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Matthias Clasen: New renderers for GTK

Dje, 28/01/2024 - 6:12md

Recently, GTK gained not one, but two new renderers: one for GL and one for Vulkan.

Since naming is hard, we reused existing names and called them “ngl” and “vulkan”. They are built from the same sources, therefore we also call them “unified” renderers.

But what is exciting about them?

A single source

As mentioned already, the two renderers are built from the same source. It is modeled to follow Vulkan apis, with some abstractions to cover the differences between Vulkan and GL (more specifically, GL 3.3+ and GLES 3.0+). This lets us share much of the infrastructure for walking the scene graph, maintaining transforms and other state, caching textures and glyphs, and will make it easier to keep both renderers up-to-date and on-par.

Could this unified approach be extended further, to cover a Metal-based renderer on macOS or a DirectX-based one on Windows? Possibly. The advantage of the Vulkan/GL combination is that they share basically the same shader language (GLSL, with some variations). That isn’t the case for Metal or DirectX. For those platforms, we either need to duplicate the shaders or use a translation tool like SPIRV-Cross.

If that is the kind of thing that excites you, help is welcome.

Implementation details

The old GL renderer uses simple shaders for each rendernode type and frequently resorts to offscreen rendering for more complex content. The unified renderers have (more capable) per-node shaders too, but instead of relying on offscreens, they will also use a complex shader that interprets data from a buffer. In game programming, this approach is known as a ubershader.

The unified renderer implementation is less optimized than the old GL renderer, and has been written with a focus on correctness and maintainability. As a consequence, it can handle much more varied rendernode trees correctly.

Here is an harmless-looking example:

repeat { bounds: 0 0 50 50; child: border { outline: 0 0 4.3 4.3; widths: 1.3; } } gl (left) ngl (right) A close-up view New capabilities

We wouldn’t have done all this work, if there wasn’t some tangible benefit. Of course, there’s new features and capabilities. Lets look at some:

Antialiasing. A big problem with the old GL renderer is that it will just lose fine details. If something is small enough to fall between the boundaries of a single line of pixels, it will simply disappear. In particular this can affect  underlines, such as mnemonics. The unified renderers handle such cases better, by doing antialiasing. This helps not just for preserving fine detail, but also prevents jagged outlines of primitives.

Close-up view of GL vs NGL

Fractional scaling. Antialiasing is also the basis that lets us handle fractional scales properly. If your  1200 × 800 window is set to be scaled to 125 %, with the unified renderers, we will use a framebuffer of size 1500 × 1000 for it, instead of letting the compositor downscale a 2400 × 1600 image. Much less pixels, and a sharper image.

Arbitrary gradients. The old GL renderer handles linear, radial and conic gradients with up to 6 color stops. The unified renders allow an unlimited number of color stops. The new renderers also apply antialiasing to gradients, so sharp edges will have smooth lines.

A linear gradient with 64 color stops

Dmabufs. As a brief detour from the new renderers, we worked on dmabuf support and graphics offloading last fall. The new renderers support this and extend it to create dmabufs when asked to produce a texture via the render_texture api (currently, just the Vulkan renderer).

Any sharp edges?

As is often the case, with new capabilities comes the potential for new gotchas. Here are some things to be aware of, as an app developer:

No more glshader nodes. Yes, they made for some fancy demos for 4.0, but they are very much tied to the old GL renderer, since they make assumptions about the GLSL api exposed by that renderer. Therefore, the new renderers don’t support them.

You have been warned in the docs:

If there is a problem, this function returns FALSE and reports an error. You should use this function before relying on the shader for rendering and use a fallback with a simpler shader or without shaders if it fails.

Thankfully, many uses of the glshader node are no longer necessary, since GTK has gained new features since 4.0, such as mask nodes and support for straight-alpha textures.

Fractional positions. The old GL renderer is rounding things, so you could get away with handing it fractional positions. The new renderers will place things where you tell it. This can sometimes have unintended consequences, so should be on the lookout and make sure that your positions are where they should be.

In particular, look out for out for cairo-style drawing where you place lines at half-pixel positions so they fill out one row of pixels precisely.

Driver problems. The new renderers are using graphics drivers in new and different ways, so there is potential for triggering problems on that side.

Please file problems you see against GTK even if they look like driver issues, since it is useful for us to get an overview how well (or badly) the new code works with the variety of drivers and hardware out there.

But is it faster?

No, the new renderers are not faster (yet).

The old GL renderer is heavily optimized for speed. It also uses much simpler shaders, and does not do the math that is needed for features such as antialiasing. We want to make the new renderers faster eventually, but the new features and correctness make them very exciting, even before we reach that goal. All of the GPU-based renderers are more than fast enough to render todays GTK apps at 60 or 144 fps.

That being said, the Vulkan renderer comes close to matching and surpassing the old GL renderer in some unscientific benchmarks. The new GL renderer is slower for some reason that we have not tracked down yet.

New defaults

In the just-released 4.13.6 snapshot, we have made the ngl renderer the new default. This is a trial balloon — the renderers need wider testing with different apps too verify that they are ready for production. If significant problems appear, we can revert back to the gl renderer for 4.14.

