Planet Igalia WebKit

July 12, 2020

Manuel Rego

Open Prioritization and CSS Containment

Igalia is a major contributor to all the open source web rendering engines (Blink, Gecko, Servo and WebKit). We have been doing different kind of contributions for years, which has led us to have an important position on the different communities. This allows us to help our customers to solve their problems through upstream contributions that also benefit the whole web community.

Implementing a feature in a rendering engine (or in several) might look very simple at first sight, but contributing them upstream can take a while depending on the standarization status, the related bugs, the browser architecture, and many other factors. You can find examples of things implemented by Igalia in the past on my previous blog posts, and you will realize about all the work behind some of those features.

There’s a common thing everywhere, people usually get really angry because that bug they reported years ago is still not fixed in a given browser. That can be for a variety of reasons, and not simply because the developers of that browser are very lazy and not paying attention to that particular bug. In many cases the answer to why that hasn’t been solved yet is pretty simple: priorities. Different companies and individuals contributing to the projects have their own interests and priorities, they prioritize the different issues and tasks and put the focus and effort on the ones that have a higher priority for them. A possible solution for that, now that major browsers are all open source, would be to look for a consulting company like Igalia that can fix that bug for you; but you as an individual, or even as a company, maybe you don’t have the budget to make that happen.

What would happen if we allow several parties to contribute together to the development of some features? That would make possible that both individuals and organizations that don’t have the power to implement them alone, could contribute their piece of the cake in order to add support for those features on the web platform.

Open Prioritization

Igalia is launching Open Prioritization, a crowd-founding campaign for the web platform. We believe this can open the door to many different people and organizations to prioritize the development of some features on the different web engines. Initially we have defined 6 tasks that can be found on the website, together with a FAQ explaining all the details of the campaign. 🚀

Let’s hope we can make this happen. If this is a success and some of these items get funded and implemented, probably there’ll be more in the future, including new things or ideas that you can share with us.

Open Prioritization by Igalia. An experiment in crowd-funding prioritization. Open Prioritization by Igalia

One of the tasks of the Open Prioritization campaign we’re starting this week is about adding CSS Containment support in WebKit, and we have experience working on that in Chromium.

Why CSS Containment in WebKit?

Briefly speaking CSS Containment is a standard focused in improving the rendering performance of web pages, it allows author to isolate DOM subtrees from the rest of the document, so any change that happens on the “contained” subtree doesn’t affect anything outside that element.

This is the spec behind the contain property, that can have a few values defining the “type of containment”: layout, paint, size and style. I’m not going to go deeper into this and I’ll refer to my introductory post or my CSSconf EU talk if you’re interested in getting more details about this specification.

So why we think this is important? Currently we have an issue with CSS Containment, it’s supported in Chromium and Firefox (except style containment) but not in WebKit. This might be not a big deal as it’s a performance oriented feature, so if you don’t have support you’ll simply have a worse performance and that’s all. But that’s not completely true as the different type of containments have some restrictions that apply to the contained element (e.g. layout containment makes the element become the containing block of positioned descendants), which might cause interoperability issues if you start to use the contain property in your websites.

The main goal of this task would be add CSS Containment support in WebKit, at least to the level that it’s spec compliant with the other implementations, and if time permits to implement some optimizations based on it. Once we have interoperability you can start using it wihtout any concern in your web pages, as the behavior won’t change between the different browsers and you might get some perf improvements (that will vary depending on each browser implementation).

In addition this will allow WebKit to implement further optimizations thanks to the information that the web authors provide through the contain property. On top of that, this initial support is a requirement in order to implement new features that are based on it; like the new CSS properties content-visibility and contain-intrinsic-size which are part of Display Locking feature.

If you think this is an important feature for you, please go ahead and do your pledge so it can get prioritized and implemented in WebKit upstream.

Really looking forward to seeing how this Open Prioritization campaign goes in the coming weeks. 🤞

July 12, 2020 10:00 PM

July 05, 2020

Frédéric Wang

Contributions to Web Platform Interoperability (First Half of 2020)

Note: This blog post was co-authored by AMP and Igalia teams.

Web developers continue to face challenges with web interoperability issues and a lack of implementation of important features. As an open-source project, the AMP Project can help represent developers and aid in addressing these challenges. In the last few years, we have partnered with Igalia to collaborate on helping advance predictability and interoperability among browsers. Standards and the degree of interoperability that we want can be a long process. New features frequently require experimentation to get things rolling, course corrections along the way and then, ultimately as more implementations and users begin exploring the space, doing really interesting things and finding issues at the edges we continue to advance interoperability.

