image processing from webcam

Hi,I have something to ask you.I have an image with a large rectangle and inside that rectangle there are a red circle and a green circle.And I want to locate the center of red circle with the origin-coordinate is one of the corners of rectangle.Everything has been finished with an image I draw in power point.But when I draw that image on a real paper and using a webcam to capture that image,I cann't do the same things as I do with ppt image.When I convert the real image to binary,the rectangle,in some parts,is not separated with background and some areas of background dont't have the same color(black or white) with others.I try to reduce threshold but it is still bad.I just want to separate the rectangle with background to "label" it and then I do anything I do with ppt image.Can you tell me how can I do that.Sorry because my bad english.

[nghia nguyen]

here are image links:

http://www.flickr.com/photos/48230201@N02/5653553606/in/photostream

http://www.flickr.com/photos/48230201@N02/5652986417/in/photostream/

Answer 1

[Sean de Wolski]

I would do this with:

I = imread('answers425.jpg'); %your_image
M = shrinkWrap(I(:,:,1),'objthresh',3000,'biggest'); %shrinkwrap it and keep only biggest object
subplot(121)
imshow(M); %show mask
subplot(122)
imshow(bsxfun(@times,uint8(M),I)); %show mask applied

FEX shrinkWrap

The result I attain:

[nghia nguyen]

Thanks a lot.But I also want to keep the rectangle from the original image.How can I do that

[nghia nguyen]

My main goal is just retain the rectangle and two circles and erase everything else.Like this:

http://www.flickr.com/photos/48230201@N02/5654383584/in/photostream

[Sean de Wolski]

Perhaps use the edge function?

[nghia nguyen]

I try to use canny method but there are so many noise.I don't know what should I do to filter those noise and how can I determine where is the rectangle's edge or circle's edge.

[nghia nguyen]

Here is the image after using canny method:

http://www.flickr.com/photos/48230201@N02/5655801861/in/photostream

And can u tell me how can I post directly images instead of links