Unwrapping
with UV Pelt
The UV Pelt
tool gives you a streamlined workflow for unwrapping UVs on organic
characters. With the knowledge that you want a single connected
mesh of UVs cut along specific seam lines, the UV Pelt tool lets
you predetermine those cut lines then stretch the UVs out from a
centerpoint. The result is a great starting point for setting up
character UVs.
The steps involved
with pelting a surface include the establishment of the cut lines
and the center polygon around which the UVs will be stretched. More
advanced techniques include the ability to adjust the spring tension
on surface points to loosen up the UVs in certain areas and to choose
a custom pelting frame which provides an alternative boundary for
stretching the UVs.
This lesson
is broken down into the following sections:
Part
One: Basic Pelting
Part Two: Painting Spring Tension
Part Three: Adding a Pelting Frame
In this lesson,
you will use a model of a monkey to generate UVs. This model has
a grid texture assigned to help you view the results of the UV pelt
operation.
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Part
One: Basic Pelting
In order to
use the pelting technique for unwrapping your UVs, you must establish
cut lines along poly edges that tell the Pelt
tool where to break apart the UVs. The location of these lines can
be chosen to follow the skinning techniques used by trappers to
create real life pelts or simply to help hide the seam lines when
animating your character. |
1.
Begin selecting the UV Pelt edges
Select the monkey
object in the Network pane then press i
to go into it. Press
tab > UV Pelt then zoom in on one of the
Monkey's feet. You will now be prompted to select boundary edges.
Before proceeding, choose Select
> Polygon Overlapping > Front Polygons
to help you focus on the front surface of the model. You can also
change this setting using the b
key to toggle between your overlapping options. You may also want
to press spacebar-t in
the perspective view to focus on the model. Later you will bring
back the UV viewport to compare the results.
Click on one
of the edges that form a line that separates the foot from the leg.
A yellow line will appear with a red arrow depicting the direction
that the edge is pointing. Press l
to edge loop the selection. The model is now
split with one side highlighted in blue. This is the side that would
be pelted if you stopped now. In fact, you have a number of cut
lines to make before accepting the selection.

Next, shift-click
on one of the edges on the same line on the
opposite foot. If you forget to press shift and lose your existing
selection then just press Ctrl-z
to undo and try again. Press l
to edge loop
this selection. It is important that the same line is chosen on
each foot in order to maintain symmetry in the resulting pelt.

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2.
Create Cut lines between the legs
Tumble and Dolly
until you are looking at the inside of the Monkey's leg. Shift-click
on part of the edge that runs between the two three-sided polygons.

Press l
to loop. The
edge loop will continue around the inside of the leg until it hits
the two edge loops created at the feet. This makes the perfect edge
selection for this case.

If you wanted
the new edge loop to ignore any other edge loops then you would
need to press Shift - l. |
3.
Create Cut lines on the Body
Tumble and dolly
until you are looking at the monkey's neck area from the back.
Shift-click on one of the edges around the neck.

Press
l to loop. Next, Shift-click
on one of the edges running down the exact center of the back.

Press
l to loop. Remember that you can use Ctrl-z
at any time you are not happy with your current selection.

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4.
Create Cut lines on the Arms
Tumble and dolly
until you are looking at the Monkey's left arm near the hand area.
Shift-click on one of the edges separating the
arm and the hand then press l to
loop.

Repeat for the
same edge loop on the right arm.

Next tumble
and dolly to the Monkey's back then Shift-click
on one of the edges partway down the back on the left side as shown.
The line you are aiming for should run directly from arm to arm.
Press l to
test the loop. If it doesn't travel all the way to the arm then
press Ctrl-z
twice then try another edge.

Since this edge
stops at the centerline of the back, Shift-click
on the next edge past the line to add it to the selection then press
l to loop.

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5.
Set the Hint Primitive
While setting
up the cut lines, the part of the geometry that is highlighted in
blue has shifted around. This is the actual area that will be pelted.
You can reset this yourself by choosing the hint primitive which
will be the center of the stretched UVs.
Press spacebar-h
to home the view then dolly into the chest area. Ctrl-LMB-click
in the center of the chest to chose the hint primitive. The body
of the monkey will now be highlighted in blue.

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6.
Complete the Pelt Move
your cursor into the Viewer pane then press
Ctrl - 6 to bring up the UV viewport along with
the perspective view.
RMB-click
to accept the chosen cut lines and hint primitive then RMB-click
again to complete without selecting a pelting frame.
In the UV view
you will see a circle with blue lines stretching the UVs just a
little bit from a centerpoint. In the Perspective view, change the
shading to Smooth shading
to hide the edge lines. The grid is too big on the surface because
the pelt needs to be stretched out more.

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7.
Adjust the Pelt Settings To
pull the UVs harder, set the Iterations
to 200 and the
Boundary Springs
to 5.The UVs
are now stretched more within the UV viewport. In
the 3D view you can now see how the grid texture is being mapped.

