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Mr
Sock Takes a Walk: a
tutorial using Realsoft3D
Part 1:
Character Set-up
By Thomas Dow -
05/12/01
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Character animation with Realsoft3D
works very well, but there are certain
rules and procedures that should be
observed in order to do it successfully.
In this tutorial I will show how to
create a bipedal character skeleton
and map it to a modeled figure. In
the second part, I will show how to
create a simple walk cycle.
The purpose of part one will be to
demonstrate some successful skeleton
building techniques and to make the
user aware of some of the pitfalls
and limitations of the skeleton system.
The purpose of part two will be to
demonstrate some elementary animation
principles and to show how to use
the choreography editor. |
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Modelling
for Animation |
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For
symmetrical figures like human beings,
it is easier to model half of the figure,
then duplicate, mirror, and stitch the
halves together. Create your model in
a neutral pose with all the limbs spaced
well apart. This will ensure that the
skeleton mapping volumes do not overlap
onto areas where they are not wanted.
Try also to make the pose as natural
as possible. In this figure, for instance,
I have the palms facing downwards. Since
that hand position is the midpoint between
the hand's extremes (prone and supine),
it minimizes the amount of twisting
on the figure when it is mapped to a
skeleton. |
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The
mesh should have enough resolution in
order to deform properly when mapped
to the skeleton. In this image the foot
does not have the proper geometry to
bend at the toe. |
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When
a skeleton is mapped to the mesh, it
is easy to see the effect of insufficient
resolution. |
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Here,
additional vertices have been added
at the bend point of the toe. The vertices
at the bottom of the foot have a greater
point weight, those at the top have
less. This makes the top of the foot
more compressible than the bottom. Point
weight can be adjusted (using alt+LMB
on the point) after the skeleton has
been mapped. |
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Rigging
the Character |
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In this section I will outline some
of the peculiarities of Realsoft's skeleton
system. It is essential to follow the
skeleton construction rules if you want
to avoid frustrating problems further
on. Here is what you need to know about
skeletons:
- Hierarchical skeletons always
follow a tree-like propagation pattern.
The parent skeleton can have one
or more root joints from which sub
skeletons branch off.
- Skeletons always branch off the
main tree from head to tail. Always
in that order. In other words, you
should not attach tails to tails
or heads to heads. Only the head
of a sub skeleton should be attached
to the parent, and then only to
a tail or middle joint of the parent.
This ensures proper IK inheritance.
- Attached child joints always inherit
the transformations of the parent
joints to which they are attached.
This is a key concept in skeleton
construction.
I will illustrate each of these concepts
visually, of course. |
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Here
is a properly constructed bipedal hierarchical
skeleton. Notice how there are hip and
shoulder skeletons, though it might
not seem necessary to build them. Also
notice that the skeleton has a tail.
The hip and shoulder skeletons are needed
in order to properly "bend" the directionality
of the joint angles from the upward
thrust of the spine towards the downward
thrust of the arms and legs. More on
that later. |
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This
is a view of the tail from above. The
tail is the root of the skeleton. It
provides the initial direction to the
skeleton and gives us a convenient grab
point for moving the entire figure.
Never attach sub skeletons to the initial
joint, only to joint 2 and below. |
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It
is clear to see how the skeleton has
basically a tree-like structure, even
though it is bent into humanoid form.
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Now
lets take an unstructured approach to
building a simple skeleton, to illustrate
what can go wrong. First, create a spine,
then two legs as shown with the head
(first joint) of the spine at the bottom,
and the head of the legs at the top.
In the select window drag the two legs
into the spine object, making the spine
the "parent" and the two legs "children".
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Select
the two legs and hit "Attach" in the
control bar. This ensures that inverse
kinematics propagate upward from the
legs to the spine. Take a close look
at the legs to see what has just happened.
The top joint has become "twisted" as
a result of an attachment that goes
against the "flow" of the hierarchy.
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Let's
take a closer look at Joint0 (the head
joint) from edit mode. Here it is before
the leg is attached. |
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And
here is the leg after it is attached.
It is apparent from the constraint
handles that joint0 has been rotated.
In fact it has been given a new set
of coordinates which will allow it
to match the joint of the parent skeleton
to which it is attached, in this case,
the head. In the process, the heading
has been moved from 3 to 147 in order
to preserve the position of the skeleton.
Open the "spec" tab of the Property
Window and you can see the change.
The problem now is that the constraints
have changed. In order to get the
constraints back to where they ought
to be, both min and max heading have
to be increased. For the min this
is no problem, but for the max the
heading will have to be increased
beyond 180. The max constraint is
user-definable (hilight the 180 and
enter a new max), but you run into
problems when you actually try to
move the heading this high. Set the
max heading to a point beyond 180,
then interactively move one of the
joints towards the max constraint.
The joint will snap back to its minimum
once it crosses the 180 threshold.
