1.
Preview the Muscles
Scrub in the
timeline to watch the animation of the arm bending and the muscles
deforming. The goal is to transfer the deformations found in the
muscles into the character's skin surface.

2.
Display the Parts
Select the skin
object and press i
to go into the object. Select View
> See All Objects from the Pane menu to turn
this option Off.
Set the
display flag
for the skinmass SOP and the template
flag for the toolgroup SOP.

The skinmass
chain is made up of a line that is deforming with the bone. This
line is then turned into a simple surface using a custom tool that
sweeps a circle along the line then caps one end. You can select
skinmass and press i
if you want to see how it is built. Be sure to press u
to go back up when you are finished. There is also a group
SOP assigned to the line to help prepare it for the Inflate.
The upper_arm
chain includes an Object
Merge SOP that brings together the bones and
muscles from other objects. Again a group
SOP is used to name these parts for use in the Inflate.
3.
Merge the Inflation tools In
the Network pane, press tab >
Merge and place the tile just under the toolgroup
SOP. Wire the output of the toolgroup SOP and the
output of the deform SOP into the input of the merge
SOP. This will make it easier to pipe these parts into the Inflate.

4.
Create the Inflate SOP In
the Network pane, press tab >
Inflate and place the tile below the merge
SOP. Wire the output of the skinmass SOP into the
first input of the new inflate SOP and the output
of the merge SOP into the second input of the new
inflate SOP.
Turn on the
display flag
for the inflate SOP. This initially results in an
error because the groups have not yet been referenced.

Select the inflate
SOP and in the Parameter pane, set the Tool
Group to toolgroup and the Center
Prim Group to centergroup. This
references the group names from the two networks and the Inflate
error is fixed. Next, turn on the Inflate
to Farthest checkbox and the Inflate
can be seen pushing the skinmass surface out to
the muscles and bones.

5.
Adjust the Skin thickness
Next, select
the skinmass SOP and in the Parameter pane, adjust
the Fat parameter.
This loosens or tightens the skin over the muscles.
6.
Preview the Results
Press
u to go up to Object level and create a Flipbook
animation preview to see the muscles deforming the new skin
surface from below.

Conclusion
The Inflate
tool lets you define a character's skin using the muscles themselves.
Rather than put the time and effort into modeling the skin, the
time would now go into the creation of realistic bones and muscles
that will create the skin procedurally using the Inflate
tool.
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