Clip QOpenGLWidget and QQuickWidget correctly

Introduce support for the widgets' clipRect(). Right now render-to-texture widgets
in scroll areas placed close to each other result in broken (non-existent) clipping.
Similarly, stack-on-top widgets fail to clip when placed inside a scroll area.

This is now corrected and the qopenglwidget example is enhanced to utilize a scroll
area.

Task-number: QTBUG-45860
Change-Id: I859a63d61a50d64ba9e87244f83c5969dce12337
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>

8 files changed, 99 insertions, 29 deletions

diff --git a/examples/opengl/qopenglwidget/glwidget.cpp b/examples/opengl/qopenglwidget/glwidget.cpp
index f93e667615..df52e3efbf 100644
--- a/examples/opengl/qopenglwidget/glwidget.cpp
+++ b/examples/opengl/qopenglwidget/glwidget.cpp
@@ -537,14 +537,14 @@ void GLWidget::setTransparent(bool transparent)
     window()->update();
 }
 
-void GLWidget::resizeGL(int w, int h)
+void GLWidget::resizeGL(int, int)
 {
     if (m_hasButton) {
         if (!m_btn) {
-            m_btn = new QPushButton("A widget on top.\nPress me!", this);
+            m_btn = new QPushButton("A widget on top.\nPress for more widgets.", this);
             connect(m_btn, &QPushButton::clicked, this, &GLWidget::handleButtonPress);
         }
-        m_btn->move(w / 2, h / 2);
+        m_btn->move(20, 80);
     }
 }
 
diff --git a/examples/opengl/qopenglwidget/mainwindow.cpp b/examples/opengl/qopenglwidget/mainwindow.cpp
index 6cf1d5d6e2..22111afdcb 100644
--- a/examples/opengl/qopenglwidget/mainwindow.cpp
+++ b/examples/opengl/qopenglwidget/mainwindow.cpp
@@ -47,6 +47,7 @@
 #include <QLabel>
 #include <QCheckBox>
 #include <QSpinBox>
+#include <QScrollArea>
 
 #include "glwidget.h"
 
@@ -69,7 +70,7 @@ MainWindow::MainWindow()
                                "and therefore an interval < 16 ms will likely lead to a 60 FPS update rate.");
     QGroupBox *updateGroupBox = new QGroupBox(this);
     QCheckBox *timerBased = new QCheckBox("Use timer", this);
-    timerBased->setChecked(true);
+    timerBased->setChecked(false);
     timerBased->setToolTip("Toggles using a timer to trigger update().\n"
                            "When not set, each paintGL() schedules the next update immediately,\n"
                            "expecting the blocking swap to throttle the thread.\n"
@@ -87,7 +88,7 @@ MainWindow::MainWindow()
     slider->setRange(0, 50);
     slider->setSliderPosition(30);
     m_timer->setInterval(10);
-    label->setText("A QOpenGLWidget");
+    label->setText("A scrollable QOpenGLWidget");
     label->setAlignment(Qt::AlignHCenter);
 
     QGroupBox * groupBox = new QGroupBox(this);
@@ -96,7 +97,10 @@ MainWindow::MainWindow()
 
     m_layout = new QGridLayout(groupBox);
 
-    m_layout->addWidget(glwidget,1,0,8,1);
+    QScrollArea *scrollArea = new QScrollArea;
+    scrollArea->setWidget(glwidget);
+
+    m_layout->addWidget(scrollArea,1,0,8,1);
     m_layout->addWidget(label,9,0,1,1);
     m_layout->addWidget(updateGroupBox, 10, 0, 1, 1);
     m_layout->addWidget(slider, 11,0,1,1);
@@ -134,7 +138,10 @@ MainWindow::MainWindow()
     connect(timerBased, &QCheckBox::toggled, this, &MainWindow::timerUsageChanged);
     connect(timerBased, &QCheckBox::toggled, updateInterval, &QWidget::setEnabled);
 
-    m_timer->start();
+    if (timerBased->isChecked())
+        m_timer->start();
+    else
+        updateInterval->setEnabled(false);
 }
 
 void MainWindow::updateIntervalChanged(int value)
@@ -170,3 +177,8 @@ void MainWindow::timerUsageChanged(bool enabled)
             w->update();
     }
 }
+
+void MainWindow::resizeEvent(QResizeEvent *)
+{
+     m_glWidgets[0]->setMinimumSize(size() + QSize(128, 128));
+}
diff --git a/examples/opengl/qopenglwidget/mainwindow.h b/examples/opengl/qopenglwidget/mainwindow.h
index f1b2c51e53..9ad8a01339 100644
--- a/examples/opengl/qopenglwidget/mainwindow.h
+++ b/examples/opengl/qopenglwidget/mainwindow.h
@@ -56,6 +56,8 @@ public:
     void addNew();
     bool timerEnabled() const { return m_timer->isActive(); }
 
