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    <p>First off, your camera matrix is wrong. If you read the <a href="http://opencv.itseez.com/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html#calibratecamera" rel="noreferrer">documentation</a>, it should look like:</p> <pre><code>fx 0 cx 0 fy cy 0 0 1 </code></pre> <p>If you look at yours, you've got it the wrong way round:</p> <pre><code>fx 0 0 0 fy 0 cx cy 1 </code></pre> <p>So first, set <code>camera_matrix</code> to <code>camera_matrix.T</code> (or change how you construct <code>camera_matrix</code>. Remember that <code>camera_matrix[i,j]</code> is <em>row</em> <code>i</code>, <em>column</em> <code>j</code>).</p> <pre><code>camera_matrix = camera_matrix.T </code></pre> <p>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):</p> <pre><code>OpenCV Error: Assertion failed (ni &gt;= 0) in collectCalibrationData, file /home/cha66i/Downloads/OpenCV-2.3.1/modules/calib3d/src/calibration.cpp, line 3161 Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; cv2.error: /home/cha66i/Downloads/OpenCV-2.3.1/modules/calib3d/src/calibration.cpp:3161: error: (-215) ni &gt;= 0 in function collectCalibrationData </code></pre> <p>I then read the <a href="http://opencv.itseez.com/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html#calibratecamera" rel="noreferrer">documentation</a> (very useful by the way) and noticed that <code>obj_points</code> and <code>img_points</code> have to be <strong>vectors of vectors</strong>, because it is possible to feed in sets of object/image points for multiple images of the same chessboard(/calibration points).</p> <p>Hence:</p> <pre><code>cv2.calibrateCamera([obj_points], [img_points],size, camera_matrix, dist_coefs) </code></pre> <p>What? I still get the same error?!</p> <p>Then, I had a look at the OpenCV python2 samples (in the folder <code>OpenCV-2.x.x/samples/python2</code>), and noticed a <code>calibration.py</code> showing me how to use the calibration functions (never underestimate the samples, they're often better than the documentation!).</p> <p>I tried to run <code>calibration.py</code> but it doesn't run because it doesn't supply the <code>camera_matrix</code> and <code>distCoeffs</code> arguments, which are necessary. So I modified it to feed in a dummy <code>camera_matrix</code> and <code>distCoeffs</code>, and hey, it works!</p> <p>The only difference I can see between my <code>obj_points</code>/<code>img_points</code> and theirs, is that theirs has <code>dtype=float32</code>, while mine doesn't.</p> <p>So, I change my <code>obj_points</code> and <code>img_points</code> 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 <code>dtype</code>):</p> <pre><code>obj_points = obj_points.astype('float32') img_points = img_points.astype('float32') </code></pre> <p>Then I try again:</p> <pre><code>&gt;&gt;&gt; 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 .... </code></pre> <p>What?! A different error at least. But I <em>did</em> supply an initial intrinsic matrix!</p> <p>So I go back to the documentation, and notice the <code>flags</code> parameter:</p> <blockquote> <p>flags – Different flags that may be zero or a combination of the following values:</p> <p><code>CV_CALIB_USE_INTRINSIC_GUESS</code> cameraMatrix contains valid initial values of fx, fy, cx, cy that are optimized further</p> <p>...</p> </blockquote> <p>Aha, so I have to tell the function <em>explicitly</em> to use the initial guesses I provided:</p> <pre><code>cv2.calibrateCamera([obj_points], [img_points],size, camera_matrix.T, dist_coefs, flags=cv2.CALIB_USE_INTRINSIC_GUESS) </code></pre> <p>Hurrah! It works!</p> <p>(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 <code>cv2</code> interface. Also, consult the examples in the <code>samples/python2</code> directory to supplement the documentation. With these two things you should be able to work out most problems.)</p>
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    1. COthank you kind sir. I had found the "vectors of vectors" in the documentation, but couldn't find a good grasp of what that meant.
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    2. CO@mathematical.coffee: Can you point me to the algorithm (paper) used in OpenCV calibration for non-coplanar rigs? I believe it is taken from Matlab camera calibration toolbox by Bouguet [link](http://www.vision.caltech.edu/bouguetj/calib_doc/htmls/ref.html). But, I am not sure whether it is Tsai's algorithm or Heikkila's algorithm?
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    3. CO@SatishKumar, I wouldn't have a clue what they use. [The documentation](http://docs.opencv.org/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html?highlight=calibratecamera#bouguetmct) has a link to the Bouget Matlab stuff. Perhaps you could read through the source code to see.
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