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From window killing to screenshot

Last week I concentrated most of my development work on screenshot support through spectacle in a KWin Wayland session. Now I am happy to announce that we merged support for capturing a screenshot of a window with the help of an external application like spectacle.

To explain why this is a great achievement we first need to look at X11. On X11 taking a screenshot of a window is easy. It’s part of the X protocol to read the pixmap data of the root window and you get the position and size of each window. Thus one is able to cut out the window and have it as a screenshot. That’s the most simple variant to do it, spectacle and previously ksnapshot do it differently. More on that later on.

This is the way to screenshot the active window. If one wants to screenshot any window the user needs to select it. Also for that the X protocol contains everything one needs: grab the mouse cursor, get the click and query the x window tree to figure out which window got clicked. Afterwards screenshot it the same way as explained.

For bringing that to Wayland we see some “problems”. Wayland is designed in a sane and secure way matching the security requirements of 2016 and not the security requirements of the 1980s. An application taking a screenshot of another window or of the complete system is nowadays not acceptable any more. And there is no built in way to take a screenshot – neither of fullscreen nor of a window.

And even if there were, it wouldn’t help much. Information about other windows are not available. One cannot get the information of which is the active window or what is the window under the current mouse position. Also an application is not able to grab the mouse and get the click anywhere on the screen. Mouse is always only available on the current window.

So overall quite some obstacles to take a screenshot and we see that it will need support from the Wayland compositor:

  • Selection of the window
  • Taking the screenshot
  • Communication protocol with the application

Luckily KWin is partially already prepared for it. Even on X11 KWin provides a screenshot functionally to spectacle. A few years ago we wanted to have something better than the standard X screenshots. We wanted to have a window completely taken without overlapping windows and the decoration with the shadows included. Shadows are in case of KWin rendered by the compositor and not part of the X windows. So in order to screenshot it needed support from the compositor, just like on Wayland.

Unfortunately we didn’t think of something like “a successor for X11” back then and designed the interaction more in a way suited for KSnapshot than for not X. We used a DBus API which passed the window id of the window to screenshot as argument and as a result sent an XPixmap to ksnapshot.

Overall not suited for Wayland, but a very good starting point as we already have the screenshot functionality available. So what we needed is a an X free DBus protocol and a way to select the window from within KWin.

Just like spectacle also KWin has a functionality to select a window through mouse interaction: the kill window functionality triggered through Ctrl+Alt+Escape. So far this functionality was only available for X11 and X11 windows, we were not able to do the same on Wayland.

For taking the screenshot I wondered whether we could use this functionality in a more generic way: a feature to interactively select a window. This required a slight refactoring. Of course the X11 way to select a window doesn’t help much, but the ideas we have there. The X11 specific code got moved into the X11-standalone Platform plugin and is now invoked through the internal Platform API. It doesn’t directly kill the window any more, but only returns the window which got selected.

A similar interaction code got added for Wayland and now the kill window functionality can be triggered on both X11 and Wayland and can kill both X11 and Wayland windows.
kwin_screenshot_bn1249

Now all we needed was making this functionality aware to the screenshot functionality. And with that we could trigger an interactive way to trigger a screenshot. This keeps the user in control of the process: the user is informed that a screenshot is being taken and informed how to cancel this. This addresses the security concerns we had for taking screenshots. By making the user perform an explicit action we know that the user agreed to taking the screenshot.

Notification about screenshot taking

Now all that was needed is adjusting spectacle. Spectacle is a rather new and modern application which had multiple windowing systems in mind when the implementation started. So far I had not done any work on spectacle and the code base was new to me. Nevertheless in about an hour I had the screenshot selection working:

Spectacle on Wayland

Not everything is supported though. Fullscreen or screen area screenshots are not yet supported. But given that the primary problem is solved now, this will also be addressed soonish.

If you like the work we are doing for Wayland consider participating in our end of year fundraiser.

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Wayland improvements since Plasma 5.8 release

Two weeks have passed since the Plasma 5.8 release and our Wayland efforts have seen quite some improvements. Some changes went into Plasma 5.8 as bug fixes, some changes are only available in master for the next release. With this blog post I want to highlight what we have improved since Plasma 5.8.

