Thread Subject: how to take live histogram on a live video?

I've already implemented object tracking using Gaussian mixture model. Now i need to take the hue histogram of the tracked object i.e the object inside rectangle. The tracking is real time. Then how can take only tracked object's histogram in real time. I've already seen 'live histogram' in matlab demos, but it gives for whole image. Can any one help?

Also I need to save these histogram values. Which format is suitable, so that I can retrieve it back for further processing.