explain the intent of HasDrawTransformedBitmap() better

Change-Id: I32d6cfb7358dae25109de4db3332797763abc7d8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/126506
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

2 files changed, 9 insertions, 4 deletions

diff --git a/vcl/inc/salgdi.hxx b/vcl/inc/salgdi.hxx
index 716c9aa934a6..db3ed6806f7e 100644
--- a/vcl/inc/salgdi.hxx
+++ b/vcl/inc/salgdi.hxx
@@ -600,7 +600,12 @@ protected:
                                     const SalBitmap* pAlphaBitmap,
                                     double fAlpha) = 0;
 
-    /// Used e.g. by canvas to know whether to cache the drawing.
+    /// Returns true if the drawTransformedBitmap() call is fast, and so it should
+    /// be used directly without trying to optimize some calls e.g. by calling drawBitmap()
+    /// instead (which is faster for most VCL backends). These optimizations are not
+    /// done unconditionally because they may be counter-productive for some fast VCL backends
+    /// (for example, some OutputDevice optimizations could try access the pixels, which
+    /// would make performance worse for GPU-backed backends).
     /// See also tdf#138068.
     virtual bool hasFastDrawTransformedBitmap() const = 0;
 
diff --git a/vcl/source/outdev/bitmapex.cxx b/vcl/source/outdev/bitmapex.cxx
index 7855d1ae5613..55a166c6e5ba 100644
--- a/vcl/source/outdev/bitmapex.cxx
+++ b/vcl/source/outdev/bitmapex.cxx
@@ -525,6 +525,8 @@ void OutputDevice::DrawTransformedBitmapEx(
     if(rtl::math::approxEqual( fAlpha, 1.0 ))
         fAlpha = 1.0; // avoid the need for approxEqual in backends
 
+    // If the backend's implementation is known to not need any optimizations here, pass to it directly.
+    // With most backends it's more performant to try to simplify to DrawBitmapEx() first.
     if(bTryDirectPaint && mpGraphics->HasFastDrawTransformedBitmap() && DrawTransformBitmapExDirect(aFullTransform, bitmapEx))
         return;
 
@@ -562,11 +564,9 @@ void OutputDevice::DrawTransformedBitmapEx(
         return;
     }
 
+    // Try the backend's implementation before resorting to the slower fallback here.
     if(bTryDirectPaint && DrawTransformBitmapExDirect(aFullTransform, bitmapEx))
-    {
-        // we are done
         return;
-    }
 
     // take the fallback when no rotate and shear, but mirror (else we would have done this above)
     if(!bRotated && !bSheared)