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Part
Two: Painting Spring Tension
The unwrapped
UVs created so far have some strengths and some weaknesses. For
instance, the arms and legs are not stretched out enough and the
texture appears stretched in these areas. By painting a spring tension
attribute on the model, you can loosen up the springs on these parts
of the model to let the UVs stretch more. |
1.
Create the Tension Attribute
RMB-click
on the shop SOP's output. From the pop-up menu,
select attribCreate
then place the tile. This node will be wired between the pelt
SOP and the shop SOP.
Change the Name
parameter to tension
and set the first Default
field to 1 and
the first Value
field to 1.
These values give you a starting point that pelts the geometry in
the same way as the default Pelt SOP. |
2.
Set up the Paint SOP
Set the
Display flag
on the attribcreate SOP. In the Viewer pane, press
tab > Paint,
press a to select
the whole monkey, then RMB-click
to accept.
In the Parameter
pane, set the following:
Under the
Operation
tab:
Foreground
Color
to 0.2 0.2 0.2;
Turn Override Color
on and set it to tension.
The first
setting chooses a low value which will be used to loosen up the
tension in the arms and legs.
Under the
Brush tab:
Radius
to 0.4;
Opacity
to 0.2.
These settings
affect the size of the brush on the monkey and how much of the
foreground color will be applied with each brush stroke.
Under the
Symmetry
tab:
Turn on
Reflective;
Axis to
1 0 0.
It is very
important to set up symmetrical painting in order to maintain
the symmetrical UV layout. You want to make sure that the two
sides of the model are working together.
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3.
Paint the Tension Attribute
Move your cursor
over the monkey's left hand and begin painting. The symmetry settings
take care of the right hand. Repeat for the legs to also loosen
these areas up. Be sure to tumble and paint on the back surfaces
as you work.

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4.
To test the results Set
the display flag for the pelt SOP then view the
UVs in a UV viewport. To see the impact of the painted attribute,
toggle the bypass
flag on and off on the paint SOP. In the UV viewport you will see
the layout change. Initially there will only be a little impact.
This is because the Boundary Springs
setting is too high.

Make sure the
bypass flag
is off then lower the Boundary Springs
parameter to about 2
in order to give the painted attribute more influence on the final
result. Toggle the bypass
flag on and off to see the impact of the tension attribute.

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Part
Three: Adding a Pelting Frame By
default the UVs are being stretched from the centerpoint out to
a circular boundary. If you want to use a different shape for the
boundary then you will have to create and assign a frame. Different
frames will provide different pelting results. |
1.
Create a Square Frame Change
the perspective view to a front view using spacebar
- 3. Zoom in to the origin until you can see
several of the grid squares. Press tab
> Curve and start clicking two grid squares
above the origin then continue as shown below. Before
finishing, Ctrl-RMB-click
and select Close Curve
from the pop-up menu then RMB-click
to complete.

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2.
Connect the Square Frame In
the Network pane, connect the output of the new curve to the second
input of the uvpelt SOP. Turn on the display flag
for the uvpelt SOP then preview the results in
the UV Viewport and the perspective view. This new shape is being
used as the boundary for the pelting.

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3.
Create an N-sided Frame Change
the perspective view to a front view using spacebar
- 3. Zoom in to the origin until you can see
several of the grid squares. Press tab
> Curve and start clicking two grid squares
above the origin then continue as shown below. Before finishing,
Ctrl-RMB-click
and select Close Curve
from the pop-up menu then RMB-click
to complete.

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4.
Connect the new Frame In
the Network pane, connect the output of the new curve to the second
input of the uvpelt SOP. Turn on the display flag
for the uvpelt SOP then preview the results in
the UV Viewport and the perspective view. This shape stretches the
pelt in a different way.

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5.
Create the Density Attribute To
further refine how the frame is affecting the pelting, you can add
a density attribute to the frame that controls the number of samples
around the frame.
RMB-click
on the curve SOP's output. From the pop-up menu,
select attribCreate
then place the tile. This node will be wired between the curve
SOP and the pelt. Change the Name
parameter to density
and set the first Default
field to 1 and
the first Value
field to 1.
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6.
Set up the Paint SOP Set
the
Display flag
on the new attribcreate SOP. In the Viewer pane,
press tab >
Paint then press a
to select the new curve then RMB-click
to accept.
In the Parameter
pane, set the following:
Under the
Operation
tab:
Foreground
Color
to 0.2 0.2 0.2;
Override Color
to density.
Under the
Symmetry
tab:
Turn on
Reflective;
Axis to
1 0 0.
It is very important
to set up symmetrical painting in order to maintain the symmetrical
UV layout. You want to make sure that the two sides of the model
are working together. |
7.
Paint the Density Attribute
Paint the new
attribute on the end points of the protruding parts of the frame.

Set the display
flag for the pelt SOP then view the UVs in a UV
viewport. To see the impact of the density attribute,
toggle the bypass
flag on and off on the new paint SOP. In the UV viewport you will
see the layout change. Where the density was lowered there are fewer
spring points that meet the frame.

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Conclusion
The UV
Pelt tool provides a powerful UV unwrapping
technique that can save you from hours of cutting and sewing different
UV projections. Of course the Pelt doesn't give you the perfect
result and further editing of the UVs using UV
Edit or the UV
Brush tool will let you create the best
UV layout possible.
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