User Guide > Modeling > UV Texturing   

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.

Load Start File


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.

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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.



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.