Resize only borders

KWin’s server side decorations have a feature that one can resize the window in the shadow area. With the Breeze window decoration this is available if one uses the border size “No Side Borders” or “No Borders”. For Wayland we just had to adjust the input area of a window slightly and honor it when evaluating the mouse pointer movements.

Global Shortcut handling

We found a few bugs related to global shortcut triggering. There is some unexpected behavior for shortcut triggering in xkbcommon, which will be addressed in the next release by adding new API. For now we had to workaround it to support some shortcuts which no longer triggered. Of course for every kind of shortcut which did not trigger we added a test case so we can also in future ensure that this works once the new xkbcommon release is available. At the moment we are not aware of any not working global shortcuts on Wayland. If you hit one, please report a bug.

Support for Keyboard LEDs through libinput

KWin did not enable the LEDs for num lock, caps lock, etc. This was mostly because I don’t have any keyboard which has such LEDs – neither my desktop keyboard nor my two notebooks have any LEDs. So I just didn’t notice that this was missing. Once we got the bug report we looked into adding this. I want to take this as an example of the “obvious bug” one doesn’t report because it’s so obvious. But if one doesn’t have such hardware it’s not so obvious any more.

Relative pointer support

A feature we added for Plasma 5.9 is support for the relative pointer protocol.

Relative Pointer Events

The protocol is implemented in KWayland 5.28 and KWin is adjusted to support the relative pointer events as can be seen in the screenshot of the input debug console. This is a rather important protocol to support games on Wayland. We also plan to add pointer confinement for Plasma 5.9.

Move windows through the widget style

Our widget styles Breeze and Oxygen have a feature to move the window when clicking in empty areas. This is a feature which needs to interact with the windowing system directly as Qt doesn’t provide an abstraction for it. On X11 it uses the NETRootInfo::moveResizeRequest, on Wayland support for triggering a window move is built into the core protocol. But so far we were not able to provide the feature on Wayland as we just didn’t have enough information from QtWayland. For example we lacked access to the wl_shell_surface on which we have to trigger the move. So some time ago I added support to QtWayland that we can access the wl_shell_surface through the native interface. Now about a year later we can start to use it. To support this feature we need to create an own wl_seat and wl_pointer object and track the serial of pointer button press. This we can then pass to the move request on the ShellSurface. The change is not KWin specific at all and will work on all Wayland compositors.

Color scheme sync to decoration

A new feature we added in KWin 5.0 is the possibility to synchronize the color scheme from the window into the window decoration and the context menu on the decoration. On X11 this works through a property which our KStyle library sets. This was the best we had back in the early days of the 5.x series as Qt didn’t expose enough information. It has the disadvantage that the sync only works with QWidget based applications and only with widget styles inheriting KStyle. For Plasma 5.9 we improved that and brought the relevant code into plasma-integration. The restriction to QWidget is gone and it works now with all kind of windows by listening to the QPlatformSurfaceEvent. This very useful event which got added in Qt 5.5. It informs us when a native window is created for a QWindow. Thus we can add our own X11 properties on the native window directly after creation and before the window is mapped.

Custom color scheme support

While adjusting this code for X11 we also added the relevant bits for Wayland. We use the Qt Surface Extension protocol to pass a property to the server. That’s a small and neat addition the Qt devs did to allow communication between a Qt based client and a Qt based Wayland compositor. As one can see in the screenshot the color scheme now updates also for Wayland applications.

Window icons

Window icon handling in Wayland is different to X11. On X11 the icons are passed as pixmaps. That has a few disadvantages nowadays because the icons provided on the window might not have a high enough resolution to work well on high-dpi systems. The icon from the icon-theme though provides higher resolution. On Wayland there is no way to pass window icons around and the compositor takes the icon from the desktop file of the application. This works well unless we don’t have a desktop file. For such windows we now started to use a generic Wayland icon as the fallback, just like we use a generic X icon as fallback for X11 windows which don’t have an icon.

Proper icons for X windows in task manager

That’s an icon which one might have noticed when using a Plasma Wayland session as every Xwayland window only had the generic X icon in the task manager. The communication between KWin and the task manager also passes the icon name around and not pixmap data. This works well for everything which isn’t Xwayland where we normally just don’t have the name. For Plasma 5.9 we addressed this problem and extended our protocol to request pixmap data for a window icon which doesn’t have a name. Thus we are now able to also support Xwayland windows, which increases the useability of the system quite a lot.

