Why the P is not identity?

[limhyon]

D: [-0.358561237166698, 0.149312912580924, 0.000484551782515636, -0.000200189442379448, 0.0] K: [615.652408400557, 0.0, 362.655454167686, 0.0, 616.760184718123, 256.67210750994, 0.0, 0.0, 1.0] R: [1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0] P: [525.978149414062, 0.0, 357.619310580667, 0.0, 0.0, 579.796447753906, 258.377118195804, 0.0, 0.0, 0.0, 1.0, 0.0]

Hi, In your example bag file, why the P is not identity? (or it should have same number as K I think) Usually, only K is effective, however, the P is not similar with K.

For example, http://wiki.ros.org/camera_calibration/Tutorials/MonocularCalibration, the P is identity.

[vgrabe]

Hi Lim,

it is great to hear from you, it has been quite some time!

That wiki page seams to be outdated - the calibration tool does provide an estimate for P now. And yes, P would be equal to K if you moved that "scale" slider all the way to the right. In that case, you have black borders but use the entire image. However, when using the default settings (slider all the way to the left), the camera zooms in to avoid black borders and P starts to differantiate from K. Therefore, in particular for lenses with a wide FoV, P will not equal K unless black image borders are desired. If you choose to let ROS rectify an image for you, it will respect P and widly ignore K.

Let me know if that helps.

Another note: The version of the ros calibration tool that is currently available on the repository for Ubuntu 14.04. is quite broken. However, all major bugs have been fixed in the source code version.

Cheers, Volker

[Fromandto]

Hello Volker,

I've been trying to figure this out the whole day, thank god there's already an issue about it.

The difference between P and K in function LEDDetector::distortPoints() confuses me still.

————————————————————————————————————————

const cv::Point2d &p = src[i]; double x = (p.x - cx_P) / fx_P; double y = (p.y - cy_P) / fy_P; double xCorrected, yCorrected;

// Correct distortion
{
  double r2 = x * x + y * y;

  // Radial distortion
  xCorrected = x * (1. + k1 * r2 + k2 * r2 * r2 + k3 * r2 * r2 * r2);
  yCorrected = y * (1. + k1 * r2 + k2 * r2 * r2 + k3 * r2 * r2 * r2);

  // Tangential distortion
  xCorrected = xCorrected + (2. * p1 * x * y + p2 * (r2 + 2. * x * x));
  yCorrected = yCorrected + (p1 * (r2 + 2. * y * y) + 2. * p2 * x * y);
}

// Project coordinates onto image plane
{
  xCorrected = xCorrected * fx_K + cx_K;
  yCorrected = yCorrected * fy_K + cy_K;
}
dst.push_back(cv::Point2d(xCorrected, yCorrected));

————————————————————————————————————————

In the codes above, P is used for unprojecting but K for projecting.

Why ?

If the difference between P and K is just 'black borders', I think we should use same matrix for unprojection and projection.

Or say, what will happen (what will become inaccurate) if I use the parameters in K for both unprojecting and projecting?

THX in advance

[eliasm]

The projection matrix is indeed not required. We solved this here https://github.com/uzh-rpg/rpg_monocular_pose_estimator/pull/7.

[mfaessle]

Thanks @eliasm for looking into this.