+    void resizeEvent(QResizeEvent *);
+
 private slots:
     void updateIntervalChanged(int value);
     void timerUsageChanged(bool enabled);
diff --git a/src/gui/painting/qplatformbackingstore.cpp b/src/gui/painting/qplatformbackingstore.cpp
index 14a9429c71..83077f38ca 100644
--- a/src/gui/painting/qplatformbackingstore.cpp
+++ b/src/gui/painting/qplatformbackingstore.cpp
@@ -86,6 +86,7 @@ struct QBackingstoreTextureInfo
     QWidget *widget; // may be null
     GLuint textureId;
     QRect rect;
+    QRect clipRect;
     QPlatformTextureList::Flags flags;
 };
 
@@ -142,6 +143,12 @@ QRect QPlatformTextureList::geometry(int index) const
     return d->textures.at(index).rect;
 }
 
+QRect QPlatformTextureList::clipRect(int index) const
+{
+    Q_D(const QPlatformTextureList);
+    return d->textures.at(index).clipRect;
+}
+
 void QPlatformTextureList::lock(bool on)
 {
     Q_D(QPlatformTextureList);
@@ -157,13 +164,15 @@ bool QPlatformTextureList::isLocked() const
     return d->locked;
 }
 
-void QPlatformTextureList::appendTexture(QWidget *widget, GLuint textureId, const QRect &geometry, Flags flags)
+void QPlatformTextureList::appendTexture(QWidget *widget, GLuint textureId, const QRect &geometry,
+                                         const QRect &clipRect, Flags flags)
 {
     Q_D(QPlatformTextureList);
     QBackingstoreTextureInfo bi;
     bi.widget = widget;
     bi.textureId = textureId;
     bi.rect = geometry;
+    bi.clipRect = clipRect;
     bi.flags = flags;
     d->textures.append(bi);
 }
@@ -198,7 +207,7 @@ void QPlatformTextureList::clear()
 
 #ifndef QT_NO_OPENGL
 
-static QRect deviceRect(const QRect &rect, QWindow *window)
+static inline QRect deviceRect(const QRect &rect, QWindow *window)
 {
     QRect deviceRect(rect.topLeft() * window->devicePixelRatio(),
                      rect.size() * window->devicePixelRatio());
@@ -219,6 +228,32 @@ static QRegion deviceRegion(const QRegion &region, QWindow *window)
     return deviceRegion;
 }
 
+static inline QRect toBottomLeftRect(const QRect &topLeftRect, int windowHeight)
+{
+    return QRect(topLeftRect.x(), windowHeight - topLeftRect.bottomRight().y() - 1,
+                 topLeftRect.width(), topLeftRect.height());
+}
+
+static void blit(const QPlatformTextureList *textures, int idx, QWindow *window, const QRect &deviceWindowRect,
+                 QOpenGLTextureBlitter *blitter)
+{
+    const QRect rectInWindow = textures->geometry(idx);
+    QRect clipRect = textures->clipRect(idx);
+    if (clipRect.isEmpty())
+        clipRect = QRect(QPoint(0, 0), rectInWindow.size());
+    const QRect clippedRectInWindow = rectInWindow & clipRect.translated(rectInWindow.topLeft());
+    const QRect srcRect = toBottomLeftRect(clipRect, rectInWindow.height());
+
+    const QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(deviceRect(clippedRectInWindow, window),
+                                                                     deviceWindowRect);
+
+    const QMatrix3x3 source = QOpenGLTextureBlitter::sourceTransform(deviceRect(srcRect, window),
+                                                                     deviceRect(rectInWindow, window).size(),
+                                                                     QOpenGLTextureBlitter::OriginBottomLeft);
+
+    blitter->blit(textures->textureId(idx), target, source);
+}
+
 /*!
     Flushes the given \a region from the specified \a window onto the
     screen, and composes it with the specified \a textures.
@@ -254,15 +289,12 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
 
     d_ptr->blitter->bind();
 
-    QRect windowRect(QPoint(), window->size() * window->devicePixelRatio());
+    const QRect deviceWindowRect = deviceRect(QRect(QPoint(), window->size()), window);
 