Multi screen effect improvements

On Wayland several of our effects broke in a multi-screen setup. This is because rendering is different. On X11 all screens are rendered together in one rendering pass and we have one OpenGL window to render to. On Wayland we have one OpenGL window per screen and have one rendering pass per screen. That’s something our effects didn’t handle well and resulted in rendering issues. For Plasma 5.9 these issues are finally resolved.

Wobbly windows

One of the affected effects is Wobbly windows. A rather important effect given that this blog is subtitled “From the land of wobbly windows”. We experienced that in a multi-screen setup the effect was only active on one screen. If the window got moved to the other screen it completely vanished.

I was quite certain that this is not a problem with the effect itself, but rather with the way how we render. As we also saw other effects having rendering issues in multi-screen setups I was quite optimistic that fixing wobbly would fix many effects.

The investigation showed that the problem in fact was an incorrect area passed to glScissor due to the general changes in rendering explained above. Rendering on other screens got clipped away. With the proper change we got wobbly working and several other effects (Present Windows, Desktop Grid, Alt+Tab for example) without having to touch the effects at all.

Screenshot

With that knowledge in place we looked into fixing other effects. E.g. the screenshot effect which allows to save a screenshot in the tmp directory. A few example of screenshots taken with this effect can be seen in this blog post. The problem with this effect was that when taking a fullscreen shot over all screens only one got captured. The assumption here was that our glBlitFramebuffer code needs adjustment to be per output and with that we can now screenshot every screen individually or all screens combined.

Multi-screen shot with blur

Blur and Background Contrast

Related to that are the blur and background contrast effect as they also interact with the frame buffer, though don’t use the glBlitFramebuffer extension. With those effects one of the biggest problems was that the viewport got restored to a wrong value after unbinding the frame buffer object. Due to that the rendering got screwed up and we had severe rendering issues with blur on multi screen. These issues are now fixed as can be seen in the screenshot above: both screens are rendered correctly even with blur enable.

Panel improvements

Plasma’s panel got some improvements for Plasma 5.9. This started from bug reports about windows can cover not working and also auto-hide not working. Another example that it is important to report bugs.

Auto hiding panel

On X11 auto hiding panels use a custom protocol with KWin to indicate that they want to be restored if the mouse cursor touches the screen edge. It uses low level X11 code thus we also need a low level Wayland protocol for it. We extended our plasma shell protocol to expose auto hiding state and implemented it in both KWin and Plasma.

Search in widget explorer

We had a bug report that search in the widget explorer doesn’t work. The investigation showed that the reason for that is that the widget explorer is a panel window and we designed panels on Wayland so that they don’t take any keyboard focus. This is correct for the normal panel, but not for this special panel. We adjusted our protocol to provide an additional hint that the panel takes focus and implemented this in kwayland-integration in a way that the widget explorer gains focus without any adjustments to it.

KRunner as a panel

Of course there are more potential users for this new feature. One being KRunner. Once we had the code in place we decided to make KRunner a Panel on Wayland which brings us quite some improvements like it will be above other windows and on all desktops.

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To EGLStream or not

The announcement of KDE Neon dev/unstable switching to Wayland by default raised quite a few worried comments as NVIDIA’s proprietary driver is not supported. One thing should be clear: we won’t break any setups. We will make sure that X11 is selected by default if the given hardware setup does not support Wayland. Nevertheless I think that the amount of questions show that I should discuss this in more detail.

NVIDIA does support Wayland – kind-of. The solution they came up with is not compatible to any existing Wayland compositor and requires patches to make it work. For the reference implementation Weston there are patches provided by NVIDIA, but those have not been integrated yet. For KWin such patches do not exist and we have no plans to develop such an adaption as long as the patches are not merged into Weston. Even if there would be patches, we would not merge them as long as they are not merged into Weston.

The solution NVIDIA came up with requires different code paths. This is unfortunate as it would require driver specific adjustments and driver specific code paths. This is bad for everybody involved. For us developers, for the driver developers and most importantly for our users. It means that we developers have to spend time on implementing and maintaining a solution for one driver – time which could be spent on fixing bugs instead. We could do such an effort for one driver, but once it goes to every driver requiring adjustment it gets not manageable.

