Fix regression in rotated bilinear sampling

Fix a mistake introduced recently and revealed by lancelot. Adds an
auto-test for rotations to catch similar errors faster in the future.

Change-Id: I028a160107d98899e723481b6201ef776f20c721
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 61 insertions, 0 deletions

diff --git a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
index d2cc7ce072..1bd7f67810 100644
--- a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
+++ b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
@@ -295,6 +295,9 @@ private slots:
 
     void QTBUG50153_drawImage_assert();
 
+    void rotateImage_data();
+    void rotateImage();
+
 private:
     void fillData();
     void setPenColor(QPainter& p);
@@ -5059,6 +5062,64 @@ void tst_QPainter::QTBUG50153_drawImage_assert()
     }
 }
 
+void tst_QPainter::rotateImage_data()
+{
+    QTest::addColumn<QImage>("image");
+    QTest::addColumn<bool>("smooth");
+
+    QImage image(128, 128, QImage::Format_RGB32);
+    for (int y = 0; y < 128; ++y) {
+        for (int x = 0; x < 128; ++x) {
+            image.setPixel(x, y, qRgb(x + y, x + y, x + y));
+        }
+    }
+
+    QTest::newRow("fast") << image << false;
+    QTest::newRow("smooth") << image << true;
+}
+
+void tst_QPainter::rotateImage()
+{
+    QFETCH(QImage, image);
+    QFETCH(bool, smooth);
+
+    QImage dest(184, 184, QImage::Format_ARGB32_Premultiplied);
+    dest.fill(Qt::transparent);
+
+    QPainter painter(&dest);
+    QTransform transform;
+    transform.translate(92, 0);
+    transform.rotate(45);
+    painter.setTransform(transform);
+    painter.setRenderHint(QPainter::SmoothPixmapTransform, smooth);
+    painter.drawImage(0, 0, image);
+    painter.end();
+
+    QRgb lastRow = qRgba(0, 0, 0, 0);
+    for (int y = 0; y < 184; ++y) {
+        QRgb row = qRgba(0, 0, 0, 0);
+        for (int x = 0; x < 184; ++x) {
+            QRgb pixel = dest.pixel(x, y);
+            if (qAlpha(pixel) < 255)
+                continue;
+            if (qAlpha(row) == 0) {
+                row = pixel;
+            } else {
+                QCOMPARE(qRed(pixel), qGreen(pixel));
+                QCOMPARE(qGreen(pixel), qBlue(pixel));
+                QVERIFY(qAbs(qRed(row) - qRed(pixel)) <= 2);
+                QVERIFY(qAbs(qGreen(row) - qGreen(pixel)) <= 2);
+                QVERIFY(qAbs(qBlue(row) - qBlue(pixel)) <= 2);
+            }
+
+        }
+        if (qAlpha(row) && qAlpha(lastRow))
+            QVERIFY(qGray(lastRow) <= qGray(row));
+        lastRow = row;
+    }
+
+}
+
 QTEST_MAIN(tst_QPainter)
 
 #include "tst_qpainter.moc"