Both AMP and Igalia are very pleased to have been able to play important roles at all stages of this process and help drive things forward. During the first half of this year, here’s what we’ve been up to…

Default Aspect Ratio of Images

In our previous blog post we mentioned our experiment to implement the intrinsic size attribute in WebKit. Although this was a useful prototype for standardization discussions, at the end there was a consensus to switch to an alternative approach. This new approach addresses the same use case without the need of a new attribute. The idea is pretty simple: use specified width and height attributes of an image to determine the default aspect ratio. If additional CSS is used e.g. “width: 100%; height: auto;”, browsers can then compute the final size of the image, without waiting for it to be downloaded. This avoids any relayout that could cause bad user experience. This was implemented in Firefox and Chromium and we did the same in WebKit. We implemented this under a flag which is currently on by default in Safari Tech Preview and the latest iOS 14 beta.


We continued our efforts to enhance scroll features. In WebKit, we began with scroll-behavior, which provides the ability to do smooth scrolling. Based on our previous patch, it has landed and is guarded by an experimental flag “CSSOM View Smooth Scrolling” which is disabled by default. Smooth scrolling currently has a generic platform-independent implementation controlled by a timer in the web process, and we continue working on a more efficient alternative relying on the native iOS UI interfaces to perform scrolling.

We have also started to work on overscroll and overscroll customization, especially for the scrollend event. The scrollend event, as you might expect, is fired when the scroll is finished, but it lacked interoperability and required some additional tests. We added web platform tests for programmatic scroll and user scroll including scrollbar, dragging selection and keyboard scrolling. With these in place, we are now working on a patch in WebKit which supports scrollend for programmatic scroll and Mac user scroll.

On the Chrome side, we continue working on the standard scroll values in non-default writing modes. This is an interesting set of challenges surrounding the scroll API and how it works with writing modes which was previously not entirely interoperable or well defined. Gaining interoperability requires changes, and we have to be sure that those changes are safe. Our current changes are implemented and guarded by a runtime flag “CSSOM View Scroll Coordinates”. With the help of Google engineers, we are trying to collect user data to decide whether it is safe to enable it by default.

Another minor interoperability fix that we were involved in was to ensure that the scrolling attribute of frames recognizes values “noscroll” or “off”. That was already the case in Firefox and this is now the case in Chromium and WebKit too.

Intersection and Resize Observers

As mentioned in our previous blog post, we drove the implementation of IntersectionObserver (enabled in iOS 12.2) and ResizeObserver (enabled in iOS 14 beta) in WebKit. We have made a few enhancements to these useful developer APIs this year.

Users reported difficulties with observe root of inner iframe and the specification was modified to accept an explicit document as a root parameter. This was implemented in Chromium and we implemented the same change in WebKit and Firefox. It is currently available Safari Tech Preview, iOS 14 beta and Firefox 75.

A bug was also reported with ResizeObserver incorrectly computing size for non-default zoom levels, which was in particular causing a bug on twitter feeds. We landed a patch last April and the fix is available in the latest Safari Tech Preview and iOS 14 beta.

Resource Loading

Another thing that we have been concerned with is how we can give more control and power to authors to more effectively tell the browser how to manage the loading of resources and improve performance.

The work that we started in 2019 on lazy loading has matured a lot along with the specification.

The lazy image loading implementation in WebKit therefore passes the related WPT tests and is functional and comparable to the Firefox and Chrome implementations. However, as you might expect, as we compare uses and implementation notes it becomes apparent that determining the moment when the lazy image load should start is not defined well enough. Before this can be enabled in releases some more work has to be done on improving that. The related frame lazy loading work has not started yet since the specification is not in place.

We also added an implementation for stale-while-revalidate. The stale-while-revalidate Cache-Control directive allows a grace period in which the browser is permitted to serve a stale asset while the browser is checking for a newer version. This is useful for non-critical resources where some degree of staleness is acceptable, like fonts. The feature has been enabled recently in WebKit trunk, but it is still disabled in the latest iOS 14 beta.