But also adjustments for one driver are problematic. The latest NVIDIA driver caused a regression in KWin. On Quadro hardware (other hardware seems to be not affected) our shader self test fails which results in compositing disabled. If one removes the shader self test everything works fine, though. I assume that there is a bug in KWin’s rendering of the self test which is triggered only with this driver. But as I don’t have such hardware I cannot verify. Yes, I did pass multiple patches for investigating and trying to fix it to a colleague with such hardware. No, please don’t donate me hardware.

In the end, after spending more than half a day on it, we had to do the worst option which is to add a driver and hardware specific check to disable the self test and ship it with the 5.7.5 release. It’s super problematic for the code maintainability to add such checks. We are hiding a bug and we cannot investigate it. We are now stuck with an implementation where we will never be able to say “we can remove that again”. Driver specific workarounds tend to stick around. E.g. we have such a check:

// Broken on Intel chips with Mesa 9.1 - BUG 313613
if (gl->driver() == Driver_Intel && gl->mesaVersion() >= kVersionNumber(9, 1) && gl->mesaVersion() < kVersionNumber(9, 2))
    return;

It's nowadays absolutely pointless to have such code around as nobody is using such a Mesa version. But the code is still there, makes it more complex and has a maintenance cost. This is why driver specific implementations are bad and is nothing we want in our code base.

People asked to be pragmatic, because NVIDIA is so important. I am absolutely pragmatic here: we don't have the resources to develop and maintain an NVIDIA specific implementation on Wayland.

Also some people complained that this is unfair because we do have an implementation for (proprietary) Android drivers. I need to point out that this does not compare at all.

First of all our Android implementation is not specific for a proprietary driver. It is written for the open source hwcomposer interface exposed through libhybris. All of that is open source. The fact that the actual driver might be proprietary is nothing we like, but also not relevant for our implementation.

In addition the implementation is encapsulated in a platform plugin and significantly reduced in functionality (only one screen, no cursor, etc.). This is something we would not be able to do for NVIDIA (you would want multi-screen, right?).

For NVIDIA we would have to add a deviation into the DRM platform plugin to create the OpenGL context in a different way. This is something our architecture does not support and was not created for. The general idea is that if creating the GBM based context fails, KWin will terminate. Adding support there for a different way to get an OpenGL context up and running would result in lots of added complexity in a very important code path. We have to ensure that KWin terminates if OpenGL fails. At the same time we have to make sure that llvmpipe is not picked if NVIDIA hardware is used. This would be a horrible mess to maintain - especially if developers are not able to test this without huge effort.

From what I understand from the patch set it would also require to significantly change the presenting of a frame on an output and by that also turn our lower level code more complex. This code is currently able to serve both our OpenGL and our QPainter based compositor, but wouldn't allow to support NVIDIA's implementation. Adding changes there would hinder us in future development of the platform plugin. This is an important area we are working on and KWin 5.8 contains a new additional implementation making use of atomic mode settings. We want to have atomic mode settings used everywhere in the stack to have every frame perfect. NVIDIA's implementation would make that difficult.

EGLStreams bring another disadvantage as the code to bind a buffer (what a window renders) to a texture (what the compositor needs to render) would need changes. Binding the buffer is currently performed by KWin core and not part of the plugin infrastructure. Given that new additional code would also be needed there. We don't need that for any other platform we currently support. E.g. for hwcomposer on Andrid libhybris takes care of allowing us to use EGL the same way as on any other platform. I absolutely do not understand why changes would be needed there. Existing code shows that it can be done differently. And here we see again why I think the situation with EGLStream does not compare at all to supporting hwcomposer.

Overall we are not thrilled by the prospect of two competing implementations. We do hope that at XDC the discussions will have a positive end and that there will be only one implementation. I don't care which one, I don't care whether one is better as the other. What I care about is only requiring one code path, the possibility to test with free drivers (Mesa) and the support for atomic mode settings. Ideally I would also prefer to not have to adjust existing code.

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A circle closes…

At Desktop Summit 2011 in Berlin I did my first presentation on Wayland and presented the idea of Wayland to the KDE community and explained to the KDE community how we are going to port to Wayland. This year at QtCon in Berlin I was finally able to tell the KDE community that the port is finished and that our code is ready for testing.

