[osg-users] about the state of the BVH plug-in, when used with CMU MoCap files

[Jan Ciger]

On Fri, Apr 1, 2016 at 4:45 PM, Christian Buchner
<christian.buchner at gmail.com> wrote:
>
>
> I've tried the OSG 3.2 branch and this version exhibited the same problem.
>
> Then I had a closer look at the BVH files supplied by that web site. I am finding that it defines
> rotational parameters always in the order Z, Y,  X.  That is different from the sequence I expect
> from various online sources that use Z, X, Y.
>
> ...
> I will have to do some more online research to understand whether the BVH spec actually allows
> to specify rotation in an arbitrary order and what the definitive answer about the sequence of applying
> the Euler angles is.
>

Actually, the order is not defined by the spec
(http://www.character-studio.net/bvh_file_specification.htm). Usually
it is assumed to be YXZ, but that's not always the case, the same as
there is no way to tell which way is "up" for the character :(

The only thing you can use to guess the order is the CHANNELS keyword
in the joint definition in the hierarchy part.

CHANNELS 3 Zrotation Xrotation Yrotation

This one defines the ZXY parsing order.  The problem is that some
software understands this as only the specification in which order are
the data written into the file, but not the order in which to compose
the transforms. So this, along with the z-up/y-up, which way is
forward, etc. must not be hardwired, because every mocap
system/animation software exports these files with different
conventions :(

Regards,

J.