https://imagemagick.org/discourse-server/posting.php?mode=reply&f=3&t=3651

diff --git a/MagickCore/fx.c b/MagickCore/fx.c
index 2f151d2fbd2a1362a2fbd18b7cc8cbe9b5dc8519..3d74b07d33adcae42a14c8d12692575c7fa1648d 100644 (file)
--- a/MagickCore/fx.c
+++ b/MagickCore/fx.c
@@ -5176,6 +5176,8 @@ MagickExport Image *SwirlImage(const Image *image,double degrees,
       swirl_image=DestroyImage(swirl_image);
       return((Image *) NULL);
     }
+  if (swirl_image->background_color.alpha_trait != UndefinedPixelTrait)
+    (void) SetImageAlpha(swirl_image,OnAlphaChannel,exception);
   /*
     Compute scaling factor.
   */