I'd like to create a rectangle (in a vector graphics format) that has a gradient in black/white. The difficult part here is that I'd want it to follow a curve similar to the gradient in the image below. The image has distinct borders/regions. Advice for creating both a gradient with distinct regions as well as a smooth gradient will be appreciated! I've not yet decided whether to go with distinct regions or not.
How do I do create a gradient like that in InDesign / Illustrator / Photoshop / Xara Designer Pro? I am not particularly skilled in any of the programs.