Even if you move the joint beyond
180 using the joint heading slider,
it will snap back during animation
playback. |
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So
the lesson, after all that, is to KEEP
ALL JOINT CONSTRAINTS BETWEEN 180 and
-180. I believe that the "tree structure"
approach which I explained above is
the best way to accomplish this. It
requires that the artist use some tricks
and workarounds, and above all to be
aware of the flow of the skeleton hierarchy.
Let's start by building a proper hip.
Start with two five joint skeletons:
one for the spine and one for the left
leg. Remember to start your spine with
a "tail handle". In the Select Window
label the spine "Spine" and the left
leg "Left_Leg". |
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Move
the leg over to the left side of the
spine. Now zoom in on the hip area.
Draw a new skeleton starting at the
second joint of the spine and ending
at the first joint of the leg. The
new skeleton should have three bones:
the first one at 45 degrees horizontal
relative to the first bone of the
spine, the second one 90 degrees horizontal
relative to the first bone, and the
last one 90 degrees vertical relative
to the second bone. The effect will
be to nudge the hierarchy's directional
flow from the horizontal spine joint
to the vertical leg joint.
With a 3 joint intermediate skeleton
like this it is possible to connect
just about any two skeletons and have
an animatable hierarchy that does
not break the 180 rule. I find that
it is easier to build skeletons by
first drawing the complete skeletons
and then moving the joints into place.
To do this, open the "spec / skeleton"
tab of your new skeleton. Change the
"Default Joint Action" from "Inverse
Kinematics" to "Move joints". It is
now easy to position the joints without
having to worry about kinematic effects.
When finished, be sure to reset the
default joint action. |
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Name
the new skeleton "Left_Hip" and drag
it under the "Spine" object. Drag "Left_Leg"
under "Left_Hip" so that there is a
parent-child hierarchy from spine to
hip to leg. Select "Left_Hip" and click
"Attach" in the Control Bar. You will
probably notice some twisting in the
hip skeleton. This is OK. Select "Left_Hip",
turn on "Move Joints" in the Property
Window, and untwist the skeleton. Use
the red control dot in the center of
the bone to perform the un-twisting,
then move the joints back into place.
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Select
the leg and attach it to the hip (using
the "attach" button). There may be twisting
in the leg skeleton. If so, untwist
the leg using the same procedure as
with the hip. Notice how with the attached
hierarchy the plane of the head joint
has moved to reflect the heading, pitch
and bearing of the parent joint. |
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A
Skeleton for Mr. Sock |
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So
now that the principles of skeleton
construction are clear, let's build
a new skeleton for an existing subdivision
surface character. For the mesh I've
created a body and attached dear old
Mr. Sock as its head. Create 3 new skeletons:
a 5 joint spine, a 5 joint leg, and
a 3 joint neck/head. Name the spine
"Spine", the leg "Leg_L", and the arm
"Arm_L". |
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The
spine should have a horizontal "handle",
then three bones. A minimum of three
is needed in order to give the spine
a naturalistic compression when it is
animated. |
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The
leg should have joints at the hip, knee,
heel and toe (middle of foot), as well
as a terminating joint. Be sure to move
the mouse throughout the range of the
leg in order to create the constraints.
You can either do this while constructing
or afterwards, using "Move Joints" as
your default joint action (Property
Window, Spec tab). |
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The
neck/head skeleton has an extra joint
in the middle of the head, connecting
to an angled bone. There are two reasons
for this: first, it allows for an additional
control point which makes it easier
to animate head turns, and second, it
will hopefully fill enough space within
the head so that the mapping volumes
do not become overwhelmingly huge. If
there were only one skeleton controlling
the neck and head the mapping volume
would have to be large enough to encompass
the head. On the neck portion of the
skeleton this would make it influence
a large part of the shoulders, which
we do not want. |
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Switch
to the front view. Select the leg and
set the "Default Joint Action" to "Move
Joints". Move the individual points
so that they line up with the leg, while
constraining the motion to the X-axis
(hold down the "x" key). Then return
"Default Joint Action" to "Inverse Kinematics".
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Still
in front view, create a new skeleton
for the arm with joints at the shoulder,
elbow, wrist, knuckle and end. Switch
to side view and move the joints into
place, as with the leg. |
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Create
a connecting hip joint as described
in the last section. Name it "Hip_L".
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Create
a similar connecting skeleton from joint3
of the spine to the base of the arm.
This skeleton serves the same purpose
as the hip skeleton, but only two bones
are needed. Name this skeleton "Shldr_L".
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In
the Select Window, drag the head, shoulder,
and hip under the spine. Then drag the
arm under the shoulder and the leg under
the hip. |
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Select
the head, shoulder and hip, then hit
the "Attach" button. Untwist any bones
which might have become twisted in the
attachment process. |
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Select
the arm and leg and hit "Attach". You
will notice that the blue constraint
arcs of the arm and leg joints do not
line up with the world coordinates (the
XYZ axis, or front and side views).
