Subject: Bones lessons learned
Date: Wed, 18 May 94 14:43:00 PDT
From: "Decker, Mark" <Decker@segaoa.COM>
First, thanks to Lesk for his bones example description. I read through it
yesterday and went home to try it out from memory when I finally got back to
my Amiga. I stumbled a couple times in the process, but I learned a few
things that might help somebody else along.
1. When grouping axes together into bones, make sure you are in Pick
Objects mode, not Pick Groups.
2. Make sure you are using the STATES Group command, not the Object Group
command.
3. Once you are in Pick Objects, pick first the parent axis, then the
child axis. In Lesk's tube example, if the axes are numbered from top to
bottom, first pick axis number two, then hold down shift and pick axis
number one and group. Next pick axis number three, then hold down shift and
pick axis number 2, and so on down, selecting the object itself followed by
the "root" bone and grouping them last. When I first tried this I was using
the bounding box to pick both axes at once, and I think it may have been
picking them in the wrong order.
4. It helps immensely to move any axes out of the way (shift M moves the
axis of an object but leaves the object in place) so that they do not
overlap while grouping. Once the grouping is correct, you can move the axis
back into place without affecting the grouping.
5. Subgroup assignment is probably the trickiest procedure and was at the
start the hardest part for me to grasp. It has not been well explained to
date, but I'll see if I can help without muddying the waters any further
:-). Each axis gets two subgroups assigned to it, helpfully referred to as
"Big" and "Small". To the best of my understanding, the Big subgroup is the
set of all faces which will be affected by motion of the axis. If a face is
not in the Big subgroup, its never going to change no matter what that
particular axis does. The Small subgroup is a subset of the Big subgroup,
which means it can only contain faces which are also in the Big subgroup,
but usually won't contain all of them. The Small subgroup moves and rotates
with the axis, but all of its faces keep their shape and orientation with
respect to each other.
So if the faces outside the Big subgroup don't change, and the faces
inside the Small subgroup don't change shape, all that's left is the faces
which ARE in the Big subgroup, but ARE NOT in the Small subgroup. These
faces form the actual joint, and actually stretch and deform to keep the
other two sets (the outside and the Small subgroup) smoothly connected.
Maybe an example will help. Think of a robotoid arm with no fingers, no
wrist. It has two parts, a forearm and an upper arm, and hence two bones
(axes). The parent axis (bone) sits at the center of the shoulder, with its
Z axis pointing at the elbow. The child axis sits at the center of the
elbow, with its Z axis pointing at what in a more highly evolved robotoid
would be the hand.
You need two subgroups for each bone. Lets start with the forearm. The
Big subgroup for the forearm will include all of the forearm and just a
little of the upper arm above the elbow, enough to allow the joint to
stretch to maintain a smooth connection. The Small subgroup will contain
most of the forearm, up to just below the elbow. The faces around the elbow
which are in the Big subgroup but are not in the Small subgroup will be the
ones which stretch and deform to allow the joint to connect smoothly.
On the upper arm, the Big subgroup will contain all of the forearm, all
of the upper arm, and a little of the shoulder it is attached to. The Small
subgroup contains all of the forearm, and most of the upper arm up to just
below the shoulder. Again, it is the faces between these two where all of
the deformation takes place.
6. Once your grouping is all set up, go back to Pick Groups and pick the
actual object which is the parent of all these bones. All the axes should
be turn blue (in the default color scheme) to indicate that they have been
picked as well. Then create your "DEFAULT" state, being sure to select both
Shape and Groups. You may need to select properties as well if you want to
do brush or texture tacking, but I'm still having problems with this myself,
so I'm not sure.
7. In order to manipulate the bones, you need to be in Pick Groups mode.
You have to pick the axis you want to manipulate, rotate (or move??) it, and
accept the change. To see the result you have to pick the base object again
(still in Group mode) and select Update Bones and it will then warp your
object to conform to the new bone positions.
This has gotten a lot more long winded than I intended, but I hope it helps
clarify a few things for someone.
Mark Decker
mark.decker@segaoa.com
Software Engineer
Sega of America Inc.
If Sega thought I had thoughts, they would certainly disavow all knowledge
of them.
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