How can I remove the background from the image?

I made a phantom image in Matlab by using this function P=phantom(256); figure, imshow(P);

if I want to remove the background color " Black " and keep the remaining picture, what can I do to remove it or make it as transparent image ???

Answer 1

[Image Analyst]

You cannot "remove" the black (outside the white ring) because an image must remain rectangular. The best you can do is to crop it to the bounding rectangle. Do you want to do that? Anyway, there is no need to remove it. You can measure whatever you want on the rest of the regions in the image without "removing" the black pixels. What do you want to do anyway? I can show you how you can do it without removing the black or making it transparent if you'll only tell me. If you want you can replace the black with something else, like a uniform gray level or even some other image. I can show you how to do that if you provide the gray level or the other background image.

[omar Alwan]

I tried to crop the image in rectangle, but it is insufficient. My project is to make a simulation projection of the image and reconstruct it as CT-scan. So I'm trying to remove the black pixels, make it transparent, or at least, crop the image to the bounding circle even if there are remaining black pixels. My goal is to make the image in the circular appearance as possible.

[Image Analyst]

Same answer. You cannot have circular images. You cannot do what you want to do. You can crop (which you say is "insufficient" for some reason) but it's really not necessary. Why do you think it is?

Why do you have a goal to make the image perfectly circular? What good will that do?

[Brian Peoples]

My question is similar @Image Analyst if you see my most recent post @ https://www.mathworks.com/matlabcentral/answers/555364-how-to-remove-black-background-from-photo-only-keep-region-of-interest

but I am okay with only using the area of interest for getting the average lightness values I desire and keeping the black background if necessary.