Correct RGB to Grayscale conversion

The existing conversions weren't handling gamma correctly and used
an ad-hoc definition of gray instead of based on true luminance.

[ChangeLog][QtGui] RGB conversions to grayscale formats are now
gamma-corrected and produce color-space luminance values

Change-Id: I88ab870c8f5e502ddb053e6a14a75102239a26f2
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

2 files changed, 25 insertions, 12 deletions

diff --git a/tests/auto/gui/image/qimage/tst_qimage.cpp b/tests/auto/gui/image/qimage/tst_qimage.cpp
index 0fb4263024..c3096bd47f 100644
--- a/tests/auto/gui/image/qimage/tst_qimage.cpp
+++ b/tests/auto/gui/image/qimage/tst_qimage.cpp
@@ -2951,21 +2951,29 @@ void tst_QImage::genericRgbConversion()
 
     QImage image(16, 16, format);
 
-    for (int i = 0; i < image.height(); ++i)
-        for (int j = 0; j < image.width(); ++j)
-            image.setPixel(j, i, qRgb(j*16, i*16, 0));
+    for (int i = 0; i < image.height(); ++i) {
+        for (int j = 0; j < image.width(); ++j) {
+            if (srcGrayscale || dstGrayscale)
+                image.setPixel(j, i, qRgb((i + j) * 8, (i + j) * 8, (i + j) * 8));
+            else
+                image.setPixel(j, i, qRgb(j * 16, i * 16, (i + j) * 8));
+        }
+    }
 
     QImage imageConverted = image.convertToFormat(dest_format);
+    uint mask = std::min(image.depth(), imageConverted.depth()) < 32 ? 0xFFF0F0F0 : 0xFFFFFFFF;
+    if (srcGrayscale || dstGrayscale)
+        mask = std::max(image.depth(), imageConverted.depth()) < 32 ? 0xFFF0F0F0 : 0xFFFFFFFF;
+    if (srcGrayscale && dstGrayscale)
+        mask = 0xFFFFFFFF;
     QCOMPARE(imageConverted.format(), dest_format);
     for (int i = 0; i < imageConverted.height(); ++i) {
         for (int j = 0; j < imageConverted.width(); ++j) {
             QRgb convertedColor = imageConverted.pixel(j,i);
-            if (srcGrayscale || dstGrayscale) {
-                QVERIFY(qAbs(qGray(convertedColor) - qGray(qRgb(j*16, i*16, 0))) < 15);
-            } else {
-                QCOMPARE(qRed(convertedColor) & 0xF0, j * 16);
-                QCOMPARE(qGreen(convertedColor) & 0xF0, i * 16);
-            }
+            if (srcGrayscale || dstGrayscale)
+                QCOMPARE(convertedColor & mask, qRgb((i + j) * 8, (i + j) * 8, (i + j) * 8) & mask);
+            else
+                QCOMPARE(convertedColor & mask, qRgb(j * 16, i * 16, (i + j) * 8) & mask);
         }
     }
 }
@@ -3014,8 +3022,7 @@ void tst_QImage::inplaceRgbConversion()
     for (int i = 0; i < imageConverted.height(); ++i) {
         for (int j = 0; j < imageConverted.width(); ++j) {
             QRgb convertedColor = imageConverted.pixel(j,i);
-            QCOMPARE(qRed(convertedColor) & 0xF0, j * 16);
-            QCOMPARE(qGreen(convertedColor) & 0xF0, i * 16);
+            QCOMPARE(convertedColor & 0xFFF0F0F0, qRgb(j * 16, i * 16, 0));
         }
     }
     if (qt_depthForFormat(format) == qt_depthForFormat(dest_format))
diff --git a/tests/auto/gui/image/qimagewriter/tst_qimagewriter.cpp b/tests/auto/gui/image/qimagewriter/tst_qimagewriter.cpp
index 907719259d..49a4504d2a 100644
--- a/tests/auto/gui/image/qimagewriter/tst_qimagewriter.cpp
+++ b/tests/auto/gui/image/qimagewriter/tst_qimagewriter.cpp
@@ -291,7 +291,13 @@ void tst_QImageWriter::writeImage2()
         QVERIFY(reader.read(&written));
     }
 
-    written = written.convertToFormat(image.format());
+    // The 8-bit input value might have turned into a fraction in 16-bit grayscale
+    // which can't be preserved in file formats that doesn't support 16bpc.
+    if (image.format() == QImage::Format_Grayscale16 &&
+        written.format() != QImage::Format_Grayscale16 && written.depth() <= 32)
+        image.convertTo(QImage::Format_Grayscale8);
+
+    written.convertTo(image.format());
     if (!equalImageContents(written, image)) {
         qDebug() << "image" << image.format() << image.width()
                  << image.height() << image.depth()