fix Skia virtual device alpha blending (tdf#129865)

The blendBitmap()/blendAlphaBitmap() stuff coming from the OpenGL code
is some undocumented crazy stuff (probably because the VirtualDevice
alpha handling itself is rather crazy). Hopefully I've finally figured
it out to work properly for Skia too. This separate alpha handling
all over the place in VCL should be just nuked.

Change-Id: I82615a9be7064e9ade00ec4970a131a80a543c14
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/86488
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/vcl/backendtest/outputdevice/bitmap.cxx b/vcl/backendtest/outputdevice/bitmap.cxx
index 66a0a64adf49..2e7e0713294a 100644
--- a/vcl/backendtest/outputdevice/bitmap.cxx
+++ b/vcl/backendtest/outputdevice/bitmap.cxx
@@ -133,10 +133,13 @@ BitmapEx OutputDeviceTestBitmap::setupDrawBlend()
         aWriteAccess->DrawRect(tools::Rectangle(3, 3, 5, 5));
     }
 
-    initialSetup(13, 13, constBackgroundColor, false, true);
+    initialSetup(13, 13, COL_TRANSPARENT, false, true);
     mpVirtualDevice->SetFillColor(constBackgroundColor);
     mpVirtualDevice->SetLineColor(constBackgroundColor);
-    mpVirtualDevice->DrawRect(maVDRectangle);
+    // Leave the outer part of the device transparent, the inner part set to the background color.
+    // This will test blending of VirtualDevice's "alpha" device (outer yellow rectangle
+    // will be blended with transparent background, inner with the grey one).
+    mpVirtualDevice->DrawRect( tools::Rectangle( Point( 3, 3 ), Size( 7, 7 )));
 
     Point aPoint(alignToCenter(maVDRectangle, tools::Rectangle(Point(), aBitmapSize)).TopLeft());
 
@@ -179,9 +182,8 @@ TestResult OutputDeviceTestBitmap::checkBlend(BitmapEx& rBitmapEx)
 
     std::vector<Color> aExpected
     {
-        constBackgroundColor, constBackgroundColor,
-        aBlendedColor, constBackgroundColor, constBackgroundColor,
-        aBlendedColor, constBackgroundColor
+        COL_WHITE, COL_WHITE, COL_YELLOW, constBackgroundColor,
+        constBackgroundColor, aBlendedColor, constBackgroundColor
     };
     Bitmap aBitmap(rBitmapEx.GetBitmap());
     return checkRectangles(aBitmap, aExpected);