Assign copies the bitmap image contained in Source to the bitmap object. If Source is not a bitmap, Assign calls the inherited Assign method, which can copy an image from any class that knows how to copy to a TBitmap object. If the bitmap needs to be changed, the actual bitmap image is copied before the changes are made (copy on write).
Note: An object of one type can always be assigned to another object of the same type. Also, the Source can be of type TPicture if the Graphic property of the picture is a bitmap.