That's answered in the FAQ:
% Create a logical image of a circle with specified
% diameter, center, and image size.
% First create the image.
imageSizeX = 2048;
imageSizeY = 2048;
[columnsInImage rowsInImage] = meshgrid(1:imageSizeX, 1:imageSizeY);
% Next create the circle in the image.
centerX = 1024;
centerY = 1024;
radius = 1024;
circlePixels = (rowsInImage - centerY).^2 ...
+ (columnsInImage - centerX).^2 <= radius.^2;
% circlePixels is a 2D "logical" array.
% Now, display it.
image(circlePixels) ;
colormap([0 0 0; 1 1 1]);
title('Binary image of a circle');
Now that you have the circle mask, you can use it to blacken outside of the original RGB or gray scale image.
% Mask image by multiplying each channel by the mask.
maskedRgbImage = rgbImage .* cast(~mask, 'like', rgbImage); % R2016b or later. Works for gray scale as well as RGB Color images.
