Cropping a circle.

i am also solving a similar problem in which i need to draw a circle and make every pixel outside that circle to a same value (o or 1 doesn't matter). Any suggestions on ? Thanks in Advance

Answer 1

[Geoff Hayes]

gill11a - if your image is given by myImage then you could do something like the following to colour every pixel outside of the circle to be the same colour. Suppose that (a,b) is the centre of the circle with radius r. Then

function plotcircle(a,b,r)

close all;

% create a simple 100x200 image of random pixels
nrows = 100;
ncols = 200;
myImg = randi(255,nrows,ncols,3);
% iterate over each pixel and determine if it is outside the circle or not
for x=1:ncols
    for y=1:nrows
        if (x-a)^2 + (y-b)^2 > r^2
           % pixel is outside of the circle, so colour it blue
           myImg(y,x,:) = [0 0 255]; 
        end
    end
end
% display the image
image(myImg/255);
axis equal

Then you can call the above function as

 plotcircle(50,100,25)
 plotcircle(100,50,40)

Try the above and see what happens!

[bhagyashri kapre]

thank you Geoff Hayes,This code is use full in my project

Answer 2

[Image Analyst]

See the FAQ: http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F

Also see my attached demos on masking with a circle.

[gill11a]

Thanks for the prompt response. I think i forgot to mention one very important aspect of the problem: I do not know the radius of the circle i wish to crop. For eg. in the picture i attached, i want the program to identify the circles themselves, and then follow the procedure you described in your link.

[Image Analyst]

1. Take any of the color channels.
2. Take the middle row of the image.
3. Call diff() to see where the value changes.
4. Knowing the middle is at (number of columns)/2, compute radius.