# Using cv::solvePnP on Lighthouse Data

Hello,

I've got a project going, were I try to get the pose of a 3d-tracker, which utilizes the lighthouse basestations from Valve.

The basestations provide laser-sweeps across the tracking-volume and my tracker records the timings when a laser-plane hits one of its ir-sensors. These timings can then be converted into degrees, based on the fact that the laser-planes rotate at exactly 3600RPM.

Since I know exactly where my sensors are placed on the tracker I should be able to get the pose using the `cv::solvePnP`

function.

But I can't figure out what kind of camera-matrix and distortion coefficients I should use.
Since a basestation has neither a lens nor a 2d-image-sensor I can't think of a way to calculate the focal-length needed for the camera-matrix.

First I've tried the `imagewidth/2 * cot(fov/2)`

formula, assuming an "image width" of 120, since this is the "domain" of my readings, which leads to a focal-length of 34.641px. But the results were completely off.
I've then tried to calculate a focal length for a given scenario (tracker 1m infront of the basestation) which gave me a focal-length of 56.62px. If I place my tracker about 1 meter in front of a basestation the results are plausible but if I move away from that "sweetspot" the results are again completely off.

But since I have no lens there should be no distortion, or am I wrong about that?

If anyone could give me a hint I would be very grateful.

If you have the direction in degrees, you're already "past" the camera matrix, as it were. I'll say more later, sorry.

@RupertVanDaCow, I am working on almost exactly the same thing as you, i'm getting crazy Z values that are way off in distance, but the general X/Y positions appear good to me.. Rotations I haven't even looked at yet, but they look crazy too. How are you converting the laser sweep time to an "image point" for feeding into SolvePNP? I'm basically saying the "camera" is 0-180 degree field of view, and tick times from sync pulse to laser are some small value, like, 90.0868 (out of 180) for about the "middle" of the image. Can you help me with getting the data from sweep times to solvePNP?