Contributions were made to improve prefetching in WebKit taking into account its cache partitioning mechanism. Before this work can be enabled some more patches have to be landed and possibly specified (for example, prenavigate) in more detail. Finally, various general Fetch improvements have been done, improving the fetch WPT score. Examples are:

What’s next

There is still a lot to do in scrolling and resource loading improvements and we will continue to focus on the features mentioned such as scrollend event, overscroll behavior and scroll behavior, lazy loading, stale-while-revalidate and prefetching.

As a continuation of the work done for aspect ratio calculation of images, we will consider the more general CSS aspect-ratio property. Performance metrics such as the ones provided by the Web Vitals project is also critical for web developers to ensure that their websites provide a good user experience and we are willing to investigate support for these in Safari.

We love doing this work to improve the platform and we’re happy to be able to collaborate in ways that contribute to bettering the web commons for all of us.

July 05, 2020 10:00 PM

July 02, 2020

Philippe Normand

Web-augmented graphics overlay broadcasting with WPE and GStreamer

Graphics overlays are everywhere nowadays in the live video broadcasting industry. In this post I introduce a new demo relying on GStreamer and WPEWebKit to deliver low-latency web-augmented video broadcasts.

Readers of this blog might remember a few posts about WPEWebKit and a GStreamer element we at Igalia worked on …

by Philippe Normand at July 02, 2020 01:00 PM

June 30, 2020

Enrique Ocaña

Developing on WebKitGTK with Qt Creator 4.12.2

After the latest migration of WebKitGTK test bots to use the new SDK based on Flatpak, the old development environment based on jhbuild became deprecated. It can still be used with export WEBKIT_JHBUILD=1, though, but support for this way of working will gradually fade out.

I used to work on a chroot because I love the advantages of having an isolated and self-contained environment, but an issue in the way bubblewrap manages mountpoints basically made it impossible to use the new SDK from a chroot. It was time for me to update my development environment to the new ages and have it working in my main Kubuntu 18.04 distro.

My mail goal was to have a comfortable IDE that follows standard GUI conventions (that is, no emacs nor vim) and has code indexing features that (more or less) work with the WebKit codebase. Qt Creator was providing all that to me in the old chroot environment thanks to some configuration tricks by Alicia, so it should be good for the new one.

I preferred to use the Qt Creator 4.12.2 offline installer for Linux, so I can download exactly the same version in the future in case I need it, but other platforms and versions are also available.

The WebKit source code can be downloaded as always using git:

git clone

It’s useful to add WebKit/Tools/Scripts and WebKit/Tools/gtk to your PATH, as well as any other custom tools you may have. You can customize your $HOME/.bashrc for that, but I prefer to have an environment script to be sourced from the current shell when I want to enter into my development environment (by running webkit). If you’re going to use it too, remember to adjust to your needs the paths used there.

Even if you have a pretty recent distro, it’s still interesting to have the latests Flatpak tools. Add Alex Larsson’s PPA to your apt sources:

sudo add-apt-repository ppa:alexlarsson/flatpak

In order to ensure that your distro has all the packages that webkit requires and to install the WebKit SDK, you have to run these commands (I omit the full path). Downloading the Flatpak modules will take a while, but at least you won’t need to build everything from scratch. You will need to do this again from time to time, every time the WebKit base dependencies change:


Now just build WebKit and check that MiniBrowser works:

build-webkit --gtk
run-minibrowser --gtk

I have automated the previous steps as go full-rebuild and

This build process should have generated a WebKit/WebKitBuild/GTK/Release/compile_commands.json
file with the right parameters and paths used to build each compilation unit in the project. This file can be leveraged by Qt Creator to get the right include paths and build flags after some preprocessing to translate the paths that make sense from inside Flatpak to paths that make sense from the perspective of your main distro. I wrote to take care of those transformations. It can be run manually or automatically when calling go full-rebuild or go update.

The WebKit way of managing includes is a bit weird. Most of the cpp files include config.h and, only after that, they include the header file related to the cpp file. Those header files depend on defines declared transitively when including config.h, but that file isn’t directly included by the header file. This breaks the intuitive rule of “headers should include any other header they depend on” and, among other things, completely confuse code indexers. So, in order to give the Qt Creator code indexer a hand, the script pre-includes WebKit.config for every file and includes config.h from it.

With all the needed pieces in place, it’s time to import the project into Qt Creator. To do that, click File → Open File or Project, and then select the compile_commands.json file that should have generated in the WebKit main directory.

