Round evenly in INV_PREMUL

Currently INV_PREMUL rounds strictly down. While PREMUL rounds evenly.

This patch adds 0x8000 to the intermediate results in INV_PREMUL before
right shifting, thereby achieving even rounding.

The rounding also makes PREMUL(INV_PREMUL()) into an identify operation,
which means we can safely convert ARGB32PM to ARGB32 and back without
ever losing color details. A test is added to verify this.

Change-Id: I1267e109caddcff0c01d726cb5c1c1e9fa5f7996
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>

2 files changed, 24 insertions, 1 deletions

diff --git a/src/gui/painting/qdrawhelper_p.h b/src/gui/painting/qdrawhelper_p.h
index 3c945338a6..0f98b07229 100644
--- a/src/gui/painting/qdrawhelper_p.h
+++ b/src/gui/painting/qdrawhelper_p.h
@@ -699,7 +699,8 @@ static Q_ALWAYS_INLINE uint INV_PREMUL(uint p) {
         return 0;
     // (p*(0x00ff00ff/alpha)) >> 16 == (p*255)/alpha for all p and alpha <= 256.
     const uint invAlpha = 0x00ff00ffU / alpha;
-    return qRgba((qRed(p)*invAlpha)>>16, (qGreen(p)*invAlpha)>>16, (qBlue(p)*invAlpha)>>16, alpha);
+    // We add 0x8000 to get even rounding. The rounding also ensures that PREMUL(INV_PREMUL(p)) == p for all p.
+    return qRgba((qRed(p)*invAlpha + 0x8000)>>16, (qGreen(p)*invAlpha + 0x8000)>>16, (qBlue(p)*invAlpha + 0x8000)>>16, alpha);
 }
 
 struct quint24 {
diff --git a/tests/auto/gui/image/qimage/tst_qimage.cpp b/tests/auto/gui/image/qimage/tst_qimage.cpp
index f7a672ad18..9ddf571dbd 100644
--- a/tests/auto/gui/image/qimage/tst_qimage.cpp
+++ b/tests/auto/gui/image/qimage/tst_qimage.cpp
@@ -166,6 +166,8 @@ private slots:
     void deepCopyWhenPaintingActive();
     void scaled_QTBUG19157();
 
+    void convertOverUnPreMul();
+
     void cleanupFunctions();
 };
 
@@ -2422,6 +2424,26 @@ void tst_QImage::scaled_QTBUG19157()
     QVERIFY(!foo.isNull());
 }
 
+void tst_QImage::convertOverUnPreMul()
+{
+    QImage image(256, 256, QImage::Format_ARGB32_Premultiplied);
+
+    for (int j = 0; j < 256; j++) {
+        for (int i = 0; i <= j; i++) {
+            image.setPixel(i, j, qRgba(i, i, i, j));
+        }
+    }
+
+    QImage image2 = image.convertToFormat(QImage::Format_ARGB32).convertToFormat(QImage::Format_ARGB32_Premultiplied);
+
+    for (int j = 0; j < 256; j++) {
+        for (int i = 0; i <= j; i++) {
+            QCOMPARE(qAlpha(image2.pixel(i, j)), qAlpha(image.pixel(i, j)));
+            QCOMPARE(qGray(image2.pixel(i, j)), qGray(image.pixel(i, j)));
+        }
+    }
+}
+
 static void cleanupFunction(void* info)
 {
     bool *called = static_cast<bool*>(info);