Fix premul conversion from ARGB32 to A2RGB30 formats.

When a premultiplied alpha changes value because it is rounded to fewer
bits the premultiplied colors may need to be recalculated with the new
value. Otherwise the color will both be wrong and potentially invalid.

Change-Id: I9ec74a22aac73cd7ffab04e180cf2bf35bb4c315
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

1 files changed, 27 insertions, 0 deletions

diff --git a/tests/auto/gui/image/qimage/tst_qimage.cpp b/tests/auto/gui/image/qimage/tst_qimage.cpp
index da29a57f98..f7c71f05bd 100644
--- a/tests/auto/gui/image/qimage/tst_qimage.cpp
+++ b/tests/auto/gui/image/qimage/tst_qimage.cpp
@@ -190,6 +190,8 @@ private slots:
 
     void devicePixelRatio();
     void rgb30Unpremul();
+    void rgb30Repremul_data();
+    void rgb30Repremul();
 
     void metadataPassthrough();
 
@@ -2946,6 +2948,31 @@ void tst_QImage::rgb30Unpremul()
     QCOMPARE(bbits[2], (3U << 30) | (201 << 20) | (393 << 10) | 777);
 }
 
+void tst_QImage::rgb30Repremul_data()
+{
+    QTest::addColumn<uint>("color");
+    for (int i = 255; i > 0; i -= 15) {
+        QTest::newRow(qPrintable(QStringLiteral("100% red=") + QString::number(i))) << qRgba(i, 0, 0, 0xff);
+        QTest::newRow(qPrintable(QStringLiteral("75% red=") + QString::number(i))) << qRgba(i, 0, 0, 0xc0);
+        QTest::newRow(qPrintable(QStringLiteral("50% red=") + QString::number(i))) << qRgba(i, 0, 0, 0x80);
+        QTest::newRow(qPrintable(QStringLiteral("37.5% red=") + QString::number(i))) << qRgba(i, 0, 0, 0x60);
+    }
+}
+
+void tst_QImage::rgb30Repremul()
+{
+    QFETCH(uint, color);
+
+    QImage a(1, 1, QImage::Format_ARGB32);
+    a.setPixel(0, 0, color);
+
+    QImage b = a.convertToFormat(QImage::Format_A2BGR30_Premultiplied);
+    b = b.convertToFormat(QImage::Format_ARGB32);
+    uint expectedColor = qUnpremultiply(qPremultiply(color));
+    uint newColor = b.pixel(0, 0);
+    QVERIFY(qAbs(qRed(newColor) - qRed(expectedColor)) <= 1);
+}
+
 void tst_QImage::metadataPassthrough()
 {
     QImage a(64, 64, QImage::Format_ARGB32);