Problems with StereoCalibrate

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Thank you for your response!

I don't think the images are the problem. With the ones provided by OpenCV the result is even worse. Besides, the "normal" calibrate function works perfectly.

The stereo camera is custom made, but we paid much attention to the parallel alignment of the cameras. Also I thought, the distortion coefficients for each camera are computed independently of each other, aren't they?

I am using 52 images from each camera with the chessboard in nearly every possible position and orientation. I am afraid, for privacy reasons, I can't post the images, because they are not my property.

That is not the case. Im also on the "stereo business" :P and opencv clearly states that the default stereo calibration calibrates each camera and the stereo rig using the same images You have to make your own code in order to firstly calibrate each camera seperately (that means different images for each camera) and then calibrate the stereo rig. Also use the dotted pattern instead of the squares one, it produces better results. I managed to have an error of 0.3 pixels for each camera and a total of 0.4 pixels this way

Sorry I don't understand what you are saying. How is it possible not to use different images for each camera?

First step is to use the function calibrateCamera() for each of your cameras. So youll end up with intrinsic and extrinsic parameters for each camera seperately. Second step is to use the function stereoCalibrate() with the flag CV_CALIB_FIX_INTRINSIC enabled. This way it will only calculate the R,T,E and F parameters.

Inside the opencv documentation it clearly says that:

"due to the high dimensionality of the parameter space and noise in the input data, the function can diverge from the correct solution. If the intrinsic parameters can be estimated with high accuracy for each of the cameras individually (for example, using calibrateCamera() ), you are recommended to do so and then pass CV_CALIB_FIX_INTRINSIC flag to the function along with the computed intrinsic parameters. "

Oh I see, I somehow missed this paragraph in the documentation.

Now it works, though it does not make sense to me. However, thank you very much!

You're welcome. I guess calibrating each camera helps because you can cover the whole FOV of the camera. Calibrating it through a stereo rig decreases this area to just the common FOV of the stereo rig.