Unity 2018 Shaders and Effects Cookbook

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You can also access the fourth component from the vert color data. If you notice, the vertColor variable we declared in the Input struct is of the float4 type. This means that we are also passing the alpha value of the vertex colors. Knowing this, you can use it to your advantage for the purpose of storing a fourth vertex color to perform effects such as transparency or giving yourself one more mask to blend two textures. It's really up to you and your production to determine if you really need to use the fourth component, but it is worth mentioning here.