How to Add Effects using Chroma Key (A.K.A., My first Blender Tutorial)

Chroma Key is what is used to create green-screen effects; basically, the green-screen image is placed on top of the image that is supposed to replace the green screen, and the chroma key erases all of a certain color that you specify, to show part of the underlying image.  Of course, there is a way to green-screen in blender, but that's not what this post will be showing you.  Take this for example; you want to put a grunge effect on an image you are using, but don't want to add it in a separate program.  You've found an image (or created one) that you want to overlay on your blender render.  This can be achieved using chroma keying in blender.  I will be using this image that I painted earlier:

Thanks to eggler for creating the scratchies brush that I used here, and lifebtb for converting it from a Photoshop brush to a GIMP one instead.  You can obtain this brush here: http://lifebtb.deviantart.com/art/GIMP-Scratchies-34170077

and you can access GIMP here: http://www.gimp.org/

         Getting back to the tutorial, let's open up blender.  I will be using Blender version 2.53 for this.  We'll start by creating a basic scene, using objects that are accessible from the 'Add' menu.  First off, delete the default cube by pressing 'X' on your keyboard.  Next, add a monkey (Shift A - Mesh - Monkey).  Rotate it along the X axis 50 degrees (R - X - 50 - Enter), and in the 'Object Tools' panel on the left, select 'Smooth' to smooth the faces for a nicer look.  It still doesn't look quite smooth enough though, so let's add a subdivision surface modifier.  On the right, click on the 'Modifiers' button (the one with the wrench), click on 'Add Modifier' and select 'Subdivision Surface' from the drop-down menu.  Click on the 'Optimal Display' button.  Next, we'll scale the monkey up 2 units (S - 2 - Enter).  Now we'll change the background to a lighter color (I'll be using light blue).  Click on the 'World' button (obviously the one with the earth on it), and click on the box under 'Horizon Color'.  Select the color you want as your background color.  If you render now, you should get something like this (F12 to render): 

It doesn't look the best; the shadows are too sharp and too dark.  Let's add some Environment Lighting.  While your still in the 'World' panel, check the box next to 'Environment Lighting' and under 'Gather' select 'Approximate'.  Finally, set the Energy to .5.  Now if we render, we should get something like this: 

That's already looking much better, and for the sake of the tutorial, we'll leave it at that.  If you wanted, you could go farther on this image, but this tutorial is on Chroma Keying.  In light of that, let's get started on the actual chroma keying.

On the bar up at the top of the window, there's a little box that says 'Default'.  Click on the button to the left of it, and select 'Compositing' from it.  This will set up our windows in a way to conveniently do the compositing necessary for the effect.  Somewhere in the middle of your screen, there should be an option that says 'Use Nodes'.  Select that, and for an easier work environment, drag the 'Render Layer' node to the upper left corner, and the 'Composite' node to the lower right.  Now add a 'Mix' node (Shift A - Color - Mix), connect the image output to the composite node input, and the render output to the first image input on the mix node.  You should now have a setup looking like this: 

Next we'll add an 'Image' node (Shift A - Input - Image), then load the image we want to overlay.  Notice that the node connected to the Mix node by default (if you had your node selected when you added your image input).  We don't want that, so simply drag the connection wire off of the second input on the Mix node.  Now add a Chroma Key node (Shift A - Matte - Chroma Key).  Connect the image output from our image node to the input on our Chroma Key node.  On the box next to 'Key Color' select the color that you want to make transparent; in my case, that color is white.  Connect the image output on the Chroma Key node to the second input on the Mix node.  Finally, click on the box that says 'Mix' on the mix node, and select 'Multiply'.  Render, and you should get this:

Hope you got the same thing, or at least something similar!  If you have any problems, questions, or comments, post them below.  Thanks for reading, and I hope you got something out of this tutorial!