Now make sure that Qt Creator has the right plugins enabled in Help → About Plugins…. Specifically: GenericProjectManager, ClangCodeModel, ClassView, CppEditor, CppTools, ClangTools, TextEditor and LanguageClient (more on that later).

With this setup, after a brief initial indexing time, you will have support for features like Switch header/source (F4), Follow symbol under cursor (F2), shading of disabled if-endif blocks, auto variable type resolving and code outline. There are some oddities of compile_commands.json based projects, though. There are no compilation units in that file for header files, so indexing features for them only work sometimes. For instance, you can switch from a method implementation in the cpp file to its declaration in the header file, but not the opposite. Also, you won’t see all the source files under the Projects view, only the compilation units, which are often just a bunch of UnifiedSource-*.cpp files. That’s why I prefer to use the File System view.

Additional features like Open Type Hierarchy (Ctrl+Shift+T) and Find References to Symbol Under Cursor (Ctrl+Shift+U) are only available when a Language Client for Language Server Protocol is configured. Fortunately, the new WebKit SDK comes with the ccls C/C++/Objective-C language server included. To configure it, open Tools → Options… → Language Client and add a new item with the following properties:

  • Name: ccls
  • Language: *.c;.cpp;*.h
  • Startup behaviour: Always On
  • Executable: /home/enrique/work/webkit/WebKit/Tools/Scripts/webkit-flatpak
  • Arguments: --gtk -c ccls --index=/home/enrique/work/webkit/WebKit

Some “LanguageClient ccls: Unexpectedly finished. Restarting in 5 seconds.” errors will appear in the General Messages panel after configuring the language client and every time you launch Qt Creator. It’s just ccls taking its time to index the whole source code. It’s “normal”, don’t worry about it. Things will get stable and start to work after some minutes.

Due to the way the Locator file indexer works in Qt Creator, it can become confused, run out of memory and die if it finds cycles in the project file tree. This is common when using Flatpak and running the MiniBrowser or the tests, since /proc and other large filesystems are accessible from inside WebKit/WebKitBuild. To avoid that, open Tools → Options… → Environment → Locator and set Refresh interval to 0 min.

I also prefer to call my own custom build and run scripts (go and instead of letting Qt Creator build the project with the default builders and mess everything. To do that, from the Projects mode (Ctrl+5), click on Build & Run → Desktop → Build and edit the build configuration to be like this:

  • Build directory: /home/enrique/work/webkit/WebKit
  • Add build step → Custom process step
    • Command: go (no absolute route because I have it in my PATH)
    • Arguments:
    • Working directory: /home/enrique/work/webkit/WebKit

Then, for Build & Run → Desktop → Run, use these options:

  • Deployment: No deploy steps
  • Run:
    • Run configuration: Custom Executable → Add
      • Executable:
      • Command line arguments:
      • Working directory:

With these configuration you can build the project with Ctrl+B and run it with Ctrl+R.

I think I’m not forgetting anything more regarding environment setup. With the instructions in this post you can end up with a pretty complete IDE. Here’s a screenshot of it working in its full glory:

Anyway, to be honest, nothing will ever reach the level of code indexing features I got with Eclipse some years ago. I could find usages of a variable/attribute and know where it was being read, written or read-written. Unfortunately, that environment stopped working for me long ago, so Qt Creator has been the best I’ve managed to get for a while.

Properly configured web based indexers such as the Searchfox instance configured in Igalia can also be useful alternatives to a local setup, although they lack features such as type hierarchy.

I hope you’ve found this post useful in case you try to setup an environment similar to the one described here. Enjoy!

by eocanha at June 30, 2020 03:47 PM

June 16, 2020

Víctor Jáquez

WebKit Flatpak SDK and gst-build

This post is an annex of Phil’s Introducing the WebKit Flatpak SDK. Please make sure to read it, if you haven’t already.

Recapitulating, nowadays WebKitGtk/WPE developers —and their CI infrastructure— are moving towards to Flatpak-based environment for their workflow. This Flatpak-based environment, or Flatpak SDK for short, can be visualized as a software sandboxed-container, which bundles all the dependencies required to compile, run and debug WebKitGtk/WPE.

In a day-by-day work, this approach removes the potential compilation of the world in order to obtain reproducible builds, improving the development and testing work flow.

But what if you are also involved in the development of one dependency?

This is the case of Igalia’s multimedia team where, besides developing the multimedia features for WebKitGtk and WPE, we also participate in the GStreamer development, the framework used for multimedia.

