How to compute disparity on a specific region of interest only?

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Is is possible, using stereo_image_proc, to compute disparities on a specific region of interest only?

The setup is as follow:

1. we detect a color blob in an image
2. we use stereo to determine its 3d position

It would greatly improve performances if we could just run the stereo on a subwindow. Is there a way to tell stereo_image_proc to do so?

We could probably just pass to stereo_image_proc a new pair of images matching the zone we want but it would be cumbersome to recompute the appropriate camera parameters and crop the images by hand...