# MATLAB Simulate 3D Camera: why is there no focal length (world units) attribute in the sensor model?

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Ram Bhaskara on 31 May 2022
Edited: Ram Bhaskara on 2 Jun 2022
In the 3D camera simulation (https://www.mathworks.com/help/driving/ref/simulation3dcamera.html) - there is no specific focal length (mm) for the camera intrinsics. I'm working on feature tracking using the camera feed and I cannot estimate the 3D location of a point without knowing the focal length or the scaling factor (w) in (https://www.mathworks.com/help/vision/ug/camera-calibration.html).
It would also be great if MATLAB provides a DEM of the UAV scenarios - ground truth for many applications.

Qu Cao on 1 Jun 2022
If you know the size of the pixel in world units, you can convert focal length in world units (usually mm) to focal length in pixels.
Qu Cao on 1 Jun 2022
Given the following equations:
X = (u - cx) * Z / fx;
Y = (v - cy) * Z / fy;
where [X, Y, Z] is the 3-D point location, [u, v] is the corresponding image point location, and [fx, fy] is the focal lengths. You should be able to compute the factor you were talking about. Note that the Depth port outputs the Z values.
Ram Bhaskara on 2 Jun 2022
Edited: Ram Bhaskara on 2 Jun 2022
For my analysis, I don't have depth info directly - but I'm computing the depth (approximately) from knowing my UAV altitude (say from GPS). If I have my feature's pixel coordinates [u, v] - I am aiming to compute the same feature coordinates in scene units. That's all I need - a vector from the optical center (not frame center) to the feature coordinates. The z-component of this vector is the focal length in mm.
P{camera frame} = [ (cx - u) * f / fx (cy - v) * f / fy f ]
From the intrinsics I devise in MATLAB simulate 3D camera, how do I get to this P (in the camera's reference frame)?

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