How are landmark coordinates really meant to be translated?

stephendbuchanan

I have a project that requires 3D pose detection, in real world coordinates, and I am basing my work off the DepthAI Blazepose project (geaxgx/depthai_blazepose). While browsing the code, I came across this curious comment block:

Beware, the y value of landmarks_world coordinates is negative for landmarks above the mid hips (like shoulders) and negative for landmarks below (like feet). The y value of (x,y,z) coordinates given by depth sensor is negative in the lower part of the image and positive in the upper part.

Is there a mistake in the first sentence? How can Y coordinates be negative for both landmarks below AND above the mid hips? Should the sentence not read:

...the y value of landmarks_world coordinates is negative for landmarks above the mid hips (like shoulders) and POSITIVE for landmarks below (like feet)

jakaskerl

stephendbuchanan
Yep, it's a typo. The center should be set as the coordinate origin.

Thanks,
Jaka

stephendbuchanan

jakaskerl

Thank you, but if it is a typo, which part needs to change? "The centre should be the origin" is not the issue - the confusion is which direction is positive? Currently, the first sentence states that landmarks ABOVE and BELOW the mid hips are BOTH NEGATIVE. Which is impossible. So which direction is in fact negative and which is meant to be positive?

jakaskerl

stephendbuchanan
The spatial location calculator uses left handed system, so Y+ would point UP. How this was implemented by geaxgx, I am unaware, but left handed system is generally what you use for localization purposes on an image. Z points forward, X right and Y up.

Thanks,
Jaka