Stereo SGBM for single pixel

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I am working in a stereo vision system with epipolar geometrics within 10º angles between cameras.

I am trying to figure out how to get the position (throught disparity) of a ROI of my image which I get using open tld system.

With the extrinsic and intrinsic matrix of my system, I prepare and launch ssgbm in order to get the disparity map. Finally, with this disparity and my Q matrix, I calculate the position of the center of my ROI and in fact, my object.

The point is, that now that I have completed all this integration, I am having timing issues.

For now, I am trying to find a way to calculate the disparity with this method only for this pixels. I have been studying the code of the opencv Stereo SGBM algorithm implementation, I was thinking, maybe I would be able to modify it to my purpose, but I was wrong and now I am more confused.

Could anyone give me a clue or tell me if I there is a easier way?

What I have:

• I have passed throught all the stereocalibration and stereorectification steps.
• I am able to retrieve the position of an object in real world coordinates with origin in left.

What I want:

• Reach a faster implementation of this method, by calculating only Stereo SGBM of my ROI discarding the rest of the images disparities which I don't mind.

Thanks for your attention and help.