In animation and graphic production we have several tools available to us. Masking, blend modes, lighting all provide their own special way to be able to blend subjects from different sources but an alpha channel absolutely has a magic all it's own.
When you combine subjects you want your result to be seamless, to look as if these scenes occurred together naturally and using an alpha channel is a very powerful tool to this end. A great exercise for seeing how powerful and basically how easy this can be is to begin with an image in Photoshop, create an alpha channel, then import it into After Effects and view the results.
It is easy to create an alpha channel and there are several tools to assist you, to fine tune your selection, to help you improve your finesse but for me, just seeing the results even on a fairly primitive exercise will make you a believer. We are the 'instant gratification' generation, right?
So let's create an alpha and see how simply this can work.
Go into Photoshop. Open the image you want to use. Obviously, if there is a nice separation in color and light from the subject you want to extract, it will make the process a bit smoother. A perfect candidate for this exercise is an image of a person. You want to extract this person from their background and place them in your After Effects composition.
Use the selection tools to highlight the image that will become your 'alpha'. The quick selection tool is pretty friendly. Click on a representative area, holding down the shift key continuing to add more, clicking on the area you want to join your selection. You see this area outlined with the famous 'marching ants', what appears like a moving marquee of dashed lines outlining your selection. Once you are satisfied with your selection, under the channels window choose the 'save selection' icon, second from the left. This will create a new 'channel', an alpha channel. If you hold down the 'alt' key when you click the new channel icon, Photoshop will present a box with options to: name your channel, choose color and opacity for your masking, and option to bind your channel to masked or selected area.
Go with the default, the 'masked' area, give it a name of your choice. To review your work, under your channels menu, click the visibility box next to your channel, an eye icon appears when you do. You will see the outline of your masking highlighted in the color display in the new channel definition box. This outline is what will be opaque when you import this image into After Effects. The opacity setting you chose (default is 100%) determines 'how opaque' your defined area will be. Turning your channel off and on is an easy way to see exactly what you've chosen, let you refine it a bit if you choose.
Save this file as a targa, 'tga' file with the 'alpha channels' option selected, and you have your image file with new alpha channel defined. Now import this file into After Effects. After Effects will recognize the file type. The import window will state "the item has an unlabeled alpha channel '; stick with the default option of' straight-unmatted '.
Create a composition to test your new creation. Populate your composition with your new image and another and you will see right away that only the portion you outlined appears. The rectangular outline of the image itself is not visible. You only see the image defined by your alpha channel. This one single technique opens endless possibilities for creating your own blends of images and compositions borne from your imagination.
Photoshop offers a rich selection of tools and techniques that will assist you in refining this technique. There are, no doubt, designers out there with PHD's in feathering and 'anti-aliasing'. Becoming more acquainted with these tools will be your next stop but using the steps outlined here are all you next to take the first plunge … and see your results right away. Enjoy!