In 2011 I used a half hour slot to mostly present the differences between X11 and Wayland and why we want Wayland. In addition I presented some of the to be expected porting steps and what we will have in the end. This year I only used a 10 min lightning talk slot to give the community an update on the work done the last year.

(Watch video on youtube, my talk starts at 15:04)

Of course the work on Wayland is not yet finished and Wayland is not yet fully ready for use. There are missing features and there must be bugs (new code base, etc.). But we are in a state to start the public beta.

What is interesting is comparing the slides from 2011 to what we have achieved. The plan presented there is to introduce “Window Manager Backends” in KWin. We wanted to identify windowing system independent areas and make our two most important classes Toplevel and Workspace X11 free and add a window manager abstraction. During the port this wasn’t really an aim, nevertheless we got there. We do have a window manager abstraction which would allow to add support for further windowing systems. Toplevel is (at runtime) X free. Workspace, though, is not yet X free, but that moved on my todo list.

Also we thoughts back in 2011 that this might be interesting for other platforms naming Android, WebOS and Microsoft Windows as examples. Android we kind of achieved by having support for Android’s hwcomposer and being able to run Wayland on top of an Android stack. Support for Android’s surfaceflinger is something we do not aim for. The example of WebOS doesn’t really fit any more as WebOS uses Wayland nowadays. And Windows is only in the area of theoretically possible (though with the new Linux support it would be interesting to try to get KWin running on it).

KWin nowadays has a platform abstraction and multiple platform plugins. This allows us to start a Wayland compositor on various software stacks. Currently we support:

  • DRM
  • fbdev
  • hwcomposer (through libhybris)
  • Wayland (nested)
  • X11 (nested)
  • virtual

Adding support for a new platform is quite straight forward and doesn’t need a lot of code. The main tasks of a Platform is to create the OpenGL context for the compositor and to present each frame on the Platform specific output. All platforms together are less than 10000 lines of code (cloc) and a single platform is around 400-3000 lines of code.

In order to add support for a new windowing system more work would be needed. It is very difficult to estimate how much code would be needed as it all depends on how well the concept can be mapped to Wayland. Ideally adding support for a new windowing system would be done by creating an external application which maps the windowing system to Wayland. Just like XWayland maps X11 to Wayland. But as we can see with XWayland this might not be enough. KWin also needs to be an X11 window manager to fully support X11 applications. Given that it really depends on the windowing system how much work is needed.

One could also add a new windowing system the same way as we added support for Wayland. This would require to implement our AbstractClient to have a representation for a managed window of the windowing system and add support for creating a texture from the window content. In addition various places in KWin need to be adjusted to also consider these windows. Not a trivial task and going through a mapping to Wayland is always the better solution. But still it’s possible and this makes KWin future proof for possible other windowing systems. In general KWin doesn’t care any more about the windowing system of a window. We can have X11 windows on Wayland and Wayland windows on X11 (only experimental branch, not yet merged).

This brings me back to my presentation from 2011. Back then we expected to have three phases of development. The first phase adding Wayland support to the existing X11 base. That was what we experimented with back then and as I just wrote still experiment with it. As it turned out that was not the proper approach for development.

As a second phase we expected to remove X and have a Wayland only system. At the moment we still require XWayland to start KWin/Wayland. During the development it showed that this is not something really needed. It was easier to move the existing X11 code to interact through XWayland – we could keep the X code and move faster.

wayland-architecture-rootless-x.png

The third and final phase was about adding back XWayland support, so that KWin can support both X11 and Wayland windows. That’s the phase we developed directly. Which is kind of interesting that we went to the final step although we thought we need easier intermediate steps.

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KDE Neon dev/unstable switching to Wayland by default

During this year’s Akademy we had a few discussions about Wayland, and the Plasma and Neon team decided to switch Neon developer unstable edition to Wayland by default soonish.

There are still a few things in the stack which need to be shaken out – we need a newer Xwayland in Neon, we want to wait for Plasma 5.8 to be released, we need to get the latest QtWayland 5.7 build, etc. etc.

This is really exciting. It’s probably the biggest step towards Wayland by default the KDE community has ever taken. I hope that other continuous delivery systems will follow so that we can get many enthusiastic users to try Wayland.

