Technical Paper - Copying and Mirroring Skin Weights

7/30/2019 Technical Paper - Copying and Mirroring Skin Weights

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Technical Paper

By Sean Smith DeRizzioMajor Project: Character Design for

AnimationCGAA Year 3


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This technical paper will aim to provide the knowhow on

how to successfully copy and mirror skin weights on a

model.

Firstly, I will start off by detailing the weight mirroring

process. I will show what it entails and display its benefits. I

will be following this method when it comes to copying the

weights.


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So you have gone through skinning all the limbs and joints on one half of your character. You are happy

with the final outcome, but you now worry that you will have to do it all again to the other side. Theoption in Maya to Mirror Skin Weights means you dont have to worry about doing this manually, as it

will automatically mirror the weights from one side of the model to the other after going through a few

options. The Mirror weights tool is commonly used to mirror weights of character limbs from one side of

the model to the other, but can also be used for other things.

I will be using Mark (pictured above) to demonstrate this.


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As you can see, Marks jumper is seen to be piercing through his jacket under his right arm. This is not

what we want, and is happening for 2 reasons;

A. The right arm is not properly skinned (where as the left arm is nearly complete).

B. Both arms have been skinned separately. Manually skinning each arm means they will no longer be

symmetrical in the fact that they hold separate skinning outcomes.


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Once you have finished skinning and weighting one side of the model

Select the model then go to

Skin>Edit Smooth Skin>

Mirror Skin Weights in the

Animations Tab and click onthe Options box.


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The Mirror Skin Weights Options come with similar option to the Mirror Joints Options, which you may

have previously used to mirror your joints across. Options are as follows.

In some cases you may not need to change any of these options and can apply the default options to

your mesh. However, the reason I have gone into options is to change the direction in which Maya

will mirror the weights. This is very important, the last thing you want to do is mirror the wrong side.


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Here I have selected the side in which I started modelling my character before mirroring the mesh.This is the positive side of the axis. This can be seen by both the black line down the middle of the grid

which separates the two sides, and the axis in the bottom left corner. With the help of the tip given

below, we can see that I have selected the mesh along the positive side of the X and Y axis.

Left = (-) Minus Axis Right = (+) Plus Axis

Tip: The directions in which each X, Y and Z axis isdisplayed is its positive (+) direction, while the opposite is

the negative (-) direction.


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In this case we will need to mirror across the YZ axis.

*If you have modelled along the positive axis, then tick the direction checkbox. If you have modelled

along the negative axis then leave this unchecked. It is always best to check up on this option

whenever youre doing this as it might not be set the same every time.

None of the other option need to be changed here, so click Apply then Close.


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Here we can see that both the arms are now responding in the same way. Both have been rotatedinwards at the elbow by 60 degrees and yield symmetrical results.

Weights from every part of the models original skinned side will be mirrored across, meaning that

we can easily skin and weight half of the model and then simply mirror it over to the other side. We

are now ready to move onto copying skin weights


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So, what is copy skin weights?

So lets say you have finally finished skinning that fine-pleated robe perfectly onto your majestic,

medieval styled wizard character, for example. Its taken you ages to get all the skin weighting right,

but youre finally ready to move onto skinning his inner-wear. You come to skinning his top, only to

realise that you have to go through it all again! Well here's a simpler solution.

There is an option in Maya called Copy Skin Weights and what this does is allow you to copy skin

weights from one model to another. It can be found in the Skin options under the Animation tab.


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For this example, we have the

previously skinned jacket..

However, I still need to skin the

jumper underneath with the same

results. I would like the jumper to

be skinned to Mark with the

same weighting as the jacket so

it reacts in sync. This is where

copy skin weights comes in

handy.


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As you can see here, the new jumper model is quite similar to the original jacket piece, but its not

the same model. It is also built over the same skeleton, which means we do not need to alter the

skeleton in any way for it to fit the new model.


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Select and isolate the 2 models you will be working with, this makes selection much easier. Select

the original model in which you want to copy the skin weights from, in this case it is the jacket, then

hold shift and select your 2nd model which will be the destination model for the copied skin weights.

Your models should be highlighted like this (white = original selection, green = secondary

selection). Now, with the models selected, go to Skin>Edit Smooth Skin> Copy Skin Weights and

click the Options box.


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Maya should open the Copy Skin Weights Options. Set your options as seen above and click apply.

If you get an error message saying..

..Its because you haven't selected the model first. Go back and try again.


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Now that skin weights are copied you should see that the 2nd model reacts very similarly to, if not in

the exact same way, as the original model. Above are some poses Ive constructed to show these

results. I am using the main controls and the jumper moves with the jacket.

Copying weights is best done on a model that has been completely skinned and weighted.As you

can see on the Front Bend image the back of the neck is not following the model, this is a clear

example of why copying weights should be left until skin weights have been finalized.

There is no problem with doing it earlier for testing purposes however, just make sure to re-copy the

finished weights when its all complete.

Back Bend Front Bend Right Twist Left Twist


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In this technical paper I have demonstrated in detail how to mirror skinned weights in a symmetrical

model from one side to the other. I have also shown how to successfully copy these skinned weights

from one model to another, showing that this process can be used on duplicate of a model and models

of similar structure.

Through following this tutorial users are able to eliminate the problems of having improper symmetry in

their model and can speed up the weighting and skinning process.

Technical Paper - Copying and Mirroring Skin Weights

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