Use buffer with alpha in VclProcessor2D::RenderUnifiedTransparencePrimitive2D

This allows UnifiedTransparencePrimitive2D to produce truly transparent image,
usable later in the stack - specifically by glow and soft edge effect.

Change-Id: I27e9bfbfb9c60856bc3fad1a483623c90d5cabff
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/94969
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/drawinglayer/source/processor2d/vclprocessor2d.cxx b/drawinglayer/source/processor2d/vclprocessor2d.cxx
index 60b372b0ef48..e64995c8b50a 100644
--- a/drawinglayer/source/processor2d/vclprocessor2d.cxx
+++ b/drawinglayer/source/processor2d/vclprocessor2d.cxx
@@ -841,13 +841,14 @@ void VclProcessor2D::RenderUnifiedTransparencePrimitive2D(
         // transparence is in visible range
         basegfx::B2DRange aRange(rTransCandidate.getChildren().getB2DRange(getViewInformation2D()));
         aRange.transform(maCurrentTransformation);
-        impBufferDevice aBufferDevice(*mpOutputDevice, aRange);
+        impBufferDevice aBufferDevice(*mpOutputDevice, aRange, true);
 
         if (aBufferDevice.isVisible())
         {
             // remember last OutDev and set to content
             OutputDevice* pLastOutputDevice = mpOutputDevice;
             mpOutputDevice = &aBufferDevice.getContent();
+            mpOutputDevice->Erase();
 
             // paint content to it
             process(rTransCandidate.getChildren());