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>

3 files changed, 67 insertions, 50 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;
diff --git a/src/gui/image/qimage_conversions.cpp b/src/gui/image/qimage_conversions.cpp
index 9e1df7058c..97a5f89e68 100644
--- a/src/gui/image/qimage_conversions.cpp
+++ b/src/gui/image/qimage_conversions.cpp
@@ -260,10 +260,17 @@ bool convert_generic_inplace(QImageData *data, QImage::Format dst_format, Qt::Im
     if (data->depth != qt_depthForFormat(dst_format))
         return false;
 
-    uint buf[BufferSize];
-    uint *buffer = buf;
     const QPixelLayout *srcLayout = &qPixelLayouts[data->format];
     const QPixelLayout *destLayout = &qPixelLayouts[dst_format];
+
+    // The precision here is only ARGB32PM so don't convert between higher accuracy
+    // formats (assert instead when we have a convert_generic_over_rgb64_inplace).
+    if (qt_highColorPrecision(data->format, !destLayout->hasAlphaChannel)
+            && qt_highColorPrecision(dst_format, !srcLayout->hasAlphaChannel))
+        return false;
+
+    uint buf[BufferSize];
+    uint *buffer = buf;
     uchar *srcData = data->data;
 
     Q_ASSERT(srcLayout->bpp == destLayout->bpp);
@@ -626,12 +633,13 @@ static bool convert_rgbswap_generic_inplace(QImageData *data, Qt::ImageConversio
 }
 
 template<QtPixelOrder PixelOrder, bool RGBA>
-static void convert_RGB_to_RGB30(QImageData *dest, const QImageData *src, Qt::ImageConversionFlags)
+static void convert_ARGB_to_A2RGB30(QImageData *dest, const QImageData *src, Qt::ImageConversionFlags)
 {
 
-    Q_ASSERT(RGBA || src->format == QImage::Format_RGB32 || src->format == QImage::Format_ARGB32);
-    Q_ASSERT(!RGBA || src->format == QImage::Format_RGBX8888 || src->format == QImage::Format_RGBA8888);
-    Q_ASSERT(dest->format == QImage::Format_BGR30 || dest->format == QImage::Format_RGB30);
+    Q_ASSERT(RGBA || src->format == QImage::Format_ARGB32);
+    Q_ASSERT(!RGBA || src->format == QImage::Format_RGBA8888);
+    Q_ASSERT(dest->format == QImage::Format_A2BGR30_Premultiplied
+             || dest->format == QImage::Format_A2RGB30_Premultiplied);
     Q_ASSERT(src->width == dest->width);
     Q_ASSERT(src->height == dest->height);
 
@@ -646,7 +654,9 @@ static void convert_RGB_to_RGB30(QImageData *dest, const QImageData *src, Qt::Im
             QRgb c = *src_data;
             if (RGBA)
                 c = RGBA2ARGB(c);
-            *dest_data = qConvertRgb32ToRgb30<PixelOrder>(c);
+            const uint alpha = (qAlpha(c) >> 6) * 85;
+            c = BYTE_MUL(c, alpha);
+            *dest_data = (qConvertRgb32ToRgb30<PixelOrder>(c) & 0x3fffffff) | (alpha << 30);
             ++src_data;
             ++dest_data;
         }
@@ -656,10 +666,10 @@ static void convert_RGB_to_RGB30(QImageData *dest, const QImageData *src, Qt::Im
 }
 
 template<QtPixelOrder PixelOrder, bool RGBA>
-static bool convert_RGB_to_RGB30_inplace(QImageData *data, Qt::ImageConversionFlags)
+static bool convert_ARGB_to_A2RGB30_inplace(QImageData *data, Qt::ImageConversionFlags)
 {
-    Q_ASSERT(RGBA || (data->format == QImage::Format_RGB32 || data->format == QImage::Format_ARGB32));
-    Q_ASSERT(!RGBA || (data->format == QImage::Format_RGBX8888 || data->format == QImage::Format_RGBA8888));
+    Q_ASSERT(RGBA || data->format == QImage::Format_ARGB32);
+    Q_ASSERT(!RGBA || data->format == QImage::Format_RGBA8888);
 
     const int pad = (data->bytes_per_line >> 2) - data->width;
     QRgb *rgb_data = (QRgb *) data->data;
@@ -670,13 +680,16 @@ static bool convert_RGB_to_RGB30_inplace(QImageData *data, Qt::ImageConversionFl
             QRgb c = *rgb_data;
             if (RGBA)
                 c = RGBA2ARGB(c);
-            *rgb_data = qConvertRgb32ToRgb30<PixelOrder>(c);
+            const uint alpha = (qAlpha(c) >> 6) * 85;
+            c = BYTE_MUL(c, alpha);
+            *rgb_data = (qConvertRgb32ToRgb30<PixelOrder>(c) & 0x3fffffff) | (alpha << 30);
             ++rgb_data;
         }
         rgb_data += pad;
     }
 
-    data->format = (PixelOrder == PixelOrderRGB) ? QImage::Format_RGB30 : QImage::Format_BGR30;
+    data->format = (PixelOrder == PixelOrderRGB) ? QImage::Format_A2RGB30_Premultiplied
+                                                 : QImage::Format_A2BGR30_Premultiplied;
     return true;
 }
 