Because of this, in our workflow we usually need to build WebKit with a fix, hack or new feature in GStreamer. Is it possible to add in Flatpak our custom GStreamer build without messing its own GStreamer setup? Yes, it’s possible.

gst-build is a set of scripts in Python which clone GStreamer repositories, compile them and setup an uninstalled environment. This uninstalled environment allows a transient usage of the compiled framework from their build tree, avoiding installation and further mess up with our system.

The WebKit scripts that wraps Flatpak operations are also capable to handle the scripts of gst-build to build GStreamer inside the container, and, when running WebKit’s artifacts, the scripts enable the mentioned uninstalled environment, overloading Flatpak’s GStreamer.

How do we unveil all this magic?

First of all, setup a gst-build installation as it is documented. In this installation is were the GStreamer plumbing is done.

Later, gst-build operations through WebKit compilation scripts are enabled when the environment variable GST_BUILD_PATH is exported. This variable should point to the directory where the gst-build tree is placed.

And that’s all!

But let’s put these words in actual commands. The following workflow assumes that WebKit repository is cloned in ~/WebKit and the gst-build tree is in ~/gst-build (please, excuse my bashisms).

Compiling WebKitGtk with symbols, using LLVM as toolchain (this command will also compile GStreamer):

$ cd ~/WebKit
% CC=clang CXX=clang++ GST_BUILD_PATH=/home/vjaquez/gst-build Tools/Scripts/build-webkit --gtk --debug

Running the generated minibrowser (remind GST_BUILD_PATH is required again for a correct linking):

$ GST_BUILD_PATH=/home/vjaquez/gst-build Tools/Scripts/run-minibrowser --gtk --debug

Running media layout tests:

$ GST_BUILD_PATH=/home/vjaquez/gst-build ./Tools/Scripts/run-webkit-tests --gtk --debug media

But wait! There’s more...

What if you I want to parametrize the GStreamer compilation. To say, I would like to enable a GStreamer module or disable the built of a specific element.

gst-build, as the rest of GStreamer modules, uses meson build system, so it’s possible to pass arguments to meson through the environment variable GST_BUILD_ARGS.

For example, I would like to enable gstreamer-vaapi 😇

$ cd ~/WebKit
% CC=clang CXX=clang++ GST_BUILD_PATH=/home/vjaquez/gst-build GST_BUILD_ARGS="-Dvaapi=enabled" Tools/Scripts/build-webkit --gtk --debug

by vjaquez at June 16, 2020 11:49 AM

June 11, 2020

Diego Pino

Renderization of Conic gradients

The CSS Images Module Level 4 introduced a new type of gradient: conic-gradient. Until then, there were only two other type of gradients available on the Web: linear-gradient and radial-gradient.

The first browser to ship conic-gradient support was Google Chrome, around March 2018. A few months after, September 2018, the feature was available in Safari. Firefox have been missing support until now, although an implementation is on the way and will ship soon. In the case of WebKitGTK (Epiphany) and WPE (Web Platform for Embedded), support landed in October 2019 which I implemented as part of my work at Igalia. The feature has been officially available in WebKitGTK and WPE since version 2.28 (March 2020).

Before native browser support, conic-gradient was available as a JavaScript polyfill created by Lea Verou.

Gradients in the Web

Generally speaking, a gradient is a smooth transition of colors defined by two or more stop-colors. In the case of a linear gradient, this transition is defined by a straight line (which might have and angle or not).

div.linear-gradient {
  width: 400px;
  height: 100px;
  background: linear-gradient(to right, red, yellow, lime, aqua, blue, magenta, red);
Linear gradient
Linear gradient

In the case of a radial gradient, the transition is defined by a center and a radius. Colors expand evenly in all directions from the center of the circle to outside.

div.radial-gradient {
  width: 300px;
  height: 300px;
  border-radius: 50%;
  background: radial-gradient(red, yellow, lime, aqua, blue, magenta, red);
Radial gradient
Radial gradient

A conical gradient, although also defined by a center and a radius, isn’t the same as a radial gradient. In a conical gradient colors spin around the circle.

div.conic-gradient {
  width: 300px;
  height: 300px;
  border-radius: 50%;
  background: conic-gradient(red, yellow, lime, aqua, blue, magenta, red);
Conic gradient
Conic gradient

Implementation in WebKitGTK and WPE

At the time of implementing support in WebKitGTK and WPE, the feature had already shipped in Safari. That meant WebKit already had support for parsing the conic-gradient specification as defined in CSS Images Module Level 4 and the data structures to store relevant information were already created. The only piece missing in WebKitGTK and WPE was painting.