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Creating a Photo-Box with the help of KWin

For a family celebration I wanted to create a “Photo-Box” or “Selfie-Box”: a place where the guests can trigger a photo of themselves without having to use tools like a selfie-stick.

The requirements for the setup were:

  • Trigger should be remote controlled
  • The remote control should not be visible or at max hardly visible
  • The guests should see themselves before taking the photo
  • All already taken photos should be presented in a slide show to the guests

The camera in question supported some but not all of the requirements. Especially the last two were tricky. While it supported showing a slide show of all taken photos, the slide show ended as soon as a new photo was taken. But the camera also has an usb-connector so the whole power of a computer could be taken in.

A short investigation showed that gphoto2 could be the solution. It allows to completely remote control the camera and download photos. With that all requirements can be fulfilled. But by using a computer a new requirement got added: the screen should be locked.

This requirement created a challenge. As the maintainer of Plasma’s lock screen infrastructure I know what it is good at and that is blocking input and preventing other applications to be visible. Thus we cannot just use e.g. Digikam with gphoto2 integration to take the photo – the lock screen would prevent the Digikam window to be visible. Also there is no way to have a slide show in the lock screen.

Which means all requirements must be fulfilled through the lock screen infrastructure. A result of that is that I spent some time on adding wallpaper plugin support to the lock screen. This allowed to reuse Plasma’s wallpaper plugins and thus also the slide show plugin. One problem solved and all Plasma users can benefit from it.

But how to trigger gphoto2? The idea I came up with is using KWin/Wayland. KWin has full control over the input stack (even before the lock screen intercepts) and also knows which input devices it is interacting with. As a remote control I decided to use a Logitech Presenter and accept any clicked button on that device as the trigger. The code looks like the following:

class PhotoBoxFilter : public InputEventFilter {
public:
    PhotoBoxFilter()
        : InputEventFilter()
        , m_tetheredProcess(new Process)
    {
        m_tetheredProcess->setProcessChannelMode(QProcess::ForwardedErrorChannel);
        m_tetheredProcess->setWorkingDirectory(QStringLiteral("/path/to/storage"));
        m_tetheredProcess->setArguments(QStringList{
                                                   QStringLiteral("--set-config"),
                                                   QStringLiteral("output=TFT"),
                                                   QStringLiteral("--capture-image-and-download"),
                                                   QStringLiteral("--filename"),
                                                   QStringLiteral("%Y%m%d-%H%M%S.jpg"),
                                                   QStringLiteral("-I"),
                                                   QStringLiteral("-1"),
                                                   QStringLiteral("--reset-interval")
        });
        m_tetheredProcess->setProgram(QStringLiteral("gphoto2"));
    }
    virtual ~PhotoBoxFilter() {
        m_tetheredProcess->terminate();
        m_tetheredProcess->waitForFinished();
    }
    bool keyEvent(QKeyEvent *event) override {
        if (!waylandServer()->isScreenLocked()) {
            return false;
        }
        auto key = static_cast(event);
        if (key->device() && key->device()->vendor() == 1133u && key->device()->product() == 50453u) {
            if (event->type() == QEvent::KeyRelease) {
                if (m_tetheredProcess->state() != QProcess::Running) {
                    m_tetheredProcess->start();
                    m_tetheredProcess->waitForStarted(5000);
                } else {
                    ::kill(m_tetheredProcess->pid(), SIGUSR1);
                }
            }
            return true;
        }
        return false;
    }

private:
    QScopedPointer m_tetheredProcess;
};

And in addition the method InputRedirection::setupInputFilters needed an adjustment to install this new InputFilter just before installing the LockScreenFilter.

photobox

The final setup:

  • Camera on tripod
  • Connected to an external screen showing the live capture
  • Connected to a notebook through USB
  • Notebook connected to an external TV
  • Notebook locked and lock screen configured to show slide show of photos
  • Logitech Presenter used as remote control

The last detail which needed adjustments was on the lock screen theme. The text input is destroying the experience of the slide show. Thus a small hack to the QML code was needed to hide it and reveal again after pointer motion.

What I want to show with this blog post is one of the advantage of open source software: you can adjust the software to your needs and turn it into something completely different which fits your needs.