@@ -2353,9 +2366,9 @@ Image_Converter qimage_converter_map[QImage::NImageFormats][QImage::NImageFormat
         0,
         0,
         0,
-        convert_RGB_to_RGB30<PixelOrderBGR, false>,
         0,
-        convert_RGB_to_RGB30<PixelOrderRGB, false>,
+        0,
+        0,
         0,
         0, 0,
         0, 0, 0, 0, 0
@@ -2381,10 +2394,10 @@ Image_Converter qimage_converter_map[QImage::NImageFormats][QImage::NImageFormat
         convert_ARGB_to_RGBx,
         convert_ARGB_to_RGBA,
         0,
-        convert_RGB_to_RGB30<PixelOrderBGR, false>,
         0,
-        convert_RGB_to_RGB30<PixelOrderRGB, false>,
+        convert_ARGB_to_A2RGB30<PixelOrderBGR, false>,
         0,
+        convert_ARGB_to_A2RGB30<PixelOrderRGB, false>,
         0, 0,
         0,
         convert_ARGB32_to_RGBA64<false>,
@@ -2634,9 +2647,9 @@ Image_Converter qimage_converter_map[QImage::NImageFormats][QImage::NImageFormat
         0,
         convert_passthrough,
         convert_passthrough,
-        convert_RGB_to_RGB30<PixelOrderBGR, true>,
         0,
-        convert_RGB_to_RGB30<PixelOrderRGB, true>,
+        0,
+        0,
         0,
         0, 0,
         0, 0, 0, 0, 0
@@ -2661,10 +2674,10 @@ Image_Converter qimage_converter_map[QImage::NImageFormats][QImage::NImageFormat
         mask_alpha_converter_RGBx,
         0,
         0,
-        convert_RGB_to_RGB30<PixelOrderBGR, true>,
         0,
-        convert_RGB_to_RGB30<PixelOrderRGB, true>,
+        convert_ARGB_to_A2RGB30<PixelOrderBGR, true>,
         0,
+        convert_ARGB_to_A2RGB30<PixelOrderRGB, true>,
         0, 0,
         0,
         convert_ARGB32_to_RGBA64<true>,
@@ -3017,9 +3030,9 @@ InPlace_Image_Converter qimage_inplace_converter_map[QImage::NImageFormats][QIma
         0,
         0,
         0,
-        convert_RGB_to_RGB30_inplace<PixelOrderBGR, false>,
         0,
-        convert_RGB_to_RGB30_inplace<PixelOrderRGB, false>,
+        0,
+        0,
         0,
         0, 0,
         0, 0, 0, 0, 0
@@ -3044,10 +3057,10 @@ InPlace_Image_Converter qimage_inplace_converter_map[QImage::NImageFormats][QIma
         convert_ARGB_to_RGBA_inplace<QImage::Format_RGBX8888>,
         convert_ARGB_to_RGBA_inplace<QImage::Format_RGBA8888>,
         0,
-        convert_RGB_to_RGB30_inplace<PixelOrderBGR, false>,
         0,
-        convert_RGB_to_RGB30_inplace<PixelOrderRGB, false>,
+        convert_ARGB_to_A2RGB30_inplace<PixelOrderBGR, false>,
         0,
+        convert_ARGB_to_A2RGB30_inplace<PixelOrderRGB, false>,
         0, 0,
         0, 0, 0, 0, 0
     }, // Format_ARGB32
@@ -3123,9 +3136,9 @@ InPlace_Image_Converter qimage_inplace_converter_map[QImage::NImageFormats][QIma
         0,
         convert_passthrough_inplace<QImage::Format_RGBA8888>,
         convert_passthrough_inplace<QImage::Format_RGBA8888_Premultiplied>,
-        convert_RGB_to_RGB30_inplace<PixelOrderBGR, true>,
         0,
-        convert_RGB_to_RGB30_inplace<PixelOrderRGB, true>,
+        0,
+        0,
         0,
         0, 0,
         0, 0, 0, 0, 0
@@ -3150,10 +3163,10 @@ InPlace_Image_Converter qimage_inplace_converter_map[QImage::NImageFormats][QIma
         mask_alpha_converter_rgbx_inplace,
         0,
         0,
-        convert_RGB_to_RGB30_inplace<PixelOrderBGR, true>,
         0,
-        convert_RGB_to_RGB30_inplace<PixelOrderRGB, true>,
+        convert_ARGB_to_A2RGB30_inplace<PixelOrderBGR, true>,
         0,
+        convert_ARGB_to_A2RGB30_inplace<PixelOrderRGB, true>,
         0, 0,
         0, 0, 0, 0, 0
     }, // Format_RGBA8888
diff --git a/src/gui/image/qimage_p.h b/src/gui/image/qimage_p.h
index 9e2d9c86bb..0930955f5a 100644
--- a/src/gui/image/qimage_p.h
+++ b/src/gui/image/qimage_p.h
@@ -276,6 +276,29 @@ inline QImage::Format qt_alphaVersion(QImage::Format format)
     return QImage::Format_ARGB32_Premultiplied;
 }
 
+inline bool qt_highColorPrecision(QImage::Format format, bool opaque = false)
+{
+    // Formats with higher color precision than ARGB32_Premultiplied.
+    switch (format) {
+    case QImage::Format_ARGB32:
+    case QImage::Format_RGBA8888:
+        return !opaque;
+    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;
+}
+
+
 inline QImage::Format qt_maybeAlphaVersionWithSameDepth(QImage::Format format)
 {
     const QImage::Format toFormat = qt_alphaVersion(format);