how to convert a gray level to another gray level in matlab?

i have an gray scale image.i want to change some gray level to another gray level.for example i want to convert all pixel with value 128 to 0 ,how can i do this?

Answer 1

[Walter Roberson]

my_image =imread('YourImage.tif');
my_image(my_image == 128) = 0;

[fariba amini]

thank you so much.it works correctly.thanks again

Answer 2

[Muhammad Usman Saleem]

my_image =imread('YourImage.tif');
for R=1:num of Rows
    for C=1:num of Columns
        pixel=my_image(R,C);
        if pixel ==128
        pixel=0;
    end
end

[Walter Roberson]

That will not save the result of the change to 0.