Merge pull request #312 from asam-ev/303-root-bone-in-osi

Add section about root bone in combination with OSI

content/geometry/object-human/human-general.adoc

@@ -18,6 +18,13 @@ In the coordinate system for human 3D geometry, the x-axis points forwards, the
 The default pose is the A-pose.
 The naming convention makes it possible to mirror one side of the skeleton to the other.
 The bones follow a specific bone orientation: The y-axis of a bone always follows the bone direction, regardless of whether it points sidewards, upwards, or forwards. The x-axis usually faces forwards accordingly.
+A bone's name, position, and orientation is defined by the end-point closer in the hierarchy to the root.
+For example, the "Lower_Arm_Left" defines the point in the left elbow of the model.
+
+The skeleton starts with a root bone positioned at the origin of the asset, the center of the bounding box projected to the ground.
+This position is static and does not change relative to the asset during simulation.
+When using an ASAM OpenMATERIAL 3D human asset in combination with ASAM OSI, the OSI field https://opensimulationinterface.github.io/osi-antora-generator/asamosi/latest/gen/structosi3_1_1MovingObject_1_1PedestrianAttributes.html#a75db7466abca2d283c8d64d424385c0a[bbcenter_to_root] must be set to the vector from the center of the bounding box to the center of the ground projection of the bounding box.
+This specifically means: x=0, y=0, z=-half of the bounding box height.
 
 <<fig-human-structure>> shows the orientation of the coordinate system of a human skeleton.
 
@@ -26,5 +33,4 @@ The bones follow a specific bone orientation: The y-axis of a bone always follow
 image::fig_human-structure.svg[,400]
 
 Bones with a local transform are indicated in the structure by a (T).
-
-This is only an indicator in the documentation and is not necessarily contained in the actual node name.
+This is only an indicator in the documentation and must not be contained in the actual node name.