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Why does kwin_wayland not start?

From time to time I get contacted because kwin_wayland or startplasmacompositor doesn’t work. With this blog post I want to show some of the most common problems and how to diagnose correctly what’s going wrong.

First test nested setup

If you want to try Wayland please always first try the nested setup. This is less complex and if things go wrong easier to diagnose than a maybe frozen tty. So start your normal X session and run a nested KWin:
export $(dbus-launch)
kwin_wayland --xwayland

This should create a black window. If it works you can send windows there, e.g.:
kwrite --platform wayland

When things go wrong

Xwayland missing

KWin terminates and you get the error message:

FATAL ERROR: failed to start Xwayland

This means you don’t have Xwayland installed. It’s a runtime dependency of KWin. Please get in contact with your distribution, they need to fix the packaging 😉

Application doesn’t start

KWin starts fine but the application doesn’t show because of an error message like:

This application failed to start because it could not find or load the Qt platform plugin “wayland”in “”.

Available platform plugins are: wayland-org.kde.kwin.qpa, eglfs, linuxfb, minimal, minimalegl, offscreen, xcb.

Reinstalling the application may fix this problem.

This means QtWayland is not installed. Please install it and try again.

Platform plugin missing

KWin terminates and you get the error message:

FATAL ERROR: could not find a backend

This means that the platform/backend plugin is not installed. KWin supports multiple platforms and distributions put them in multiple packages. For X11 you need e.g. on Debian based systems the package kwin-wayland-backend-x11. For the “real thing” you need: kwin-wayland-backend-drm.

XDG_RUNTIME_DIR not set

KWin terminates and you get the error message:

FATAL ERROR: could not create Wayland server

This means that KWin failed to create the Wayland server socket. The most common reason for this is that your environment does not contain the XDG_RUNTIME_DIR environment variable. This should be created and set by your login system. Please get in contact with your distribution. The debug output should also say something about XDG_RUNTIME_DIR.

Platform fails to load

KWin terminates and you get the error message:

FATAL ERROR: could not instantiate a backend

You hit the jackpot. Something somewhere went horribly wrong. Please activate all KWin related debug categories, run it again and report a bug against KWin – ideally for the platform you used. The debug output hopefully contains more information on why it failed. If not we need to try to look into your specific setup.

Trying on the tty

KWin works fine in the nested setup: awesome. In most cases this means that KWin will also work on the DRM device. Before trying: make sure that you don’t use the NVIDIA proprietary driver, if you do: sorry that’s not yet supported. If you are on Mesa drivers everything should be fine.

Log in to a tty and setup similar to nested setup – I recommend the exit-with-session command line option to have a nice defined setup to exit again:

export $(dbus-launch)
export QT_QPA_PLATFORM=wayland
kwin_wayland --xwayland --exit-with-session=kwrite

Ideally this should turn your screen black, put the mouse cursor into the center of the screen and after a short time show kwrite. You should be able to interact with it and when closing kwrite KWin should terminate. If that all works you are ready to run startplasmacompositor. It will work.

But what if not. This is the tricky situation. The most often problem I have heard is that KWin freezes at this point and gdb shows KWin is in the main event loop. This most likely means that KWin tries to interact with logind DBus API, but your system does not provide this DBus interface. Please get in contact with your distribution on how to get the logind DBus interface installed (this does neither require using systemd nor logind). I would like to handle this better, but I don’t have a system without logind to test. Patches welcome.

Running KWin on the weird systems

Yes one can go crazy and try running KWin on devices like the Nexus 5, Virtual Machines or NVIDIA based systems. Here the experience differs and I myself don’t know exactly what is supported on which hardware and in which setting combination. Best get in contact with us to check what works and if you are interested: please help in adding support for it.

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Animating auto-hiding panels

With Plasma 5 a change regarding auto-hiding panels was introduced. The complete interaction was moved from Plasma to KWin. This was an idea which we had in mind for a long time. The main idea is to have only one process to reserve the interaction with the screen edges (which is needed to show the panel when hidden) and to prevent conflicts there. Also it allowed to have only one place for providing the hint that there is the panel.

On an implementation level that uses an x11-property protocol between KWin and Plasma to indicate when the panel should be hidden. KWin then does the interaction to hide it and to show on screen edge activation.

