Tip of the Week - 3D

The past weeks I've discussed 2D compositing tips and tricks. During the next several weeks I will start going into 3D compositing tips, which you may encounter in your work.

As a general overview, your job as a compositor is to integrate a variety of 2D elements and photographic plates into one seamless composite that appears to be shot at once, in camera. These 2D elements can range from steam and smoke, to large vistas and basic bluescreen and greenscreens. What happens when you're given several different layers of 3D rendered elements? How do you integrate these into a 2D composite? While some packages allow you to build your comp in three dimensions, we still project our finished images on a 2D surface, mainly a film or TV screen.

Your goal here is to visualize the screen in front of you in 3D dimensions. This can be made even harder since your 3D elements are in on a 2D plane! You have to visualize the depth (Z-depth) of the image. Most current image formats comprise of five channels; red, green, blue, alpha, and Z. Depending on the file format, some of them may omit the alpha and Z channels. Pick up a book on image formats to learn more about which channels each file format can hold.

The Z depth channel will hold a grayscale version of the objects in your image. Depending on the encoding, closer objects will be black, while futher objects will be white. In essence, the grayscale image will look like all the objects are in fog.

Some compositing packages allow easier tweaking of the Z channel. Combustion uses 3DS MAX's rpf file format to accurately layer different 3D renders in comp, necessitating only the rerender of one 3D render if something goes wrong. In other packages, we can use the Z depth channel as an alpha, and effectively mask certain areas of the image that we know are behind an object.