Rigging & Weighting

Demo I: Optimize Shrinkwrap with a Weight Map

This is our roadmap for demo I:

• Load the Tank top Base model
• Add a Shrinkwrap modifier
• Add a Mirror modifier
• Inspect the model
• Create a Shrinkwrap weight map
• Use the Levels tool to adjust the weight map
• Use weight Paint brushes for fine adjustments
• Use the Vertex Group Editor to adjust single vertex weights
• Do some final tests to see shrinkwrap and Avastar shape sliders in action

Load the model and add a Shrinkwrap modifier

So let us add the shrinkwrap modifier as indicated here:

For Avastar users:

• Open the .blend file of the tank-top model
• Add -> Avastar
• Select the Tank-Top
• Add the Shrinkwrap modifier as indicated above

Note: Avastar-1.0 has an additional set of hidden “shadow” meshes. These meshes are linked copies of the Avastar meshes, but without an armature modifier. These shadow meshes should be used as targets for the Shrinkwrap modifier. This will have advantages when you later rig your attachment (For further explanation, see demo II)

For Workbench users:

• Open the Avatar workbench
• File->Append the Tank-Top object from the  .blend file of the tank-top model
• Add shrinkwrap modifier as indicated above (use the meshes available from the workbench character as shrinkwrap targets)

Add a Mirror Modifier

Add the modifier with “merge” and “clipping” enabled. And please move the Mirror modifier to the top of the Modifier stack. That ensures the resulting mesh keeps clean and does not get holes along the mirror plane.

A detailed inspection

When we examine the mesh in detail, then we identify 4 issues caused by the shrinkwrap modifier:

1. The cleavage looks completely wrong. The shirt should not fit tight to the body here, but it should stretch over the cleavage part and at least partially hide the form of the breasts.
2. The same problem occurs at the lower backside. Here the Shirt should also fall down loose instead of getting glued to the body.
3. The bottom hem should be less tight as well.
4. The poke through parts (where the underlaying mesh pokes through the tank-top) need to be fixed. However this is not necessarily done by tweeking the shrinkwrap. We will see how to use a mask modifier (or an alpha mask in the game engine)

Of course all of the above issues occur because the shrinkwrap modifier tries to wrap the shirt as tight as possible to the target meshes. So the easy solution would be to just forget the shrinkwrap and then adjust the vertices of the tank-top mesh by hand. That would be not much of additional work (see the undeformed mesh above)

But i want to keep the Shrinkwrap modifier intact (for the reason see below). And fortunately there is a way to tell the shrinkwrap modifier to become less radical at certain parts of the mesh. this is done by using a Weight-Map.

So, What do i gain from the Shrinkwrap ?

Due to its nature a shrinkwrapped mesh will always follow its target mesh(es) as precise as possible.

So, when you modify the target mesh (the Avastar character in our case), then the shrinked mesh will follow the changes. Hence:

The Avastar Shape sliders take effect on your mesh as long as the Shrinkwrap Modifier is active in the Modifier Stack.

Note: As soon as the character shape differs significantly from the shrinkwrapped mesh, you might see distortions. Then you need to adjust the mesh again in edit mode.

Shrinkwrapped Tank-Top reacts on Avastar sliders

Note: This method does not work “out of the box” with bone length changing sliders . But you can get this to work as well in combination with “Custom shape Sliders“)

The Shrinkwrap Weightmap

So what is that, a Shrinkwrap weight map ? The shrinkwrap weight map is actually a vertex group containing all vertices of the tank-top. Now, please remember that weights can have a value in the range [0,1]. And typically a value of 0 means “no influence” and a value of 1 means “full influence”.

So, let’s create a weightmap which initially gives full influence to the modifier, thus we create the map with all weights set to 1.0.

• Locate the Vertex Groups list in the Object Data properties (the list is currently empty)
• Create a new vertex group and name it however you like, e.g. “shrinkwrap-map”
• Ensure that the weight value is set to 1.0
• Ensure that you are in Edit mode and all vertices are selected.
• Finally “Assign” the selected vertices to the map (see upper image on the right)
• Open the Modifier Stack and in the Shrinkwrap-Modifier add the “shrinkwrap-map” in the Vertex Group field
• Please keep the Offset value at 0.005 for a moment.

Visually nothing has changed by now. But we will change that in a moment.

First let’s go to Weight Paint mode and see how the shrinkwrap map looks.

Please ensure that the shrinkwrap-map is still selected in the Vertex Group list, otherwise your mesh will be rendered fully in blue. But when the map is selected, then the mesh turns fully red.

Now lets get rid of the offset value in the shrinkwrap modifier. However, as soon as we set the offset to 0.0 now, we will get a massive amount of poke through (see next image)

But now we have the shrinkwrap map and actually we can use the map to define different offset values at different locations of the mesh. When the weight is 1.0 then the offset is zero. So lets reduce the overall weight of the map and then adjust it as needed.

