Adding The Background
To start off, open up your 3D image in Photoshop. If you want to follow along using this exact file, then you can get it here.
It's a good habit to right away duplicate your base layer, so you're always working on top of a copy of the original (left click on layer, choose duplicate layer). Then, lock and hide your base layer, just to be safe.
Right away the solid-light-blue-negative-space attracts the eye, so that's got to be replaced by something a little more divided. If you're ultracool then you'll have created the background in 3d and can skip this step, but if your not ultracool then you can do things this way.
Choosing an background image:
The background image is only meant to take up negative space, meaning it should be something that does not attract attention and the more it blends in, the better. For this reason, a blurrier diffused image will be used.
I'm going to choose this picture by 3dTotal's WebGuru Tom for my background, simply because it has elements that work for my piece and Tom will probably get a kick outa this. Now, as it is, this image won't work so well as a background...
But after heavy doctoring it'll be perfect! By cropping, rotating the image, blurring, and adjusting the levels I was able to create something that sufficiently broke up the background area for me. (I planned ahead a little with this, so trust me, it should look OK in the end). You can grab this image to work with here.
Creating a background will be a little different in every scenario. You can use just about anything to create something that blend in and no one will notice. In fact, the less it's noticed, the better.
Once you've got your background ready, then copy and paste it into a new layer on your 3d piece. Now open your Channels Palette and hold the Control button while left clicking on the "Alpha" or "Alpha 1" Channel.
This loads the channel as a selection. (Ctrl+click works for layers and masks as well, by the way).
You may need to invert your selection, depending on whether or not your alpha channel had the background as black or white. Ctrl+click will select whatever is white in the channel, which you can see (in the tiny thumbnail in the picture just above) is the foreground, not the background.
If it's necessary to select the invert, simple hold Control and Shift while you tap the "I" key. Regardless, you want a selection of what your background will be.
Now, click back on the Layers Palette and click on your layer with the "background" (be careful not to mix this up with the "Background" layer, which is Photoshop's default name for the lowest layer in your stack of layers) By clicking the little mask layer icon, you'll automatically create a mask over the layer in the shape of your alpha channel.
This is what you'd end up with.
It doesn't look like much, but it's a start. Now's a good time to invest in some good habits. Rename your layers so you won't get confused later on. I renamed my base layer as "original", my base copy as "work copy" and my masked 'background' image as "negative space". You may need to unlock the bottom layer to change it's name.
It's also a good idea to unlink the mask from the layer. Do this by clicking the little link/unlink symbol. After you unlink the mask, click on the little layer thumbnail to manipulate just the layer. Click on the mask thumbnail to manipulate just the mask. You would link the mask and layer when you want to manipulate them together.
The final good habit to get into is to save. A lot. Save this file as a .psd to preserve all the channels and layers.
Now that the background is in place, we need to adjust it so that it looks like it fits better. The original negative space was a light blue that I had selected to match the image, so by lowering the layer opacity I can let a little bit of that blue shine through.
By changing the blending mode of the layer to overlay, I was able to increase the negative space's illuminance.
I brightened the opacity again too, finally settling on 85%. This more luminous feel will become helpful later on when we begin to work with the backlighting glow.
For now the background is about done, and we're ready to move on to the next step, Glows and Highlights.