From 665e05d711f814b04845abc5833b8899242efd54 Mon Sep 17 00:00:00 2001
From: Vlad Zahorodnii <vlad.zahorodnii@kde.org>
Date: Mon, 3 Jul 2023 09:53:47 +0300
Subject: [PATCH] plugins/qpa: Simplify backing store buffer handling

The main reasoning behind double buffering was to avoid InternalWindow's
copy of m_frontBuffer detaching. But it already detaches when the qpa
calls blitImage(). So remove double buffering to simplify the backing
store.
---
 src/plugins/qpa/backingstore.cpp | 39 ++++++++------------------------
 src/plugins/qpa/backingstore.h   |  3 +--
 2 files changed, 11 insertions(+), 31 deletions(-)

diff --git a/src/plugins/qpa/backingstore.cpp b/src/plugins/qpa/backingstore.cpp
index c685d7e576..d47c68433a 100644
--- a/src/plugins/qpa/backingstore.cpp
+++ b/src/plugins/qpa/backingstore.cpp
@@ -28,28 +28,25 @@ BackingStore::~BackingStore() = default;
 
 QPaintDevice *BackingStore::paintDevice()
 {
-    return &m_backBuffer;
+    return &m_buffer;
 }
 
 void BackingStore::resize(const QSize &size, const QRegion &staticContents)
 {
-    if (m_backBuffer.size() == size) {
+    if (m_buffer.size() == size) {
         return;
     }
 
     const QPlatformWindow *platformWindow = static_cast<QPlatformWindow *>(window()->handle());
     const qreal devicePixelRatio = platformWindow->devicePixelRatio();
 
-    m_backBuffer = QImage(size * devicePixelRatio, QImage::Format_ARGB32_Premultiplied);
-    m_backBuffer.setDevicePixelRatio(devicePixelRatio);
-
-    m_frontBuffer = QImage(size * devicePixelRatio, QImage::Format_ARGB32_Premultiplied);
-    m_frontBuffer.setDevicePixelRatio(devicePixelRatio);
+    m_buffer = QImage(size * devicePixelRatio, QImage::Format_ARGB32_Premultiplied);
+    m_buffer.setDevicePixelRatio(devicePixelRatio);
 }
 
 void BackingStore::beginPaint(const QRegion &region)
 {
-    if (m_backBuffer.hasAlphaChannel()) {
+    if (m_buffer.hasAlphaChannel()) {
         QPainter p(paintDevice());
         p.setCompositionMode(QPainter::CompositionMode_Source);
         const QColor blank = Qt::transparent;
@@ -59,20 +56,6 @@ void BackingStore::beginPaint(const QRegion &region)
     }
 }
 
-static QRect scaledRect(const QRect &rect, qreal devicePixelRatio)
-{
-    return QRect(rect.topLeft() * devicePixelRatio, rect.size() * devicePixelRatio);
-}
-
-static void blitImage(const QImage &source, QImage &target, const QRegion &region)
-{
-    QPainter painter(&target);
-    painter.setCompositionMode(QPainter::CompositionMode_Source);
-    for (const QRect &rect : region) {
-        painter.drawImage(rect, source, scaledRect(rect, source.devicePixelRatio()));
-    }
-}
-
 void BackingStore::flush(QWindow *window, const QRegion &region, const QPoint &offset)
 {
     Window *platformWindow = static_cast<Window *>(window->handle());
@@ -81,17 +64,15 @@ void BackingStore::flush(QWindow *window, const QRegion &region, const QPoint &o
         return;
     }
 
-    blitImage(m_backBuffer, m_frontBuffer, region);
-
     QRegion bufferDamage;
     for (const QRect &rect : region) {
-        bufferDamage |= QRect(std::floor(rect.x() * m_backBuffer.devicePixelRatio()),
-                              std::floor(rect.y() * m_backBuffer.devicePixelRatio()),
-                              std::ceil(rect.width() * m_backBuffer.devicePixelRatio()),
-                              std::ceil(rect.height() * m_backBuffer.devicePixelRatio()));
+        bufferDamage |= QRect(std::floor(rect.x() * m_buffer.devicePixelRatio()),
+                              std::floor(rect.y() * m_buffer.devicePixelRatio()),
+                              std::ceil(rect.width() * m_buffer.devicePixelRatio()),
+                              std::ceil(rect.height() * m_buffer.devicePixelRatio()));
     }
 
-    internalWindow->present(m_frontBuffer, bufferDamage);
+    internalWindow->present(m_buffer, bufferDamage);
 }
 
 }
diff --git a/src/plugins/qpa/backingstore.h b/src/plugins/qpa/backingstore.h
index a204e4213f..6331eda0ac 100644
--- a/src/plugins/qpa/backingstore.h
+++ b/src/plugins/qpa/backingstore.h
@@ -30,8 +30,7 @@ public:
     void beginPaint(const QRegion &region) override;
 
 private:
-    QImage m_backBuffer;
-    QImage m_frontBuffer;
+    QImage m_buffer;
 };
 
 }