Color Correction and Sky Replacement

I’d like to follow up yesterday’s post by giving a few examples of ways that simple and cheap digital effects can improve the average low-budget film. The most obvious and probably the most useful tool in post production is digital color correction. Almost every software-based non-linear editor can adjust the hue, saturation, and levels of any video clip. This is important for those filmmakers shooting on video who would like to emulate, or at least simulate, the warmth and depth of film.

And as handy as those simple controls are, compositing programs like After Effects and Shake can accomplish much more. Individual parts of a scene can by isolated and tweaked. Sharpening or blurring can emphasize or obscure certain details. Minor camera shakes can be fixed. Glows and blooms can be added to highlights. Specific colors can even be altered, turning green spring foliage into a warm autumnal environment.

But sometimes changing what already exists isn’t enough, and new image data must be added. Video captures much less color depth than film, so when the camera is exposed for an actor’s face, the sky behind him will probably be completely blown out. With the right equipment and good polarizing filters, some of the sky’s color may be salvageable, but with highly compressed 8-bit DV video, it’s more likely to be pure white. This is where sky replacement can come in.

For years now, Hollywood effects houses have been adding new sky and cloud elements to locked-off plates when the images need more dramatic kick, or they needed to simulate weather that wasn’t available on the set. However, this is now a very simple procedure for anyone with a basic working knowledge of almost any compositing package, and skies can now be tracked to moving and even hand-held shots. Andrew Kramer has an excellent tutorial on how this is done.

Adding an image as simple as a blue sky or a few clouds can instantly make an image much richer and more filmic, and with a little planning on the set, you can shoot images that will integrate with dramatic sunsets and moving clouds, as shown below:

On the left is an image shot with a Canon XL2 on a nice, cloudy Texas day. Even though the sky is overcast, it’s still bright enough to overpower the image. I exposed for the flag, blowing the sky out and leaving the soldiers almost silhouetted against it. Obviously, it left a little to be desired. To create the image on the right, I combined three pictures of clouds I had taken earlier behind the video footage. Each cloud layer was panned and skewed at a different speed to simulate depth, and with a some color adjustment and stabilization, the shot was much improved.

With a little imagination, it’s easy to see how useful this technique is. But why stop with skies and clouds? Why not add hills and mountains, cities and castles? If we can easily track other images into our footage and correct them for color and lighting, then the sky isn’t the limit at all. I hope to post an article on matte paintings tomorrow, which is the next logical step, and yet in many ways the precursor to the methods I’ve discussed here. I suggest anyone interested in these tricks read up on matte painting’s history, Peter Ellenshaw, and look at some of the recent work of Matte World Digital.