You actually found a bug, it's not a feature. It should be white instead of transparent. This will be fixed in the next release. You can do this:
I will create a static property MagickColor.Transparent so you won't have to use System.Drawing.
using (MagickImage image = new MagickImage(sPath + "Render3D1.jpg") { image.Resize(300, 300); image.Format = MagickFormat.Png; // There is an implicit cast from System.Drawing.Color to MagickColor. image.Extent(300, 300, Gravity.Center, System.Drawing.Color.Transparent); image.Write(sPath + "Render3D1_300_WthGeometry.png"); }