applying kmeans for ROI in Image

I have a image below

http://imgur.com/RsRHy2V

here i have detected surf features and have draw circle over it,now i want to apply k means for that surf features inside that blue circle,please assist

for whole image i used code

cform = makecform('srgb2lab');
lab_he = applycform(I,cform);

he=I;
ab = double(lab_he(:,:,2:3));
nrows = size(ab,1);
ncols = size(ab,2);
ab = reshape(ab,nrows*ncols,2);

nColors = 3;
% repeat the clustering 3 times to avoid local minima
[cluster_idx cluster_center] = kmeans(ab,nColors,'distance','sqEuclidean', ...
                                      'Replicates',3);

but how to apply for surffeatures inside circle

[Lester Lim]

http://www.princeton.edu/~kung/ele571/571-MatLab/571BP_Chad/kmeans.m

[FIR]

how to select those surf features

[Lester Lim]

use roipoly, after drawing the ROI, double click it to give mask...

[FIR]

but rioploy gives a binary image , i need to apply k means surf features

Answer 1

[Image Analyst]

Get a mask for each circle (say by using code from the FAQ) and then get the pixel values inside by doing:

pixelValuesInsideCircle = grayImage(circleMask);