     // Textures for renderToTexture widgets.
     for (int i = 0; i < textures->count(); ++i) {
-        if (!textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop)) {
-            QRect targetRect = deviceRect(textures->geometry(i), window);
-            QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(targetRect, windowRect);
-            d_ptr->blitter->blit(textures->textureId(i), target, QOpenGLTextureBlitter::OriginBottomLeft);
-        }
+        if (!textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop))
+            blit(textures, i, window, deviceWindowRect, d_ptr->blitter);
     }
 
     funcs->glEnable(GL_BLEND);
@@ -272,6 +304,7 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
     // semi-transparency even when it is not wanted.
     funcs->glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE);
 
+    // Backingstore texture with the normal widgets.
     GLuint textureId = 0;
     QOpenGLTextureBlitter::Origin origin = QOpenGLTextureBlitter::OriginTopLeft;
     if (QPlatformGraphicsBuffer *graphicsBuffer = this->graphicsBuffer()) {
@@ -307,7 +340,6 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
             origin = QOpenGLTextureBlitter::OriginBottomLeft;
         textureId = d_ptr->textureId;
     } else {
-        // Backingstore texture with the normal widgets.
         TextureFlags flags = 0;
         textureId = toTexture(deviceRegion(region, window), &d_ptr->textureSize, &flags);
         d_ptr->needsSwizzle = (flags & TextureSwizzle) != 0;
@@ -316,7 +348,7 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
     }
 
     if (textureId) {
-        QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(QRect(QPoint(), d_ptr->textureSize), windowRect);
+        QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(QRect(QPoint(), d_ptr->textureSize), deviceWindowRect);
         if (d_ptr->needsSwizzle)
             d_ptr->blitter->setSwizzleRB(true);
         d_ptr->blitter->blit(textureId, target, origin);
@@ -326,11 +358,8 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
 
     // Textures for renderToTexture widgets that have WA_AlwaysStackOnTop set.
     for (int i = 0; i < textures->count(); ++i) {
-        if (textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop)) {
-            QRect targetRect = deviceRect(textures->geometry(i), window);
-            QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(targetRect, windowRect);
-            d_ptr->blitter->blit(textures->textureId(i), target, QOpenGLTextureBlitter::OriginBottomLeft);
-        }
+        if (textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop))
+            blit(textures, i, window, deviceWindowRect, d_ptr->blitter);
     }
 
     funcs->glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
diff --git a/src/gui/painting/qplatformbackingstore.h b/src/gui/painting/qplatformbackingstore.h
index ae7314b6d0..5fa7e6dac0 100644
--- a/src/gui/painting/qplatformbackingstore.h
+++ b/src/gui/painting/qplatformbackingstore.h
@@ -82,12 +82,14 @@ public:
     bool isEmpty() const { return count() == 0; }
     GLuint textureId(int index) const;
     QRect geometry(int index) const;
+    QRect clipRect(int index) const;
     QWidget *widget(int index);
     Flags flags(int index) const;
     void lock(bool on);
     bool isLocked() const;
 
-    void appendTexture(QWidget *widget, GLuint textureId, const QRect &geometry, Flags flags = 0);
+    void appendTexture(QWidget *widget, GLuint textureId, const QRect &geometry,
+                       const QRect &clipRect = QRect(), Flags flags = 0);
     void clear();
 
  Q_SIGNALS:
diff --git a/src/platformsupport/platformcompositor/qopenglcompositor.cpp b/src/platformsupport/platformcompositor/qopenglcompositor.cpp
index 3fd6c999a2..2e386532e2 100644
--- a/src/platformsupport/platformcompositor/qopenglcompositor.cpp
+++ b/src/platformsupport/platformcompositor/qopenglcompositor.cpp
@@ -169,6 +169,29 @@ struct BlendStateBinder
     bool m_blend;
 };
 
+static inline QRect toBottomLeftRect(const QRect &topLeftRect, int windowHeight)
+{
+    return QRect(topLeftRect.x(), windowHeight - topLeftRect.bottomRight().y() - 1,
+                 topLeftRect.width(), topLeftRect.height());
+}
+
+static void clippedBlit(const QPlatformTextureList *textures, int idx, const QRect &targetWindowRect, QOpenGLTextureBlitter *blitter)
+{
+    const QRect rectInWindow = textures->geometry(idx);
+    QRect clipRect = textures->clipRect(idx);
+    if (clipRect.isEmpty())
+        clipRect = QRect(QPoint(0, 0), rectInWindow.size());
+
+    const QRect clippedRectInWindow = rectInWindow & clipRect.translated(rectInWindow.topLeft());
+    const QRect srcRect = toBottomLeftRect(clipRect, rectInWindow.height());
+
+    const QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(clippedRectInWindow, targetWindowRect);
+    const QMatrix3x3 source = QOpenGLTextureBlitter::sourceTransform(srcRect, rectInWindow.size(),
+                                                                     QOpenGLTextureBlitter::OriginBottomLeft);
+
+    blitter->blit(textures->textureId(idx), target, source);
+}
+
 void QOpenGLCompositor::render(QOpenGLCompositorWindow *window)
 {
     const QPlatformTextureList *textures = window->textures();
@@ -181,7 +204,6 @@ void QOpenGLCompositor::render(QOpenGLCompositorWindow *window)
 
