Why depth is divided by 5000 in slambook2/ch7/pose_estimation_3d2d.cpp in PnP Code implemantation?

[fettahyildizz]

In order to find 3d feature points coordinates in world, we divide depth by 5000. What is the reason for that? What are the units of 3d coordinates (mm, cm, m) ? I want to implement PnP algorithm using Intel RealSense D435 but I am not sure how much it should be divided by?

[nicolasrosa]

If I remember, this value is the conversion value between the depth values (float, metric) and the values saved on the PNG (16-bits, int, [0, 65535]) from TUM Dataset.

[fettahyildizz]

I have a depth value int, milimeter. When the point is 1 meter away from camera, it gives me 1000 depth value. What kind of conversion do I need?
@nicolasrosa

[nicolasrosa]

Was this the value read directly from the 16-bit png image? In your case, I believe you already are in the metric system, I can do the calculation in millimeters if you want. I would not recommend that for numeric reasons (avoid calculation with large numbers).

Just pay attention during the conversion process of pixel coordinates to metric coordinates. Besides, always check the units of the calibration parameters of the dataset you are using.

The following link may help you.

https://vision.in.tum.de/data/datasets/rgbd-dataset/file_formats

[gaoxiang12]

@fettahyildizz
The depth unit of the example picture used in our book is 5000 so we should divide the depth reading by 5000 here. The image is from TUM rgb-d dataset, which is captured by Kinect 1.
Most of the rgb-d cameras have depth unit settings in their API/SDK, and most of them use 1000 as the depth unit. I'm not sure the exact unit of RealSense, please check its datasheet or documents.

[fettahyildizz]

@nicolasrosa @gaoxiang12 Thanks for such a quick response. I am really grateful for your effort.
After dividing by 1000, my 3dpoints vector holds values for 3d coordinates in meter unit. For example if a point is 1 meter away from camera and point is in same x and y plane from camera, it gives me [0, 0, 1] point, which I assume this value is correct for calculations.