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>

1 files changed, 6 insertions, 1 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;