how to convert the background(white) of a image to blaclk.

I obtained a image which is output of my intermediate process.It contains white background and small gray patches(grayscale image).I want the background to be black.Please suggest me a solution.

Answer 1

[Image Analyst]

You mean like this:

grayPatchGrayLevel = 150; % or whatever it is.
% Turn anything brighter than that to black.
grayImage(grayImage >= (grayPatchGrayLevel +1)) = 0;

Or, if you know for a fact that all your "white" is actually exactly 255, then:

grayImage(grayImage == 255) = 0;