Fix accuracy of ARGB32->A2RGB30 conversions

It was converted over ARGB32PM, when it should have been directly
converted to not lose accuracy, instead there was an unnecessary direct
ARGB32->RGB30 conversion, which was converted to the necessary type.

This also improves the selection of conversion over ARGB32PM or RGBA64PM
for ARGB32 and RGBA8888 by using 32-bit conversion when alpha is not
relevant.

Change-Id: I5990d8a23b2909d3910d8c1213fa46477742b052
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 3 insertions, 22 deletions

diff --git a/src/gui/image/qimage.cpp b/src/gui/image/qimage.cpp
index 2779b97fbd..d8ed0829af 100644
--- a/src/gui/image/qimage.cpp
+++ b/src/gui/image/qimage.cpp
@@ -2060,27 +2060,6 @@ QImage::Format QImage::format() const
     \sa {Image Formats}
 */
 
-static bool highColorPrecision(QImage::Format format)
-{
-    // Formats with higher color precision than ARGB32_Premultiplied.
-    switch (format) {
-    case QImage::Format_ARGB32:
-    case QImage::Format_RGBA8888:
-    case QImage::Format_BGR30:
-    case QImage::Format_RGB30:
-    case QImage::Format_A2BGR30_Premultiplied:
-    case QImage::Format_A2RGB30_Premultiplied:
-    case QImage::Format_RGBX64:
-    case QImage::Format_RGBA64:
-    case QImage::Format_RGBA64_Premultiplied:
-    case QImage::Format_Grayscale16:
-        return true;
-    default:
-        break;
-    }
-    return false;
-}
-
 /*!
     \internal
 */
@@ -2092,9 +2071,11 @@ QImage QImage::convertToFormat_helper(Format format, Qt::ImageConversionFlags fl
     if (format == Format_Invalid || d->format == Format_Invalid)
         return QImage();
 
+    const QPixelLayout *destLayout = &qPixelLayouts[format];
     Image_Converter converter = qimage_converter_map[d->format][format];
     if (!converter && format > QImage::Format_Indexed8 && d->format > QImage::Format_Indexed8) {
-        if (highColorPrecision(format) && highColorPrecision(d->format)) {
+        if (qt_highColorPrecision(d->format, !destLayout->hasAlphaChannel)
+                && qt_highColorPrecision(format, !hasAlphaChannel())) {
             converter = convert_generic_to_rgb64;
         } else
             converter = convert_generic;