Unfortunately from a visual perspective this created a regression. In Plasma 4 the auto-hiding panel was animated with our Sliding-Popups effect, but in Plasma 5 this doesn’t work any more. The reason for that is that our effect system can only animate when a window gets mapped and unmapped. With the new protocol the window doesn’t get unmapped when hidden and not mapped when shown, so our effects are not able to react on it. Technically our Client object is kept instead of being released on unmapped.

Thanks to Wayland this will be working again starting with Plasma 5.8. For Wayland windows KWin keeps the object around when a window gets unmapped and uses the same object when it gets shown again. KWin keeps more state for Wayland windows than for X11 windows. But this also means that our animations are not working for Wayland windows. Last week I addressed this and extended the internal effects API to also support hiding/showing again of Wayland windows. As the effects API does not differentiate between Wayland and X11 windows this change also enables the auto-hiding panel animation. All that was needed was emitting two signals at the right place.

Here Wayland shows another strength: not only does it help to bring features to X11 it also allows us to automate the testing. With a pure X11 system we would have had a hard time to properly auto-test this. But for Wayland we have a nice isolated integration test-framework which allowed to add a test-case for auto-hiding panels.

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Synchronizing the X11 and Wayland clipboard

One of the greatest annoyances after switching my work systems to Wayland is the lack of clipboard synchronization between the X11 and Wayland windowing system. X11 clipboard is only available to X11 windows and the Wayland clipboard is only available to Wayland windows. It’s the useability problem of the two X11 clipboard (primary and secondary) just exposed on a much higher and more annoying way. Instead of “just” remembering whether you copied or selected text, you need to remember which windowing system you copied from and which windowing system the current window is in. And if they are not in the same, there is just no way.

So getting this problem resolved got quite a priority. It was something I had never really spent any thought on and incorrectly assumed it’s something Xwayland would take care of. But after thinking about the problem I realized that the X11 clipboard is part of ICCCM and not a core feature of the X-Server. It’s a communication protocol between clients.

This means that KWin had to learn to communicate with the X11 clipboard in order to provide the synchronization. Not an easy task as ICCCM is quite complex on this topic and implementing an X11 clipboard is nothing I’m looking forward to. Luckily I remembered that we have a clipboard manager for X11 (klipper) and that this one works without having X11 specific code thanks to Qt’s abstraction.

And this makes the whole thing way easier: by using Qt’s X11 clipboard implementation we only need to interact with Wayland’s clipboard which is thanks to KWayland quite straight forward. The solution involves a small helper binary which gets started by KWin. It’s forced to the xcb windowing system to be able to use Qt’s X11 clipboard and manually connects with KWin through KWayland. The task of the binary is quite simple: whenever the X11 clipboard changes it gets the data and sets it on the Wayland clipboard. Similar whenever the Wayland clipboard changes it gets set on the X11 clipboard.

Unlike the X11 clipboard on Wayland clients are normally not informed about clipboard changes. Only the application currently having keyboard focus gets informed by the Wayland server about what kind of data is available on the clipboard. This means we don’t need to inform all Wayland clients about changes in the X11 clipboard and similar we don’t need to inform X11 about clipboard changes in Wayland all the time. Instead KWin helps the process: when an X11 window gains keyboard focus our helper process gets informed about the current Wayland clipboard content, so that it can be available for the focused X11 window and while X11 windows are focused clipboard changes from the helper application are forwarded to Wayland. So when the next Wayland window gets focused it has the clipboard content set on X11. A small and simple solution which works quite well in practice. At the moment only the “real” clipboard is synchronized. There is not yet the concept of the selection based clipboard on Wayland. A solution might be to just synchronize it to the normal clipboard. But this would again introduce the useability problems of this non-explicit clipboard. So I’m not yet convinced what’s the proper way.

In my last blog post I pointed to our current donation campaign to support the Randa developer sprint. A lot of readers donated after reading this. I want to thank everybody who donated. It’s so important for a free software community to meet in person and we need donations for getting developers to sprints and conferences. Last week I was at openSUSE conference and had a talk about how Wayland improves the quality of Plasma. These are important things to talk about at a conference, but it’s only possible if we have the donations to send people there. Please help to make such developer sprints possible. Thank you!