What is Compositing?

Compositing is the foundation of the visual effects industry. In many ways, if visual effects is a game, then compositing is the last level where you fight the biggest baddy. If you win, you get to watch the spectacular cut-scene.


Adobe After Effects

What is Compositing?

If I had to pin down a definition for compositing, I would say it is the art of marrying pixels.

You can combine two pixels in infinite ways. What separates compositing from other visual effects and computer generated imagery is the fact that you always need two pixels to composit.

If you only change the values of one pixel, it isn’t compositing.

Examples of Compositing

What are some of the simple ways in which you can marry pixels?

You can add them:

Compositing Adding

If you put a green pixel on top of a blue pixel, you only see the green pixel.

You can subtract them:

Compositing Minus

You can multiply them:

Compositing Multiply

The key to all this ‘magic’ is mathematics. In its essence, every pixel holds nothing but numbers. All said and done, a good compositor can play music with pixels.

The most famous compositing task is keying, more popularly known as chroma keying. Keying is simply the act of selecting data based on certain criteria (like all green pixels for green screen):

Compositing Keying

If you ‘key’ an image, i.e., if you can selectively isolate a part of the image that you want to work on, you can do many things with it. In the above example, we just replaced the background. You can also selectively color grade only the ‘keyed’ part, or you can even paint over it, relight it, erase parts of it – whatever you want.

Keying doesn’t have to be chroma based. You can even key off luma values, or anything else.

The person who composits is called a Compositor.

So, what does a compositor do? A compositor is like a chef, who takes ingredients from many places or people, and mixes them all together to produce the end result. Sometimes, the carrots aren’t fresh – maybe it’s a poor season. Still, a good chef will have the knowledge, experience and skills to make the dish taste great.

In a visual effects studio, artists will produce computer generated imagery (CGI). The DP will have shot an actor against a green or blue background. They might also have shot the same actor in different clothes – the evil twin, in his lair.

You take the green away from the good guy and add him to the shot with the evil twin. Then you place the CGI dinosaur produced by the visual effects team. Then you look at the picture with the eye of an artist, and do whatever it takes to make it look real.

Then you do it over and over and over and over.

What does it take to be a Compositor?

The challenge in compositing is to make the marriage work. In other words, when you combine images from different sources, the end result must look real. But that’s not all.

Artists who use Photoshop do exactly this. The added challenge that compositors face is they also have to match the shots to either the ones preceding it or succeeding it. In addition, they also have to ensure the result works even when the elements are in motion. Oh… there’s one more challenge.

A Photoshop artist might get a whole day to work on one image, to perfect it. A compositor must display the same level of skill, but can’t afford to spend a whole day on a frame. A movie running at 24 frames a second will have 1,440 frames per minute. Not only must a compositor be good, she must also be fast.

It takes a whole lot of skill to be a good compositor. Here are some general requirements:

  • An eye for art, photography and cinematography
  • Knowledge of lighting
  • Knowledge of color grading
  • Keen eye for storytelling and timing

Anybody can be taught to composit in a few days. The general principles of compositing (or comping) are simple, just like learning to use a camera or an NLE. To master it, so that you have the magic touch to make a scene come alive – that’s the hallmark of a true artist.


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