Safari leverages many of its graphical painting operations on CoreGraphics library, which counts with a primitive for conic gradient painting (CGContextDrawConicGradient). Something similar happens in Google Chrome, although in this case the graphics library underneath is Skia (CreateTwoPointConicalGradient). WebKitGTK and WPE use Cairo for many of their graphical operations. In the case of linear and radial gradients, there’s native support in Cairo. However, there isn’t a function for conical gradient painting. This doesn’t mean Cairo cannot be used to paint conical gradients, it just means that is a little bit more complicated.

Mesh gradients

Cairo documentation states is possible to paint a conical gradient using a mesh gradient. A mesh gradient is defined by a set of colors and control points. The most basic type of mesh gradient is a Gouraud-shading triangle mesh.

cairo_mesh_pattern_begin_patch (pattern)

cairo_mesh_pattern_move_to (pattern, 100, 100);
cairo_mesh_pattern_line_to (pattern, 130, 130);
cairo_mesh_pattern_line_to (pattern, 130,  70);

cairo_mesh_pattern_set_corner_color_rgb (pattern, 0, 1, 0, 0);
cairo_mesh_pattern_set_corner_color_rgb (pattern, 1, 0, 1, 0);
cairo_mesh_pattern_set_corner_color_rgb (pattern, 2, 0, 0, 1);

cairo_mesh_pattern_end_patch (pattern)
Gouraud-shaded triangle mesh
Gouraud-shaded triangle mesh

A more sophisticated patch of mesh gradient is a Coons patch. A Coons patch is a quadrilateral defined by 4 cubic Bézier curve and 4 colors, one for each vertex. A Bézier curve is defined by 4 points, so we have a total of 12 control points (and 4 colors) in a Coons patch.

cairo_mesh_pattern_begin_patch (pattern);

cairo_mesh_pattern_move_to (pattern, 45, 12);
cairo_mesh_pattern_curve_to(pattern, 69, 24, 173, -15, 115, 50);
cairo_mesh_pattern_curve_to(pattern, 127, 66, 174, 47, 148, 104);
cairo_mesh_pattern_curve_to(pattern, 65, 58, 70, 69, 18, 103);
cairo_mesh_pattern_curve_to(pattern, 42, 43, 63, 45, 45, 12);

cairo_mesh_pattern_set_corner_color_rgb (pattern, 0, 1, 0, 0); // red
cairo_mesh_pattern_set_corner_color_rgb (pattern, 1, 0, 1, 0); // green
cairo_mesh_pattern_set_corner_color_rgb (pattern, 2, 0, 0, 1); // blue
cairo_mesh_pattern_set_corner_color_rgb (pattern, 3, 1, 1, 0); // yellow

cairo_mesh_pattern_end_patch (pattern);
Coons patch gradient
Coons patch gradient

A Coons patch comes very handy to paint a conical gradient. Consider the first quadrant of a circle, such quadrant can be easily defined with a Bézier curve.

cairo_mesh_pattern_begin_patch (pattern);

cairo_mesh_pattern_move_to (pattern, 0, 200);
cairo_mesh_pattern_line_to (pattern, 0, 0);
cairo_mesh_pattern_curve_to (pattern, 133, 0, 200, 133, 200, 200);
cairo_mesh_pattern_line_to (pattern, 0, 200);

cairo_mesh_pattern_set_corner_color_rgb (pattern, 0, 1, 0, 0); // red
cairo_mesh_pattern_set_corner_color_rgb (pattern, 1, 0, 1, 0); // green
cairo_mesh_pattern_set_corner_color_rgb (pattern, 2, 0, 0, 1); // blue
cairo_mesh_pattern_set_corner_color_rgb (pattern, 3, 1, 1, 0); // yellow

Coons patch of the first quadrant of a circle
Coons patch of the first quadrant of a circle

If we just simply use two colors instead, the final result resembles more to how a conical gradient looks.

cairo_mesh_pattern_set_corner_color_rgb (pattern, 0, 1, 0, 0); // red
cairo_mesh_pattern_set_corner_color_rgb (pattern, 1, 1, 0, 0); // red
cairo_mesh_pattern_set_corner_color_rgb (pattern, 2, 1, 1, 0); // yellow
cairo_mesh_pattern_set_corner_color_rgb (pattern, 3, 1, 1, 0); // yellow
Coons patch of the first quadrant of a circle (2 colors)
Coons patch of the first quadrant of a circle (2 colors)

Repeat this step 3 times more, with a few more stop colors, and you have a nice conical gradient.

