, in a different color with an opacity and bigger font size.
surface.fill_text(
    Point::from_xy(0.0, 50.0),
    Fill {
        paint: rgb::Color::new(255, 0, 0).into(),
        opacity: NormalizedF32::new(0.5).unwrap(),
        rule: Default::default(),
    },
    font.clone(),
    16.0,
    &[],
    "This text has font size 16!",
    false,
    None
);

// Finish the page.
surface.finish();
page.finish();

// Load an SVG.
let svg_tree = {
    let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
        .join("assets/svgs/custom_integration_wikimedia_coat_of_the_arms_of_edinburgh_city_council.svg");
    let data = std::fs::read(&path).unwrap();
    usvg::Tree::from_data(&data, &usvg::Options::default()).unwrap()
};

// Start a new page, with the same dimensions as the SVG.
let svg_size = svg_tree.size();
let mut page = document.start_page_with(PageSettings::new(svg_size.width(), svg_size.height()));
let mut surface = page.surface();
// Draw the SVG.
surface.draw_svg(&svg_tree, svg_size, SvgSettings::default());

// Finish up and write the resulting PDF.
surface.finish();
page.finish();
let pdf = document.finish().unwrap();
std::fs::write("target/example.pdf", &pdf).unwrap();

Safety

This crate has one direct usage of unsafe, which is needed when loading a font via fontdb. The only reason this is unsafe is that fonts can be memory-mapped, in which case other programs could tamper with that memory. However, this part of the code will only be invoked if you use the svg or fontdb feature.

Other than that, this crate has no unsafe code, although it relies on crates such as bytemuck and yoke that do.

License

This crate is dual-licensed under the MIT and Apache 2.0 licenses.