Fix blending of RGB32 on RGB32 with partial opacity

The alpha channel of an RGB32 image was not properly ignored when doing
blending with partial opacity.

Now the alpha value is properly ignored, which is both more correct
and faster. This also makes SSE2 and AVX2 implementations match NEON
which was already doing the right thing (though had dead code for
doing it wrong).

Change-Id: I4613b8d70ed8c2e36ced10baaa7a4a55bd36a940
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 18 insertions, 12 deletions

diff --git a/src/gui/painting/qblendfunctions.cpp b/src/gui/painting/qblendfunctions.cpp
index 0a5d458532..a4a091a29f 100644
--- a/src/gui/painting/qblendfunctions.cpp
+++ b/src/gui/painting/qblendfunctions.cpp
@@ -385,19 +385,25 @@ void qt_blend_rgb32_on_rgb32(uchar *destPixels, int dbpl,
             destPixels, dbpl, srcPixels, sbpl, w, h, const_alpha);
     fflush(stdout);
 #endif
-
-    if (const_alpha != 256) {
-        qt_blend_argb32_on_argb32(destPixels, dbpl, srcPixels, sbpl, w, h, const_alpha);
-        return;
-    }
-
     const uint *src = (const uint *) srcPixels;
     uint *dst = (uint *) destPixels;
-    int len = w * 4;
-    for (int y=0; y<h; ++y) {
-        memcpy(dst, src, len);
-        dst = (quint32 *)(((uchar *) dst) + dbpl);
-        src = (const quint32 *)(((const uchar *) src) + sbpl);
+    if (const_alpha == 256) {
+        const int len = w * 4;
+        for (int y = 0; y < h; ++y) {
+            memcpy(dst, src, len);
+            dst = (quint32 *)(((uchar *) dst) + dbpl);
+            src = (const quint32 *)(((const uchar *) src) + sbpl);
+        }
+        return;
+    } else if (const_alpha != 0) {
+        const_alpha = (const_alpha * 255) >> 8;
+        int ialpha = 255 - const_alpha;
+        for (int y=0; y<h; ++y) {
+            for (int x=0; x<w; ++x)
+                dst[x] = INTERPOLATE_PIXEL_255(dst[x], ialpha, src[x], const_alpha);
+            dst = (quint32 *)(((uchar *) dst) + dbpl);
+            src = (const quint32 *)(((const uchar *) src) + sbpl);
+        }
     }
 }
 
@@ -414,7 +420,7 @@ struct Blend_RGB32_on_RGB32_ConstAlpha {
     }
 
     inline void write(quint32 *dst, quint32 src) {
-        *dst = BYTE_MUL(src, m_alpha) + BYTE_MUL(*dst, m_ialpha);
+        *dst = INTERPOLATE_PIXEL_255(src, m_alpha, *dst, m_ialpha);
     }
 
     inline void flush(void *) {}