     for (int i = 0; i < textures->count(); ++i) {
         uint textureId = textures->textureId(i);
-        QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(textures->geometry(i), targetWindowRect);
         const float opacity = window->sourceWindow()->opacity();
         if (opacity != currentOpacity) {
             currentOpacity = opacity;
@@ -191,24 +213,25 @@ void QOpenGLCompositor::render(QOpenGLCompositorWindow *window)
         if (textures->count() > 1 && i == textures->count() - 1) {
             // Backingstore for a widget with QOpenGLWidget subwidgets
             blend.set(true);
+            const QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(textures->geometry(i), targetWindowRect);
             m_blitter.blit(textureId, target, QOpenGLTextureBlitter::OriginTopLeft);
         } else if (textures->count() == 1) {
             // A regular QWidget window
             const bool translucent = window->sourceWindow()->requestedFormat().alphaBufferSize() > 0;
             blend.set(translucent);
+            const QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(textures->geometry(i), targetWindowRect);
             m_blitter.blit(textureId, target, QOpenGLTextureBlitter::OriginTopLeft);
         } else if (!textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop)) {
             // Texture from an FBO belonging to a QOpenGLWidget
             blend.set(false);
-            m_blitter.blit(textureId, target, QOpenGLTextureBlitter::OriginBottomLeft);
+            clippedBlit(textures, i, targetWindowRect, &m_blitter);
         }
     }
 
     for (int i = 0; i < textures->count(); ++i) {
         if (textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop)) {
-            QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(textures->geometry(i), targetWindowRect);
             blend.set(true);
-            m_blitter.blit(textures->textureId(i), target, QOpenGLTextureBlitter::OriginBottomLeft);
+            clippedBlit(textures, i, targetWindowRect, &m_blitter);
         }
     }
 
diff --git a/src/platformsupport/platformcompositor/qopenglcompositorbackingstore.cpp b/src/platformsupport/platformcompositor/qopenglcompositorbackingstore.cpp
index 3caec468a6..4cf64e61da 100644
--- a/src/platformsupport/platformcompositor/qopenglcompositorbackingstore.cpp
+++ b/src/platformsupport/platformcompositor/qopenglcompositorbackingstore.cpp
@@ -175,7 +175,8 @@ void QOpenGLCompositorBackingStore::composeAndFlush(QWindow *window, const QRegi
 
     m_textures->clear();
     for (int i = 0; i < textures->count(); ++i)
-        m_textures->appendTexture(textures->widget(i), textures->textureId(i), textures->geometry(i), textures->flags(i));
+        m_textures->appendTexture(textures->widget(i), textures->textureId(i), textures->geometry(i),
+                                  textures->clipRect(i), textures->flags(i));
 
     updateTexture();
     m_textures->appendTexture(Q_NULLPTR, m_bsTexture, window->geometry());
diff --git a/src/widgets/kernel/qwidgetbackingstore.cpp b/src/widgets/kernel/qwidgetbackingstore.cpp
index 485cf82078..5752317924 100644
--- a/src/widgets/kernel/qwidgetbackingstore.cpp
+++ b/src/widgets/kernel/qwidgetbackingstore.cpp
@@ -962,7 +962,8 @@ static void findTextureWidgetsRecursively(QWidget *tlw, QWidget *widget, QPlatfo
         QPlatformTextureList::Flags flags = 0;
         if (widget->testAttribute(Qt::WA_AlwaysStackOnTop))
             flags |= QPlatformTextureList::StacksOnTop;
-        widgetTextures->appendTexture(widget, wd->textureId(), QRect(widget->mapTo(tlw, QPoint()), widget->size()), flags);
+        const QRect rect(widget->mapTo(tlw, QPoint()), widget->size());
+        widgetTextures->appendTexture(widget, wd->textureId(), rect, wd->clipRect(), flags);
     }
 
     for (int i = 0; i < wd->children.size(); ++i) {