OpenCV 2.3 camera calibration

camera-calibration, computer-vision, matlab, opencv, vision

Solution

First off, your camera matrix is wrong. If you read the documentation, it should look like:

fx  0 cx
 0 fy cy
 0  0  1

If you look at yours, you've got it the wrong way round:

fx  0  0
 0 fy  0
cx cy  1

So first, set `camera_matrix` to `camera_matrix.T` (or change how you construct `camera_matrix`. Remember that `camera_matrix[i,j]` is row `i`, column `j`).

camera_matrix = camera_matrix.T

Next, I ran your code and I see that "can't seem to get it to work" means the following error (by the way - always say what you mean by "can't seem to get it to work" in your questions - if it's an error, post the error. If it runs but gives you weirdo numbers, say so):

OpenCV Error: Assertion failed (ni >= 0) in collectCalibrationData, file /home/cha66i/Downloads/OpenCV-2.3.1/modules/calib3d/src/calibration.cpp, line 3161
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
cv2.error: /home/cha66i/Downloads/OpenCV-2.3.1/modules/calib3d/src/calibration.cpp:3161: error: (-215) ni >= 0 in function collectCalibrationData

I then read the documentation (very useful by the way) and noticed that `obj_points` and `img_points` have to be vectors of vectors, because it is possible to feed in sets of object/image points for multiple images of the same chessboard(/calibration points).

Hence:

cv2.calibrateCamera([obj_points], [img_points],size, camera_matrix, dist_coefs)

What? I still get the same error?!

Then, I had a look at the OpenCV python2 samples (in the folder `OpenCV-2.x.x/samples/python2`), and noticed a `calibration.py` showing me how to use the calibration functions (never underestimate the samples, they're often better than the documentation!).

I tried to run `calibration.py` but it doesn't run because it doesn't supply the `camera_matrix` and `distCoeffs` arguments, which are necessary. So I modified it to feed in a dummy `camera_matrix` and `distCoeffs`, and hey, it works!

The only difference I can see between my `obj_points`/`img_points` and theirs, is that theirs has `dtype=float32`, while mine doesn't.

So, I change my `obj_points` and `img_points` to also have dtype float32 (the python2 interface to OpenCV is funny like that; often functions don't work when matrices don't have a `dtype`):

obj_points = obj_points.astype('float32')
img_points = img_points.astype('float32')

Then I try again:

>>> cv2.calibrateCamera([obj_points], [img_points],size, camera_matrix, dist_coefs)
OpenCV Error: Bad argument 
(For non-planar calibration rigs the initial intrinsic matrix must be specified) 
in cvCalibrateCamera2, file ....

What?! A different error at least. But I did supply an initial intrinsic matrix!

So I go back to the documentation, and notice the `flags` parameter:

flags – Different flags that may be zero or a combination of the following values:

`CV_CALIB_USE_INTRINSIC_GUESS` cameraMatrix contains valid initial values of fx, fy, cx, cy that are optimized further

...

Aha, so I have to tell the function explicitly to use the initial guesses I provided:

cv2.calibrateCamera([obj_points], [img_points],size, camera_matrix.T, dist_coefs,
                    flags=cv2.CALIB_USE_INTRINSIC_GUESS)

Hurrah! It works!

(Moral of the story - read the OpenCV documentation carefully, and use the newest version (i.e. on opencv.itseez.com) if you're using the Python `cv2` interface. Also, consult the examples in the `samples/python2` directory to supplement the documentation. With these two things you should be able to work out most problems.)

Problem

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