docview: greatly improve scrolling speed when help topic contains images

We always did StretchDraw() which is rather CPU intensive. Now we check
the image dimensions first, then decide if StretchDraw() is really
needed or not. If not, then do the much faster DrawImage() call.

1 files changed, 13 insertions, 4 deletions

diff --git a/docview/components/richtext/RichTextDisplayUnit.pas b/docview/components/richtext/RichTextDisplayUnit.pas
index 2e62a511..482a587b 100644
--- a/docview/components/richtext/RichTextDisplayUnit.pas
+++ b/docview/components/richtext/RichTextDisplayUnit.pas
@@ -231,10 +231,19 @@ ProfileEvent('DEBUG:  DrawRichTextLine >>>');
             BitmapRect.Bottom := Trunc(BitmapRect.Top
                               + Bitmap.Height * Layout.VerticalImageScale);
 
-            FontManager.Canvas.StretchDraw(BitmapRect.Left, BitMapRect.Top,
-                BitmapRect.Right-BitMapRect.Left, BitMapRect.Bottom-BitMapRect.Top, Bitmap);
-
-            inc( X, trunc( Bitmap.Width * Layout.HorizontalImageScale ) );
+            if ((BitMapRect.Right - BitMapRect.Left) = Bitmap.Width) and
+               ((BitMapRect.Bottom - BitMapRect.Top) = Bitmap.Height) then
+            begin
+              // no stretching required
+              FontManager.Canvas.DrawImage(BitmapRect.Left, BitMapRect.Top, Bitmap);
+              inc(X, Bitmap.Width);
+            end
+            else
+            begin
+              FontManager.Canvas.StretchDraw(BitmapRect.Left, BitMapRect.Top,
+                  BitmapRect.Right-BitMapRect.Left, BitMapRect.Bottom-BitMapRect.Top, Bitmap);
+              inc(X, trunc(Bitmap.Width * Layout.HorizontalImageScale));
+            end;
           end;
         end
         else