Mix Shapes

This is a tutorial to do a quick and easy mix shape for your blend shapes. For example if you have a smile and a sneer, but when they're together the results don't mix well. This is a case to create another blend shape that only kicks in when the two are combined. If you can get Jason Osipa's book "Stop Staring" I strongly recommend it. It covers complete blend shape driven facial setups, and has some really neat scripts to accomplish mix shapes. This tutorial covers a completely different way of doing that if you have to set it up manually quickly.

I'm just using cubes to illustrate this example, but it works well for an mix of blend shapes. To start out I have 4 cubes. From screen right to left they are the final geometry, the two target shapes, and finally the mix shape.
Here you can see the two target shapes, which could analogous to a left and right lip. But when they're together, we want the middle to raise as well. In the second image I sculpted the fix shape so that when it's added in, the final shape is raised fairly evenly. You can see that there are 3 inputs on the blendShape now.

So now we need to set it up so that when either the left and the right is utilized, the mix stays off. But as soon as we start to mix the right and the left, the mix starts working. Probably the easiest and cheapest way of doing this is a blendColors node. After creating it, we'll connect the outPut R to the weight of the mix shape. This makes whatever value comes out of the blendColor node drive the mix.
We then connect the weight of either the right or the left (which one doesn't matter) to the Blender attribute on the blendColor. We then connect the weight of the other (either right or left) to the inputColor 1R, while keeping the inputColor2R at 0. And that's it.
What this does is take the weight of one of the shapes and stores it in the blendColor, with the other weight controlling the blender. As long as the blender (or one of the weights) is at 0, the outColor will take its value from input2, and remain at 0, forcing the mix shape to stay at 0, regardless of what the input1 is. Likewise, regardless of what the blender is, as long as input1 is 0, the outColor will remain 0. Only when the two input shapes are above 0 does the mix shape start kicking in.
Try it out and see how it works. Hope it helps.