A conic gradient made by composing mesh patches
A conic gradient made by composing mesh patches

Bézier curve as arcs

At this point the difficulty of painting a conical gradient has been reduced to calculating the shape of the Bézier curve of each mesh patch.

Computing the starting and ending points is straight forward, however calculating the position of the other two control points of the Bézier curve is a bit much harder.

Bézier curve approximation to a circle quadrant
Bézier curve approximation to a circle quadrant

Mozillian Michiel Kamermans (pomax) has a beautifully written essay on Bézier curves. Section “Circles and cubic Bézier curves” of such essay discusses how to approximate a Bézier curve to an arc. The case of a circular quadrant is particularly interesting because it allows painting a circle with 4 Bézier curves with minimal error. In the case of the quadrant above the values for each point would be the following:

S = (0, r), CP1 = (0.552 * r, r), CP2 = (r, 0.552 * r), E = (r, 0) 

Even though on its most basic form a conic gradient is defined by one starting and one ending color, painting a circle with two Bézier curves is not a good approximation to a semicircle (check the interactive examples of pomax’s Bézier curve essay). In such case, the conic gradient is split into four Coon patches with middle colors interpolated.

Also, in cases were there are more than 4 colors, each Coons patch will be smaller than a quadrant. It’s necessary a general formula that can compute the control points for each section of the circle, given an angle and a radius. After some math, the following formula can be inferred (check section “Circle and cubic Bézier curves” in pomax’s essay):

cp1 = {
   x: cx + (r * cos(angleStart) - f * (r * sin(angleStart),
   y: cy + (r * sin(angleStart)) + f * (r * cos(angleStart))
cp2 = {
   x: cx + (r * cos(angleEnd)) + f * (r * sin(angleEnd)),
   y: cy + (r * sin(angleEnd)) - f * (r * cos(angleEnd))

where f is a variable computed as:

f = 4 * tan((angleEnd - angleStart) / 4) / 3;

For a 90 degrees angle the value of f is 0.552. Thus, if the quadrant above had a radius of 100px, the values of the control points would be: CP1(155.2, 0) and CP2(200, 44.8) (considering top corner left as point 0,0).

And that’s basically all that is needed. The formula above allows us to compute a circular sector as a Bézier line, which when setup as a Coons patch creates a section of a conical gradient. Adding several Coons patches together creates the final conical gradient.

Wrapping up

It has been a long time since conic gradients for the Web were first drafted. For instance, the current bug in Firefox’s Bugzilla was created by Lea Verou five years ago. Fortunately, browsers have started shipping native support and conical gradients have been available in Chrome and Safari since two years ago. In this post I discussed the implementation, mainly rendering, of conic gradients in WebKitGTK and WPE. And since both browsers are WebKit based, they can leverage on the implementation efforts led by Apple when bringing support of this feature to Safari. With Firefox shipping conic gradient support soon this feature will be safe to use in the Web Platform.

June 11, 2020 12:00 AM

June 09, 2020

Philippe Normand

WebKitGTK and WPE now supporting videos in the img tag

Using videos in the <img> HTML tag can lead to more responsive web-page loads in most cases. Colin Bendell blogged about this topic, make sure to read his post on the cloudinary website. As it turns out, this feature has been supported for more than 2 years in Safari, but …

by Philippe Normand at June 09, 2020 04:00 PM

June 08, 2020

Philippe Normand

Introducing the WebKit Flatpak SDK

Working on a web-engine often requires a complex build infrastructure. This post documents our transition from JHBuild to Flatpak for the WebKitGTK and WPEWebKit development builds.

For the last 10 years, WebKitGTK has been relying on a custom JHBuild moduleset to handle its dependencies and (try to) ensure a reproducible …

by Philippe Normand at June 08, 2020 04:50 PM

June 05, 2020

Paulo Matos

JSC: what are my options?

Compilers tend to be large pieces of software that provide an enormous amount of options. We take a quick look at how to find what JavaScriptCore (JSC) provides.


by Paulo Matos at June 05, 2020 12:23 PM