clustering of intensities in an image by k-means clustering

I want to cluster the intensities of an image into three clusters: light color, middle and dark with similiar intensities. hw can I find the centroids and then wht abt the threshold. I've read a lot bt not able to implement it practically.this will be used in my research work. Is there anybdy who is understanding my problem. pls help me out.its urgnt.

Answer 1

[Image Analyst]

See this demo: http://www.mathworks.com/products/demos/image/color_seg_k/ipexhistology.html