[wayland] Add support for a "software" cursor

At least the framebuffer backend does not have support for an overlay
cursor. Thus the cursor needs to be rendered by the scene. This change
allows a backend to set that it needs a software cursor which triggers
tracking in the AbstractBackend. A repaint for the old cursor region is
triggered whenever the cursor pos changes.

So far only the QPainter/framebuffer scene is adjusted to render the
software cursor. This is done after rendering a frame with the up to
date cursor position.

There is one problem, though: the KWin internal cursors don't work
as we need to get it from the theme. Using wayland-cursor doesn't help
as it gives us a (client) wl_buffer* and we cannot read the memory back.

abstract_backend.cpp

@@ -18,7 +18,13 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *********************************************************************/
 #include "abstract_backend.h"
+#include "composite.h"
+#include "cursor.h"
 #include "wayland_server.h"
+// KWayland
+#include <KWayland/Server/buffer_interface.h>
+#include <KWayland/Server/seat_interface.h>
+#include <KWayland/Server/surface_interface.h>
 
 namespace KWin
 {
@@ -36,6 +42,27 @@ AbstractBackend::~AbstractBackend()
 
 void AbstractBackend::installCursorFromServer()
 {
+    if (!m_softWareCursor) {
+        return;
+    }
+    if (!waylandServer() || !waylandServer()->seat()->focusedPointer()) {
+        return;
+    }
+    auto c = waylandServer()->seat()->focusedPointer()->cursor();
+    if (!c) {
+        return;
+    }
+    auto cursorSurface = c->surface();
+    if (cursorSurface.isNull()) {
+        return;
+    }
+    auto buffer = cursorSurface.data()->buffer();
+    if (!buffer) {
+        return;
+    }
+    triggerCursorRepaint();
+    m_cursor.hotspot = c->hotspot();
+    m_cursor.image = buffer->data().copy();
 }
 
 void AbstractBackend::installCursorImage(Qt::CursorShape shape)
@@ -59,4 +86,32 @@ QPainterBackend *AbstractBackend::createQPainterBackend()
     return nullptr;
 }
 
+void AbstractBackend::setSoftWareCursor(bool set)
+{
+    if (m_softWareCursor == set) {
+        return;
+    }
+    m_softWareCursor = set;
+    if (m_softWareCursor) {
+        connect(Cursor::self(), &Cursor::posChanged, this, &AbstractBackend::triggerCursorRepaint);
+    } else {
+        disconnect(Cursor::self(), &Cursor::posChanged, this, &AbstractBackend::triggerCursorRepaint);
+    }
+}
+
+void AbstractBackend::triggerCursorRepaint()
+{
+    if (!Compositor::self() || m_cursor.image.isNull()) {
+        return;
+    }
+    Compositor::self()->addRepaint(m_cursor.lastRenderedPosition.x() - m_cursor.hotspot.x(),
+                                   m_cursor.lastRenderedPosition.y() - m_cursor.hotspot.y(),
+                                   m_cursor.image.width(), m_cursor.image.height());
+}
+
+void AbstractBackend::markCursorAsRendered()
+{
+    m_cursor.lastRenderedPosition = Cursor::pos();
+}
+
 }

abstract_backend.h

@@ -20,6 +20,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #ifndef KWIN_ABSTRACT_BACKEND_H
 #define KWIN_ABSTRACT_BACKEND_H
 #include <kwin_export.h>
+#include <QImage>
 #include <QObject>
 
 namespace KWin
@@ -41,8 +42,29 @@ public:
     virtual OpenGLBackend *createOpenGLBackend();
     virtual QPainterBackend *createQPainterBackend();
 
+    bool usesSoftwareCursor() const {
+        return m_softWareCursor;
+    }
+    QImage softwareCursor() const {
+        return m_cursor.image;
+    }
+    QPoint softwareCursorHotspot() const {
+        return m_cursor.hotspot;
+    }
+    void markCursorAsRendered();
+
 protected:
     explicit AbstractBackend(QObject *parent = nullptr);
+    void setSoftWareCursor(bool set);
+
+private:
+    void triggerCursorRepaint();
+    bool m_softWareCursor = false;
+    struct {
+        QPoint hotspot;
+        QImage image;
+        QPoint lastRenderedPosition;
+    } m_cursor;
 };
 
 }

fb_backend.cpp

@@ -39,6 +39,7 @@ namespace KWin
 FramebufferBackend::FramebufferBackend(QObject *parent)
     : AbstractBackend(parent)
 {
+    setSoftWareCursor(true);
 }
 
 FramebufferBackend::~FramebufferBackend()

scene_qpainter.cpp

@@ -21,6 +21,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 // KWin
 #include "client.h"
 #include "composite.h"
+#include "cursor.h"
 #include "deleted.h"
 #include "effects.h"
 #include "main.h"
@@ -79,6 +80,11 @@ void QPainterBackend::setFailed(const QString &reason)
     m_failed = true;
 }
 
+void QPainterBackend::renderCursor(QPainter *painter)
+{
+    Q_UNUSED(painter)
+}
+
 #if HAVE_WAYLAND
 //****************************************
 // WaylandQPainterBackend
@@ -311,6 +317,21 @@ bool FramebufferQPainterBackend::usesOverlayWindow() const
     return false;
 }
 
+void FramebufferQPainterBackend::renderCursor(QPainter *painter)
+{
+    if (!m_backend->usesSoftwareCursor()) {
+        return;
+    }
+    const QImage img = m_backend->softwareCursor();
+    if (img.isNull()) {
+        return;
+    }
+    const QPoint cursorPos = Cursor::pos();
+    const QPoint hotspot = m_backend->softwareCursorHotspot();
+    painter->drawImage(cursorPos - hotspot, img);
+    m_backend->markCursorAsRendered();
+}
+
 #endif
 
 //****************************************
@@ -380,6 +401,7 @@ qint64 SceneQPainter::paint(QRegion damage, ToplevelList toplevels)
     }
     QRegion updateRegion, validRegion;
     paintScreen(&mask, damage, QRegion(), &updateRegion, &validRegion);
+    m_backend->renderCursor(m_painter.data());
 
     m_backend->showOverlay();
 

scene_qpainter.h

@@ -93,6 +93,7 @@ public:
 
     virtual QImage *buffer() = 0;
     virtual bool needsFullRepaint() const = 0;
+    virtual void renderCursor(QPainter *painter);
 
 protected:
     QPainterBackend();
@@ -166,6 +167,7 @@ public:
     bool usesOverlayWindow() const override;
     void prepareRenderingFrame() override;
     void present(int mask, const QRegion &damage) override;
+    void renderCursor(QPainter *painter) override;
 
 private:
     QImage m_renderBuffer;