Using this method you’ll be able to hide the things you don’t want people to see and highlight the things you do want people to see. Even if they’re a part of the same object, without messing around with complex modelling where it’s not strictly necessary. When combined with Mix Maps it opens a lot of doors for you that had possibly been slammed shut before due to time constraints or complexity.
This tutorial doesn’t show you how to use it within a project; I’ll leave that to you! It will tell you how to use alphas and opacity to your advantage.
What is an Alpha Map?
An alpha map lets you use one material to hide and/or show different areas of an object; to create holes in your objects by texturing for example. Changing the Alpha or ‘Opacity’ settings determine how visible the texture (and the object it’s applied to) is. This is determined by how white or black the map is; the darker the grey the less opaque the object.
To demonstrate this technique I’ve already set up a scene with a sphere, a cube, and a plane; each is going to show a different use of alphas. There aren’t any lights here, and all the materials assigned to the objects are using the default settings.
Background Alphas; Using Presets
We’re going to work on the sphere first; inside your material editor click the button next to ‘Opacity’. From this menu you can select any type you like, but for this example we’re going to use ‘Smoke’. [Fig. 1]
As you can see from the image, parts of the sphere have disappeared. Changing the colours will alter which parts of the object are visible. This is a very basic alpha map and most often used for less detailed objects. [Fig. 2]
Coping with Repetition; Patterned Alphas
Now we’re going to work on the cube; inside its material select the same button next to opacity. This time click on ‘Bitmap’ and select a pre-prepared file. [Fig. 1] I won’t show you how to create these this time, but any black and white pattern image would do (you’re able to use any graphics package to make your own). Let’s use a rough brick texture. [Fig 2]
Even though it uses the same method as the last type of alpha we get a different result. Using patterned bitmaps makes it easy to tile the texture for large areas; especially useful with environments and objects requiring lots of repetition. [Fig 3]
Using Detailed Maps; Think of the possibilities!
Adapting the method again slightly, we can use any black and white image to create the illusion of form. I’ve already UVW mapped the plane and taken it into Photoshop. There I copied a black and white leaf onto it with the white part being the body of the leaf. [Fig 1] When we apply it, the plane appears to take the shape of the leaf while remaining the same object. [Fig 2]
I use this method the most; it lends itself to more detailed work, and also to working with particle systems. The opportunities are virtually endless; results are determined by how you use them, and how much time you’re willing to take on your maps.
Go Forth and Create!
After following through this tutorial and playing with the techniques, can you see the opportunities for making detail without complex and time consuming modelling? For things like leaves, signs, or even graffiti on a wall you can use alpha maps to determine the shape instead. You also now have a tool that you can use in conjunction with other effects to bring down your render time and reduce lag in the program (By not modelling the entire shape).
As an aside; you can also animate Alpha Maps – just use a movie instead of a bitmap, or animate a gradient (I’ll show you how sometime if you’re uncertain). Imagine the possibilities!