Changeset 153522 in webkit

Timestamp:

Jul 31, 2013 7:39:38 AM (11 years ago)

Author:

allan.jensen@digia.com

Message:

[Qt] Images are down-scaled badly
​https://bugs.webkit.org/show_bug.cgi?id=119263

Reviewed by Jocelyn Turcotte.

QPainter only does bilinear filtering, which means it will downscaling beyond
0.5x will start skipping pixel and start to degrade the end result. Scaling in
QImage and QPixmap however uses a better but much more expensive sampling that
counts all pixels.

To get the high quality downscaling we must therefore prescale the images before
painting them. To avoid a performance impact on repeated paints the prescaled
image are saved in the QPixmapCache.

• platform/graphics/qt/ImageQt.cpp:

(WebCore::BitmapImage::draw):

Location:

trunk/Source/WebCore

Files:

• ChangeLog (modified) (1 diff)
• platform/graphics/qt/ImageQt.cpp (modified) (3 diffs)

trunk/Source/WebCore/ChangeLog

@@ +1 @@
+2013-07-31  Allan Sandfeld Jensen  <allan.jensen@digia.com>
+
+        [Qt] Images are down-scaled badly
+        https://bugs.webkit.org/show_bug.cgi?id=119263
+
+        Reviewed by Jocelyn Turcotte.
+
+        QPainter only does bilinear filtering, which means it will downscaling beyond
+        0.5x will start skipping pixel and start to degrade the end result. Scaling in
+        QImage and QPixmap however uses a better but much more expensive sampling that
+        counts all pixels.
+
+        To get the high quality downscaling we must therefore prescale the images before
+        painting them. To avoid a performance impact on repeated paints the prescaled
+        image are saved in the QPixmapCache.
+
+        * platform/graphics/qt/ImageQt.cpp:
+        (WebCore::BitmapImage::draw):
+
 2013-07-31  Grzegorz Czajkowski  <g.czajkowski@samsung.com>
 

trunk/Source/WebCore/platform/graphics/qt/ImageQt.cpp

@@ -48 +48 @@
 #include <QPainter>
 #include <QPixmap>
+#include <QPixmapCache>
 #include <QTransform>
+#include <private/qhexstring_p.h>
 
 #include <math.h>
@@ -246 +248 @@
     if (!image)
         return;
+    QPixmap prescaled;
 
     if (mayFillWithSolidColor()) {
@@ -255 +258 @@
     normalizedSrc = adjustSourceRectForDownSampling(normalizedSrc, image->size());
 #endif
+
+    QPainter* painter = ctxt->platformContext();
+    // The quality of down scaling at 0.5x and below in QPainter is not very good
+    // due only using bilinear sampling, so for high quality scaling we need to
+    // perform scaling ourselves.
+#if QT_VERSION >= QT_VERSION_CHECK(5, 1, 0)
+    const float pixelRatio = painter->device()->devicePixelRatio();
+#else
+    const float pixelRatio = 1;
+#endif
+    if (painter->renderHints() & QPainter::SmoothPixmapTransform
+        && (normalizedDst.width() * 2 * pixelRatio < normalizedSrc.width()
+            || normalizedDst.height() * 2 * pixelRatio < normalizedSrc.height())) {
+        // This may not work right with subpixel positions, but that can not currently happen.
+        QRect pixelSrc = normalizedSrc.toRect();
+        QSize scaledSize(normalizedDst.width() * pixelRatio, normalizedDst.height() * pixelRatio);
+        QString key = QStringLiteral("qtwebkit_prescaled_")
+            % HexString<qint64>(image->cacheKey())
+            % HexString<int>(pixelSrc.x()) % HexString<int>(pixelSrc.y())
+            % HexString<int>(pixelSrc.width()) % HexString<int>(pixelSrc.height())
+            % HexString<int>(scaledSize.width()) % HexString<int>(scaledSize.height());
+        if (!QPixmapCache::find(key, &prescaled)) {
+            if (pixelSrc != image->rect())
+                prescaled = image->copy(pixelSrc).scaled(scaledSize, Qt::IgnoreAspectRatio, Qt::SmoothTransformation);
+            else
+                prescaled = image->scaled(scaledSize, Qt::IgnoreAspectRatio, Qt::SmoothTransformation);
+            QPixmapCache::insert(key, prescaled);
+        }
+        normalizedSrc = QRectF(QPointF(), prescaled.size());
+        image = &prescaled;
+    }
 
     CompositeOperator previousOperator = ctxt->compositeOperation();