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problems with the ray in projectPixelTo3dRay

asked 2011-05-03 11:06:03 -0500

Javier Romero
66 ●2 ●5 ●8

updated 2014-01-28 17:09:37 -0500

ngrennan
1 ●1 ●1

According to the API, the return value is "the unit vector in the camera coordinate frame in the direction of rectified pixel" (from 1.4 is not unit, but with unit z component). I expected that multiplying such a vector by a scalar z would give me the 3D point with depth z that projects into the input pixel. However, when i back-project such a vector the result is not the input pixel:

cv::Point2d pixel(r,c);
cv::Point3d ray =  model.projectPixelTo3dRay(pixel);
cv::Point3d point = ray * z;
cv::Point2d back_pixel = model.project3dToPixel(point);

I would expect back_pixel be equal to pixel for any z, but it only happens for z=1. Have I misunderstood something?

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answered 2011-05-03 12:34:50 -0500

Javier Romero
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OK, it's pretty straightforward if you look at the code of the functions. The problem is that I was looking to the second camera of a stereo pair, and therefore the components Tx is non-zero. Therefore you have to make a couple of translations:

cv::Point2d pixel(r,c);
cv::Point3d ray =  model.projectPixelTo3dRay(pixel);
float tx_fx = model.Tx()/model.fx();
cv::Point3d point(z*(ray.x+tx_fx)-tx_fx,z*ray.y,z*ray.z);
cv::Point2d back_pixel = model.project3dToPixel(point);

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Asked: 2011-05-03 11:06:03 -0500

Seen: 1,465 times

Last updated: May 03 '11

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problems with the ray in projectPixelTo3dRay