We decided not make the Vulkan renderer the default yet, since it is behind the GL renderers in a few application integration aspects: the webkit GTK4 port works with GL, not with Vulkan, and GtkGLArea and GtkMediaStream currently both produce GL textures that the Vulkan renderer can’t directly import. All of these issues will hopefully be addressed in the not-too-distant future, and then we will revisit the default renderer decision.

If you are using GTK on very old hardware, you may be better off with the old GL renderer, since it makes fewer demands on the GPU. You can override the renderer selection using the GSK_RENDERER environment variable:

GSK_RENDERER=gl Future plans and possibilities

The new renderers are a good foundation to implement things that we’ve wanted to have for a long time, such as

  • Proper color handling (including HDR)
  • Path rendering on the GPU
  • Possibly including glyph rendering
  • Off-the-main-thread rendering
  • Performance (on old and less powerful devices)

Some of these will be a focus of our work in the near and medium-term future.


The new renderers have some exciting features, with more to come.

Please try them out, and let us know what works and what doesn’t work for you.

Hans de Goede: A fully open source stack for MIPI cameras

Pre, 26/01/2024 - 5:42md
Many recent Intel laptops have replaced the standard UVC USB camera module with a raw MIPI camera-sensor connected to the IPU6 found in recent Intel laptop chips.

Both the hw interface of the ISP part of the IPU6 as well as the image processing algorithms used are considered a trade secret and so far the only Linux support for the IPU6 relies on an out of tree kernel driver with a proprietary userspace stack on top, which is currently available in rpmfusion.

Both Linaro and Red Hat have identified the missing ISP support for various ARM and X86 chips as a problem. Linaro has started a project to add a SoftwareISP component to libcamera to allow these cameras to work without needing proprietary software and Red Hat has joined Linaro in working on this.


Bryan O'Donoghue (Linaro) and I are giving a talk about this at FOSDEM.

Fedora COPR repository

This work is at a point now where it is ready for wider testing. A Fedora COPR repository with a patched kernel and libcamera is now available for users to test, see the COPR page for install and test instructions.

This has been tested on the following devices:
  • Lenovo ThinkPad X1 yoga gen 8 (should work on any ThinkPad with ov2740 sensor)
  • Dell Latitude 9420 (ov01a1s sensor)
  • HP Spectre x360 13.5 (2023 model, hi556 sensor)

Description of the stack
  1. Kernel driver for the camera sensor, for the ov2740 used on current Lenovo designs (excluding MTL) I have landed all necessary kernel changes for this upstream.
  2. Kernel support for the CSI receiver part of the IPU6 Intel is working on upstreaming this and has recently posted v3 of their patch series for this upstream and this is under active review.
  3. A FOSS Software ISP stack inside libcamera to replace the missing IPU6 ISP (processing-system/psys) support. Work on this is under way. I've recently send out v2 of the patch-series for this.
  4. Firefox pipewire camera support and support for the camera portal to get permission to access the camera. My colleague Jan Grulich has been working on this, see Jan's blogpost. Jan's work has landed in the just released Firefox 122.


GNOME Accessibility Blog: Automated testing of GNOME accessibility features

Pre, 26/01/2024 - 3:16md

GNOME is partipating in the December 2023 – February 2024 round of Outreachy. As part of this project, our interns Dorothy Kabarozi and Tanju Achaleke have extended our end-to-end tests to cover some of GNOME’s accessibility features.

End-to-end testing, also known as UI testing, involves simulating user interactions with GNOME’s UI. In this case we’re using a virtual machine which runs GNOME OS, so the tests run on the latest, in-development version of GNOME built from the gnome-build-meta integration repo. The tests send keyboard & mouse events to trigger events in the VM, and use fuzzy screenshot comparisons to assert correct behavior. We use a tool called openQA to develop and run the tests.

Some features are easier to test than others. So far we’ve added tests for the following accessibility features:

  • High contrast theme
  • Large text theme
  • Always-visible scrollbars
  • Audio over-amplification (boost volume above 100%)
  • Visual alerts (flash screen when the error ‘bell’ sound plays)
  • Text-to-speech using Speech Dispatcher
  • Magnifier (zoom)
  • On-screen keyboard

In this screenshot you can see some of the tests:

Here’s a link to the actual test run from the screenshot:

These tests run every time the gnome-build-meta integration repo is updated, so we can very quickly detect if a code change in the ‘main’ branch of any GNOME module has unintentionally caused a regression in some accessibility feature.

  • GNOME’s accessibility features are seeing some design and implementation improvements at the moment, thanks to several volunteer contributors, investments from the Sovereign Tech Fund and Igalia, and more. As improvements land, the tests will need updating too. Screenshots can be updated using openQA’s web UI, available at, there are instructions available. The tests themselves live in openqa-tests.git and are simple Perl programs using openQA’s testapi. Of course merge requests to extend and improve the tests are very welcome.

One important omission from the testsuite today is Orca, the GNOME screen reader. Tanju spent a long time trying to get this to work, and we do have a test that verifies text-to-speech using Speech Dispatcher. Orca itself is more complicated and we’ll need to spend more time to figure out how best to set up end-to-end tests for screen reading.

If you have feedback on the tests, we’d love to hear from you over on the GNOME Discourse forum.