UVs
Maya destroys UVs when geometry topology changes. To fix that, a UV Retaining system is developed in the Hair Strand Builders.
For this system to work, please avoid manually editing on parts of UV components, and avoid using any UV editing tools (cut, sew, align, flip, unfold etc).
UV Retaining System
The 
icons are the status of the UV Retaining System on strands.
When a strand is selected, and if it's , that means the UV Retaining system is not active. This happens when you first created the strand, or, when you manually turn tracking off.
On the other hand, when you see on a strand, it means the UV Retaining is active.
What does the UV Retaining System actually do?
When using certain functions*, and if the UV Retaining is on , the tool will automatically detect if there's a change in the topology, and when it does, it will unwrap the UV of the affected strands, remap the position back to the previously recorded coordinates.
*These functions include:
Moving/Rotating/Translating the UVs in the UV Editor. (The system will only record UV positions)
Copy/Pasting UVs from strand to strands.
Copy/Pasting Shape (as the profile CVs, and the amount of mid profile curves, can vary, causing topology between strands to mismatch).
Randomly Pasting UVs (topology between strands can mismatch).
Moving the Subdivision slider in the Strand Editor.
Mirroring or Flipping strands (only if the Mirror/Flip UV Method is set to Retain UV).
Saving a Strand Preset will also save the Retaining System status from the strand.
When you know for sure that the UV will not change ever again, and if you are doing any of the above operation a lot, you can turn the Retaining System off . This will increase the speed. However, I recommend leaving the Retaining System active at all times to avoid confusions.
See :Situation to Watch Out For
Example
When UV Retaining System is active , we are bullet proof that the UV will be re-stored after changing the Subdivision slider.
In this example, I rotated the UVs, and moved it. The tool will record these UV editing actions.
Upon changing the subdivision and releasing the slider, the UV Retaining System automatically remap the messed up UVs back to its recorded position.
When UV Retaining System is NOT active , the tool will not do anything. But the UV position is already recorded. (See previous .gif)
So, when you are done with the strand, toggling it back will fix your UV.
Situation to Watch Out For
If you turn off the UV Retaining System, and THEN move the UV, the new UV position will NOT be recorded. To avoid this, it's recommended to leave the Retaining System on at all times.
Tube Strands
- Important Notes for UV retaining to work on tubes strands:
UVs must be set to either Stripe or Grid mode.
You should only use the translate and the scale transform tools and only do so on the entire shell.
At the moment UV Retaining does not support rotation on tube strands.
Do not use any other editing functions from Maya UV editor (cut, sew, align, flip, unfold etc).
Automatic Unwrap Tubes
The easiest way to unwrap a card, is to click on the button on top of the Strand Editor. There are four types of UV type available. Strip, Grid and Unfold. If you unwrap from the icon at the top, the UV type will always be set to Strip.
UVs will be automatically unwrapped to be vertical ( UV top = root, UV bottom = tip ) at the moment and supports moving and scaling. Rotation is not supported.
UV Seams and Resetting UVs
For Tubes, if a UV seam need to be changed, you can turn on the Toggle CV Labels, select a CV label, and click on Create/Reset UV.
Please note, for tubes, resetting the UV seam will reset your UV
Card Strands
Important Notes for UV retaining to work on tubes strands:
You can use the translate, the scale and the rotation transform tools, but only do so on the entire shell.
Do not use any other editing functions from Maya UV editor (cut, sew, align, flip, unfold etc).
Automatic Unwrap Cards
The easiest way to unwrap a card, is to click on the button on top of the Strand Editor. There are two types of UV type available. Strip and Grid. If you unwrap from the icon at the top, the UV type will always be set to Strip.
To change the type to Grid, scroll down and set the Type to Grid
Resetting UV
You can reset the UV by clicking on the Create/Reset UV button. This is useful if something is out of order or when you simply want a fresh new UV.
When resetting, you will need to have the Type chosen first. You can not reset a Grid UV back to a Strip UV without switching the Type back to Strip first.
V-Axis Treatment on Card UVs
(Vertical UVs)
The Hair Card Builder is built with a unique system that allows the insertion of profile curves and CV points. This feature comes with a technical change when it comes to the automatic unwrap when you click on the Create/Reset UV button in the Strand Editor.
Depending on the curvature of your strand, and the spans of the cards, the center vertical UVs can sometimes 'mis-align' and become off center. You can the example below, when we have a 3 point card, the Create/Reset UV button unwraps the UV nicely but in the center (the vertical line), there's a slight offset to the right of the center axis in the UV space. The effect is small enough to ignore, but important enough to mention about it.
We do have an option for how it treats these vertical edges and it's under the Menu -> Set Scene Presets -> UV V-Axis Treatment
By default, it's set to "Respect CV Distance"
Setting it to 'Stretch Equally' will fix this issue. But, please note, using this option is only suitable if your profile CVs are symmetrical.
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