• Open the Tool shelf
• Locate the Weight Tools Panel
• Select the “Levels” tool

• Look at the Operator panel (it is in the tool Shelf at the very bottom) and reduce the gain gradually from 1.0 until the major amount of poke through is fixed (around the breasts) I found a value of 0.6 to be convenient. Now the mesh looks green.

The shirt is now deformed “halfway” between the undeformed Shirt shape and the target character (Avastar shape) Slide the gain between 0 and 1 to see what i mean exactly.

Fixing the cleavage

We will now adjust the weightmap around the cleavage to our taste. And we will use a weight paint brush from the Tool shelf (brushes are available  in Weight Paint Mode) for this purpose:

We use the Draw brush. Please set the Weight to something around 0.2, and the strength around 0.1 These settings allow us to modify the weightmap gradually.

Each stroke on a specific area will now move the affected weight values gradually toward the set weight value (here 0.2) This allows us to add decent weight changes.

Hint: Also try using the other brush types

For example the Darken Brush would be a good alternative to using the Draw brush. Then only those vertices are affected which are too light (have too much weight).

Or try the Subtract brush, which would remove weight until the value reaches 0 zero. Actually you should experiment with all brush types and see how you can make use of them.

With the Draw brush i can set the “target” value and gradually approach my weights towards that value with each stroke. Actually it is more a matter of “taste and believe” what is the better approach in our case.

My final weightmap looks like in the image on the right and the cleavage now looks more like it should be. some remarks:

• You see the center of the cleavage area is painted in a light blue to reduce the shrinkwrap and keep the garment stretched more naturally.
• Along the top hem the weighting is more yellow, thus the garment is stretched towards the body and “closes” the top of the cleavage.
• I have also reduced the weight on the sides, so they appear in lighter green/blue This widens up the sides of the shirt and let it fall down more naturally as well.
• I reduced the poke through by painting light blue on top of the “poked” areas.

On the backside:

• I reduced the shrinkwrap influence to almost 0 (zero) at the top hem.
• I removed weight from where i saw poke through areas,
• and i took care to avoid the shrinking of the lower hem.
• Finally i reduced the weight of the entire lower hem, so that it now hangs down more loose instead of fitting tight to the body.

Hint: I also made use of the Blur brush. This brush is used to make smoother transitions between areas of different weighting. I used this brush especially to blur the weight of the lower hem towards the upper part of the shirt.

Fine adjustements

By now we have allready a very good weightmap for our Shrinkwrap Modifier. But lets say we are not fully satisfied with the cleavage. So we now go and adjust the weights for each vertex separately.

We can even display the weight colors in edit mode, but due to a current limitation in Blender we first have to disable the mirror modifier (disable the eye icon in the modifiers head line:

• Now change the tank-top into edit mode.
• open the properties sidebar (“n”)
• locate the “Mesh Display” Tab
• And enable “Show weights”
• Now select one single vertex on the mesh
• And locate the Vertex Groups panel,it should look like this:

Note there is one vertex selected! Now lets go to side view (use CONTROL + 3)  and move the vertex by dragging the weight slider, until the vertex shows up just above the silhouette.

You can do this for all vertices in this region until the fabric stretches just like you want it to be and gets into exactly the form you want. you even could add folds if you wanted to do so.

before adjustment, fabric sinks in a bit

fine adjusted, garment stretches straight.

Admitedly the adjustment can hardly be seen from the screenshots here. However, when you play a bit with it you will see quickly how adjusting single vertex weights can be very helpful to achieve the anticipated shrinkwrap effect.

So we have demonstrated by now:

• Using Brushes for weight map painting
• Using the Levels tool (to adjust the entire weight map strength
• Assigning numeric values to sets of vertices
• Adjust single vertices.

Now let us re enable the mirror modifier and see how our weighted Shrinkwrap Modifier behaves when we change the Avastar shape…

The final test with Avastar shape sliders

So lets try it out and move the Avastar Shape sliders. And indeed the shirt changes in a convenient way and stretches more or less as expected (well keeping aside that this particular shape may be not exactly realistic…) .

However, technically we could not get rid of poke through here. This is what i meant before, when i mentioned that we get issues when the target mesh deviates too much from the shrinked mesh. Hence if we wanted to fix that as well, then we needed to modify the Shirt mesh directly. Or get it partially fixed by re adding the offset value to the Shrinkwrap modifier.

But actually poke through is not such a big problem as it might look on first sight. In fact poke through can be removed more efficiently by using a mask modifier in Blender (and using an alpha mask in OpenSim). I made a tip top tool tip about how to do that.

So we are now at the end of Demo I. We have worked on a simple weight map. By “simple” i mean that it is very easy to see what happens when the weight changes, because the influence of the map on each vertex is clearly described as:

• Weight=0: vertex location not affected.
• Weight=1: vertex pulled on surface of target mesh.

In Demo II we will see a much more complex situation where one vertex is influenced in different ways from different Weight maps and the entire thing will also be animated.

So are you ready ? then let’s proceed…