Understanding .bvh (What does it mean)

This will be a rudimentary explination of what a .bvh file is and how it applies to Second life.

As most of you know BVH is a file format utilized by Biovisions Motion Capture program/software. This was and still is one of the Industry leading standards which many programs that deal in 3d animation still rely.

how is a .bvh file set up.

In the first segment of the file it is broken down into a hiearchy of offsets.

The way the offsets are read are in a Y-X-Z sequence.

the Heiarchy is as follows for Secondlife.

Root [hip] offset 0.00 0.00 0.00 this being the primary link parent the rest of the avatar follows the comands of the link structure from there as follows.

Note:This is the root or base set up frame that sets your avatar to its start point from there all follow on frames are animation frames only referenced by numeric ordering.


[hip]--(3) offset #s
......[abdomen]--(3) offset #s
.....................[collar]--(3) offset #s
..............................[left shoulder]--(3) offset #s
.................................................[left fore arm]--(3) offset #s
.................................................[left hand]--(3) offset #s
..............................[right shoulder]--(3) offset #s
.................................................[right fore arm]--(3) offset #s
.................................................[right hand]--(3) offset #s
......[left thigh]--(3) offset #s
.....................[left shin]--(3) offset #s
.....................[left foot]--(3) offset #s
......[right thigh]--(3) offset #s
.....................[right shin]--(3) offset #s
.....................[right foot]--(3) offset #s



What does this all mean.

Each body part has its offset from the parent root of the [hip] noted in Y X Z format Plus it has two additional offset numbers for the Y & X for rotation. Meaning in one Frame of an animation there can be up to 78 primary and secondary offsets.
But no less than 48 primary offsets if no rotations of any kind were used.

The offsets function primarly from the hip not from any base plane within Second life.

What does this mean. It means the Cartwheel and the other animations the run into the ground are being converted to where the hip is offsetting from the orginal start position that it lowers each time it moves. Can this be corrected.

(Yes) but trust me its tedious and very hard work. Unless of course you are used to cnc programing and love xyz numbers then it will be a cake walk for you.

Can the .bvh file be edited and manipulated by numbers. Yes, but be prepaired if you dont have a way to view it to be spending a lot of money uploading tests into SL. Oh and you will recieve some very WILD results if you do.

Now to explain in simplest of terms

Think of having a box in Second life. you put a sphere on each corner and one in the center.

each sphere on the corners have a relative offset from the one in the center. The one in the center defines the center location and the offsets of the ones on the corner are listed relative to that point.

IE you have a 2 meter box
each sphere on each corner is 1 meter x 1 meter y and 1 meter z away from the center thus your theoretical .bvh would look simlar to this. (this is also to show if the animation moved the box up 2 meters)


Root [center sphere] 0.00 0.00 0.00
............................[F left top corner sphere] -1.00 1.00 1.00
............................[F right top corner sphere] 1.00 1.00 1.00
............................[F left bot corner sphere] -1.00 1.00 -1.00
............................[F right bot corner sphere] 1.00 1.00 -1.00
............................[R left top corner sphere] -1.00 -1.00 1.00
............................[R right top corner sphere] 1.00 -1.00 1.00
............................[R left bot corner sphere] -1.00 -1.00 -1.00
............................[R right bot corner sphere] 1.00 -1.00 -1.00

(note not listed but I will bracket (Frame 1) and (Fame 2) for definition purpose only.)

(Frame 1) 0.00 1.00 0.00 -1.00 1.00 1.00 1.00 1.00 1.00 -1.00 1.00 -1.00 -1.00 -1.00 1.00 1.00 -1.00 1.00 -1.00 -1.00 -1.00 1.00 -1.00 -1.00
(Frame 2) 0.00 2.00 0.00 -1.00 1.00 1.00 1.00 1.00 1.00 -1.00 1.00 -1.00 -1.00 -1.00 1.00 1.00 -1.00 1.00 -1.00 -1.00 -1.00 1.00 -1.00 -1.00



This give you a preliminary Idea of how the .bvh file works the primary issue is each component as it moves from frame to frame will have its own dramatic offset and rotation variables applied allowing for motion.

For those of you following what I have written here thank you for understanding. For those that didnt feel free to ask questions here and I will try my best to clear up any misconceptions.

Thank you for your time and patience.

Sincerely, Shadow.