If left in this state, when you animate
the figure you will have to move the
arm along two local axes in order to
get a front to back swing. This will
end up complicating your animation unnecessarily,
especially when it comes to editing
the animation curves. Better to set
things up now so that animation is easier
later on. To make the local axes of
the arm and leg correspond to the world
coordinates, we need to align the constraint
axes. |
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Step
one in aligning the arm axes is to select
the shoulder. Since Joint0 of the arm
inherits the alignment of the tail joint
of the shoulder, we need to rotate the
tail joint of the shoulder. This is
done by moving the red dot in the middle
of the end bone in the shoulder skeleton.
That red dot is called the banking control.
Make sure you are in front or back view
when you do this, with perspective turned
off. Once Joint0 of the arm is aligned,
select the arm and align the remaining
joints by using the banking control
of the preceding bone. Of course this
will make the skeleton's position become
misaligned from the mesh, so once the
joints are aligned with the world axis,
set "Move Joints" as the default joint
action and move the joints back into
place. Repeat this process with the
leg. |
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Select
the spine and hit "DupMir". Delete the
second head skeleton that is created
and rename the new skeletons Shldr_R,
Arm_R, Hip_R, and Leg_R. Select the
spine and visually center the skeleton
over the mesh. Select the head, arms
and legs individually and center the
coordinate handles over Joint0, the
head joint, of each skeleton using the
move tool, "target: handles". |
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Mapping
the Skeleton to the Mesh |
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Now
let's prepare the mapping volumes. The
first thing to do is to take another
look at the mesh. Select the mesh, and
in the Property Window / Spec tab, under
"rendering", change "type" to "Polygonal"
A subdivision mesh is actually an interpolated
polygonal mesh. This means that skeleton
mapping will affect the polygon vertices,
which generally lie outside the boundary
of the subdivided mesh itself. For accurate
mapping it is necessary to work from
the un-subdivided polygon shape. |
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Select
the mesh, and in the Property Window
/ Wire tab set "Forced Visibility" to
around "0.3". This will allow you to
see the model more easily with mapping
volumes in place using OpenGL. Select
each skeleton and turn on "Map Volume".
Use "Adj. Radius" to increase the mapping
volume until it surrounds the mesh.
On this model it becomes apparent that
there is a mapping problem around the
neck area. Because the neck skeleton
mapping volume overlaps onto the shoulder
it will cause the shoulders to move
awkwardly whenever the head is moved.
I had hoped that the extra bone in the
head would have prevented this. Apparently
not. |
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The
solution is to add two new skeletons
(mirrored) attached to the tail joint
of the head skeleton. Label these "skull_L"
and "skull_R". Select the head joint
of each skeleton, the part that attaches
to the tail of the head, and select
the "tail" option in the control bar.
The purpose of this strange procedure,
calling a head a tail, is to prevent
IK propagation along the new skull skeletons.
If the skeletons were not made rigid
like this the head would squash and
deform unpleasantly. |
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With
the new skull in place, you can now
decrease the mapping volume of the head
skeleton to get it clear of the shoulders.
Increase the mapping volume of the skull
until it encompasses the rest of the
head. |
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Turn
the mapping volumes back on for all
skeletons and adjust each radius until
the mesh is surrounded. It might be
necessary to move the skeleton joints
with mapping volumes on or, as a last
resort, adjust your mesh vertices.
it is absolutely necessary, though,
to get every last vertex inside the
map volume, or else the figure cannot
be animated.
Finally, select all the skeletons
together and check the spec tab of
the Property Window. If "default joint
Action" shows up as "Inverse Kinematics"
in red letters, or as anything other
than "Inverse Kinematics", then re-select
"Inverse Kinematics" from the pull-down
menu. This is very easy to overlook.
Do the same with the "Map Volume"
option. Each skeleton in your hierarchy
must have "Map Volume" selected, otherwise
Realsoft will not properly map the
mesh to the skeleton. On my system
Realsoft crashed when this happened. |
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And
now for the moment of truth. Select
your mesh, then the spine. Go to "Lattice
Mapping" in the Tool Bar and hit "Multi
Map", then "Accept". The mesh should
now be mapped to the skeleton. Select
the skeletons and turn off the map
volumes. Give a little tug on the
spine. If the mesh moves without distorting,
you have a fully posable, animatable
character!
One topic I have not covered is joint
constraints. As you pose your character,
you may notice that limbs begin to
twist if they are moved too far. This
is a result of the limb hitting a
joint constraint. You may want to
open up the constraint and expand
the range of motion either by moving
it in the View or the Property Window.
Another useful feature are joint anchors.
Anchoring a joint allows you to move
lower joints more easily. Both anchors
and constraints can be manipulated
without having to re-map the skeleton.
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In the next section, I will show
how to animate this character using
the choreography features of Realsoft
3D.
Get the R3D file for part 1 of this
tutorial here.
Credits:
The Real3D
animation tutorials by Rob Dollase
and Dave Corcoran provided some excellent
clues about the skeleton system of
Realsoft V4. While you are there,
check out the Real3D masterwork: Spike's
Saga.
I hope Hellmut Tümmler will forgive
me for using Mr.
Sock |
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