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When postworking an image, things are made much easier when you have an alpha channel to make sure you don't paint areas that aren't supposed to be painted. Here's an easy way to generate an alpha channel using lights in Poser.
The support file contains a light preset that you can use. First, setup and render your scene as usual (I'm using Little_Dragon's Furrette 2 and morphing beanbag chair in this example). After you've saved your render, set the background color to black, and load up the light set included in the support file, or just make your own. What you're doing is putting so much light in the scene, that everything will be REALLY bright. Hide any background objects like Cycloramas that don't need to be in the alpha channel. Also, set the difuse color of everything to white. In this image, I haven't yet set the diffuse colors. This is just to illustrate what the lights are doing.
You don't HAVE to set the colors to white, but it makes it easier to select the “blank” (black) areas of the channel if the objects are all white. Also, if there are any transparant textures in the image, either turn off their transparancy or hide them. They won't show up properly with this method.
Go to render options, and select the Poser 4 renderer (if you're using P5). Make sure the image resolution is the same as the image you rendered before, otherwise this won't work.
Uncheck ALL the checkboxes. Now hit render. The result will be that everything in the scene will appear as flat white sillhouettes, and the background will be black. Now you have a perfect alpha channel!
You can also render alpha channels for individual objects to have more control in postwork. My most recent images were done with this method, check out my galleries for examples!
A Faster Method: —Comment by MPelham: November 24, 2008—
This is a good tutorial, however I would recommend deleting all lights from scene, setting background to white and rendering as usual. This requires less edits to render window, less time for setup and less overall destruction to your 3D materials. Inverting the channel is an easy process in most image editors.