StereoBM_create.compute() with different images shape

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first, you're supposed to run blockmatching on images from a calibrated stereo pair (and also, that you undistort the images, using the params from the calibration), it seems, you're not doing so. using arbitrary, "found" images won't give you any accuracy here, unfortunately.

1. no, not possible (just believe the error msg.)
2. no, not a valid idea , either. whatever you do to the images, it will change the relative position captured in them, and make the disparity worse, again see the comment above.
3. ofc. there are alternative ways to acquire depth images of some sort, use specialized cameras (kinect, etc), images from multiple poses (structure from motion), if you can control the lighting, and you're focussed on fairly small, indoor objects, also "structured light / threephase" and "photogrammetric stereo".