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Round evenly in INV_PREMUL

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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>
This commit is contained in:
Allan Sandfeld Jensen 2014-01-23 16:09:20 +01:00 committed by The Qt Project
parent fa83803119
commit 0226795cf3
2 changed files with 24 additions and 1 deletions
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3
src/gui/painting/qdrawhelper_p.h
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@ -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 {

22
tests/auto/gui/image/qimage/tst_qimage.cpp
View File

@ -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);