Fix DirectWrite subpixel antialiasing on BGR screens

On monitors where the subpixel arrangement is blue, green, red, our DirectWrite rendering would give the wrong subppixel antialiasing, causing color fringes on text. Like we do with Freetype, we determine subpixel arrangement of the primary screen and use this as the default. Pick-to: 6.8 6.9 Change-Id: I9ce7025449106a2376bd0ed02ce07b59c79438bd Reviewed-by: Eirik Aavitsland <[email protected]>
qt · Dec 16, 2024 · d5cef74 · d5cef74
1 parent a90d9f4
commit d5cef74
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Showing 2 changed files with 17 additions and 3 deletions.
diff --git a/src/gui/text/windows/qwindowsfontenginedirectwrite.cpp b/src/gui/text/windows/qwindowsfontenginedirectwrite.cpp
@@ -217,6 +217,12 @@ QWindowsFontEngineDirectWrite::QWindowsFontEngineDirectWrite(IDWriteFontFace *di
     m_fontEngineData->directWriteFactory->AddRef();
     m_directWriteFontFace->AddRef();
 
+    IDWriteRenderingParams *renderingParams = nullptr;
+    if (SUCCEEDED(m_fontEngineData->directWriteFactory->CreateRenderingParams(&renderingParams))) {
+        m_pixelGeometry = renderingParams->GetPixelGeometry();
+        renderingParams->Release();
+    }
+
     fontDef.pixelSize = pixelSize;
     collectMetrics();
     cache_cost = m_xHeight.toInt() * m_xHeight.toInt() * 2000;
@@ -1029,6 +1035,9 @@ void QWindowsFontEngineDirectWrite::renderGlyphRun(QImage *destination,
                         float blueAlpha = a * *src++ / 255.0;
                         float averageAlpha = (redAlpha + greenAlpha + blueAlpha) / 3.0;
 
+                        if (m_pixelGeometry == DWRITE_PIXEL_GEOMETRY_BGR)
+                            qSwap(redAlpha, blueAlpha);
+
                         QRgb currentRgb = dest[x];
                         dest[x] = qRgba(qRound(qRed(currentRgb) * (1.0 - averageAlpha) + averageAlpha * r),
                                         qRound(qGreen(currentRgb) * (1.0 - averageAlpha) + averageAlpha * g),
@@ -1052,10 +1061,14 @@ void QWindowsFontEngineDirectWrite::renderGlyphRun(QImage *destination,
                     BYTE *src = alphaValues + width * 3 * y;
 
                     for (int x = 0; x < width; ++x) {
-                        dest[x] = *(src + 0) << 16
-                                | *(src + 1) << 8
-                                | *(src + 2);
+                        BYTE redAlpha   = *(src + 0);
+                        BYTE greenAlpha = *(src + 1);
+                        BYTE blueAlpha  = *(src + 2);
+
+                        if (m_pixelGeometry == DWRITE_PIXEL_GEOMETRY_BGR)
+                            qSwap(redAlpha, blueAlpha);
 
+                        dest[x] = qRgb(redAlpha, greenAlpha, blueAlpha);
                         src += 3;
                     }
                 }

diff --git a/src/gui/text/windows/qwindowsfontenginedirectwrite_p.h b/src/gui/text/windows/qwindowsfontenginedirectwrite_p.h
@@ -136,6 +136,7 @@ class Q_GUI_EXPORT QWindowsFontEngineDirectWrite : public QFontEngine
     FaceId m_faceId;
     QString m_uniqueFamilyName;
     QList<QFontVariableAxis> m_variableAxes;
+    DWRITE_PIXEL_GEOMETRY m_pixelGeometry = DWRITE_PIXEL_GEOMETRY_RGB;
 };
 
 QT_END_NAMESPACE