plugins: add effect that shows when compositing is active

It just shows "Compositing" in the corner and doesn't block direct scanout.
When direct scanout is active, the "Compositing" text isn't visible anymore.

This is useful for debugging performance or power draw issues.

src/plugins/CMakeLists.txt

@@ -91,6 +91,7 @@ add_subdirectory(screentransform)
 add_subdirectory(sessionquit)
 add_subdirectory(shakecursor)
 add_subdirectory(sheet)
+add_subdirectory(showcompositing)
 add_subdirectory(showfps)
 add_subdirectory(showpaint)
 add_subdirectory(slide)

src/plugins/showcompositing/CMakeLists.txt

@@ -0,0 +1,19 @@
+# SPDX-FileCopyrightText: 2022 Arjen Hiemstra <ahiemstra@heimr.nl>
+# SPDX-FileCopyrightText: 2024 Xaver Hugl <xaver.hugl@gmail.com>
+#
+# SPDX-License-Identifier: BSD-3-Clause
+
+set(showcompositing_SOURCES
+    main.cpp
+    showcompositing.cpp
+)
+
+kwin_add_builtin_effect(showcompositing ${showcompositing_SOURCES})
+
+target_link_libraries(showcompositing PRIVATE
+    kwin
+    KF6::I18n
+    Qt::Quick
+)
+
+install(DIRECTORY qml DESTINATION ${KDE_INSTALL_DATADIR}/kwin/effects/showcompositing)

src/plugins/showcompositing/main.cpp

@@ -0,0 +1,18 @@
+/*
+    SPDX-FileCopyrightText: 2021 Vlad Zahorodnii <vlad.zahorodnii@kde.org>
+
+    SPDX-License-Identifier: GPL-2.0-or-later
+*/
+
+#include "showcompositing.h"
+
+namespace KWin
+{
+
+KWIN_EFFECT_FACTORY_SUPPORTED(ShowCompositingEffect,
+                              "metadata.json.stripped",
+                              return ShowCompositingEffect::supported();)
+
+} // namespace KWin
+
+#include "main.moc"

src/plugins/showcompositing/metadata.json

@@ -0,0 +1,9 @@
+{
+    "KPlugin": {
+        "Category": "Tools",
+        "Description": "Display if compositing is active",
+        "EnabledByDefault": false,
+        "License": "GPL",
+        "Name": "Show Compositing"
+    }
+}

src/plugins/showcompositing/qml/main.qml

@@ -0,0 +1,15 @@
+/*
+    SPDX-FileCopyrightText: 2022 Arjen Hiemstra <ahiemstra@heimr.nl>
+    SPDX-FileCopyrightText: 2024 Xaver Hugl <xaver.hugl@gmail.com>
+
+    SPDX-License-Identifier: GPL-2.0-or-later
+*/
+
+import QtQuick
+
+Text {
+    color: Qt.rgba(1.0, 0.0, 0.0, 1.0)
+    text: i18nc("@label", "Compositing")
+    fontSizeMode: Text.Fit
+    font.pointSize: 100
+}

src/plugins/showcompositing/showcompositing.cpp

@@ -0,0 +1,55 @@
+/*
+    SPDX-FileCopyrightText: 2006 Lubos Lunak <l.lunak@kde.org>
+    SPDX-FileCopyrightText: 2021 Vlad Zahorodnii <vlad.zahorodnii@kde.org>
+    SPDX-FileCopyrightText: 2022 Arjen Hiemstra <ahiemstra@heimr.nl>
+    SPDX-FileCopyrightText: 2024 Xaver Hugl <xaver.hugl@gmail.com>
+
+    SPDX-License-Identifier: GPL-2.0-or-later
+*/
+
+#include "showcompositing.h"
+#include "core/output.h"
+#include "core/renderviewport.h"
+#include "effect/effecthandler.h"
+
+namespace KWin
+{
+
+ShowCompositingEffect::ShowCompositingEffect()
+{
+}
+
+ShowCompositingEffect::~ShowCompositingEffect() = default;
+
+void ShowCompositingEffect::prePaintScreen(ScreenPrePaintData &data, std::chrono::milliseconds presentTime)
+{
+    effects->prePaintScreen(data, presentTime);
+    if (!m_scene) {
+        m_scene = std::make_unique<OffscreenQuickScene>();
+        m_scene->setSource(QUrl::fromLocalFile(QStandardPaths::locate(QStandardPaths::GenericDataLocation, QStringLiteral("kwin/effects/showcompositing/qml/main.qml"))));
+    }
+}
+
+void ShowCompositingEffect::paintScreen(const RenderTarget &renderTarget, const RenderViewport &viewport, int mask, const QRegion &region, Output *screen)
+{
+    effects->paintScreen(renderTarget, viewport, mask, region, screen);
+    const auto rect = viewport.renderRect();
+    m_scene->setGeometry(QRect(rect.x() + rect.width() - 300, 0, 300, 150));
+    effects->renderOffscreenQuickView(renderTarget, viewport, m_scene.get());
+}
+
+bool ShowCompositingEffect::supported()
+{
+    return effects->isOpenGLCompositing();
+}
+
+bool ShowCompositingEffect::blocksDirectScanout() const
+{
+    // this is intentionally wrong, as we want direct scanout to change the image
+    // with this effect
+    return false;
+}
+
+} // namespace KWin
+
+#include "moc_showcompositing.cpp"

src/plugins/showcompositing/showcompositing.h

@@ -0,0 +1,35 @@
+/*
+    SPDX-FileCopyrightText: 2006 Lubos Lunak <l.lunak@kde.org>
+    SPDX-FileCopyrightText: 2021 Vlad Zahorodnii <vlad.zahorodnii@kde.org>
+    SPDX-FileCopyrightText: 2022 Arjen Hiemstra <ahiemstra@heimr.nl>
+    SPDX-FileCopyrightText: 2024 Xaver Hugl <xaver.hugl@gmail.com>
+
+    SPDX-License-Identifier: GPL-2.0-or-later
+*/
+
+#pragma once
+
+#include "effect/effect.h"
+#include "effect/offscreenquickview.h"
+
+namespace KWin
+{
+
+class ShowCompositingEffect : public Effect
+{
+    Q_OBJECT
+public:
+    ShowCompositingEffect();
+    ~ShowCompositingEffect() override;
+
+    void prePaintScreen(ScreenPrePaintData &data, std::chrono::milliseconds presentTime) override;
+    void paintScreen(const RenderTarget &renderTarget, const RenderViewport &viewport, int mask, const QRegion &region, Output *screen) override;
+    bool blocksDirectScanout() const override;
+
+    static bool supported();
+
+private:
+    std::unique_ptr<OffscreenQuickScene> m_scene;
+};
+
+} // namespace KWin