Use libjpeg's builtin scaler for implementing setScaledSize()

The libjpeg library has builtin support for scaling to 1/2, 1/4, and 1/8
the original size very quickly.  Use this in the implementation of
setScaledSize() to get close to the desired size and then scale
with QImageSmoothScaler the rest of the way.

Task-number: QT-2023
Reviewed-by: Daniel Pope

2 files changed, 11 insertions, 4 deletions

diff --git a/src/plugins/imageformats/jpeg/qjpeghandler.cpp b/src/plugins/imageformats/jpeg/qjpeghandler.cpp
index aa239ecb92..8da6610b6b 100644
--- a/src/plugins/imageformats/jpeg/qjpeghandler.cpp
+++ b/src/plugins/imageformats/jpeg/qjpeghandler.cpp
@@ -758,10 +758,9 @@ static bool read_jpeg_image(QIODevice *device, QImage *outImage,
             quality = 75;
 
 #ifndef QT_NO_IMAGE_SMOOTHSCALE
-        // If high quality not required, shrink image during decompression
-        if (scaledSize.isValid() && !scaledSize.isEmpty() && quality < HIGH_QUALITY_THRESHOLD) {
+        if (!scaledSize.isEmpty()) {
             cinfo.scale_denom = qMin(cinfo.image_width / scaledSize.width(),
-                                     cinfo.image_width / scaledSize.height());
+                                     cinfo.image_height / scaledSize.height());
             if (cinfo.scale_denom < 2) {
                 cinfo.scale_denom = 1;
             } else if (cinfo.scale_denom < 4) {
diff --git a/tests/auto/qimagereader/tst_qimagereader.cpp b/tests/auto/qimagereader/tst_qimagereader.cpp
index 15b1c1c540..630cc034a3 100644
--- a/tests/auto/qimagereader/tst_qimagereader.cpp
+++ b/tests/auto/qimagereader/tst_qimagereader.cpp
@@ -322,7 +322,15 @@ void tst_QImageReader::setScaledSize_data()
     QTest::newRow("PPM: test") << "test.ppm" << QSize(10, 10) << QByteArray("ppm");
     QTest::newRow("XBM: gnus") << "gnus" << QSize(200, 200) << QByteArray("xbm");
 #ifdef QTEST_HAVE_JPEG
-    QTest::newRow("JPEG: beavis") << "beavis" << QSize(200, 200) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis A") << "beavis" << QSize(200, 200) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis B") << "beavis" << QSize(175, 175) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis C") << "beavis" << QSize(100, 100) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis D") << "beavis" << QSize(100, 200) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis E") << "beavis" << QSize(200, 100) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis F") << "beavis" << QSize(87, 87) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis G") << "beavis" << QSize(50, 45) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis H") << "beavis" << QSize(43, 43) << QByteArray("jpeg");
+    QTest::newRow("JPEG: beavis I") << "beavis" << QSize(25, 25) << QByteArray("jpeg");
 #endif // QTEST_HAVE_JPEG
 #ifdef QTEST_HAVE_GIF
     QTest::newRow("GIF: earth") << "earth" << QSize(200, 200) << QByteArray("gif");