Mastering VFX terminology: Bringing images together with compositing

Knowing where you want to fit in the pipeline of the movie making process can be tough when entering the visual effects (VFX) industry. Have no fear, this article is part of a series that covers terminology and workflows that are essential to every VFX artist. For this article, we'll be looking at VFX from a compositing standpoint. The film has been shot, characters have been animated and rendered and now it’s up to you to bring it all together in a blaze of glory! So with that in mind, let’s get some basic vocabulary out of the way.

In the beginning, there was a union of two images

To composite something means to combine two or more images to make a single picture. These pictures are made up of a collection of simple images called channels. A basic image will have a channel to store the red information, one for the blue, and one for the green. A screen only displays in these channels so this is how we get a color image. If you want to bring two images together, you would remove the background of one image and then it would be placed onto a new background. The variations on this idea and the technical challenges behind it are the vast and magical land of compositing. Let’s take a look at four of the key ways this can occur. When one image is going to be placed on another, we take for granted the fact that the picture on top needs some mode by which its background is removed. This information is called an alpha channel. It is the information by which the computer removes the area of the image that would cover the region where you want the new background to show through.

Alpha channels and chroma keys

Often, the alpha channel of a CG image is stored in the image’s data. When this happens, it's easy to take it for granted because the image just looks like it doesn't have a background. However, you may be working with an alpha channel that isn't stored with the RGB channel data and instead you have a simple black and white image. It’s good to know you can use this image in many ways to cut away information in an image. However, sometimes when you're not working with a CG image, for example, green screen footage, you have to create that alpha information by pulling a chroma key, also called an alpha matte. When you pull a chroma key, you're telling the computer to look at an isolated color that is usually blue or green, and remove the areas of the footage that have that color. Doing this will allow you to place your subject on any background you choose. This green or bluescreen technique is used everywhere from big blockbuster movies to your local news station.

Blending modes

Another way you can composite images together is through blending modes. There are a lot of different blending modes to choose from, but when boiled down to their core functionality they are simply mathematical algorithms that bring the images together in a way that blends the color information together. One of the most common blending modes is the Multiply blending mode. Before I explain the awesomeness of this blending mode, you first need to understand the color scale within the RGB channels. It is on a scale of 255 colors with 0 being black and 255 being white. So let’s say that I have a beautiful nature scene and I have an image of a red leaf. I want to place the leaf on top of the nature scene and use the multiply blending mode to unite them. So you change the blending mode of the leaf to multiply, and now all the color values in the nature scene below the leaf will be multiplied by the color value of the leaf. Then they will be divided by 255. Following the above process, you'll essentially create an effect where the whitest pixels in the image become transparent. This creates a darkened effect on the leaf so that the pixels underneath in the image show through. There are a lot of other blending modes with similar operations that can be chosen depending on the look you want. In fact, every union of images you have has a blending mode. It’s just that if you don’t set it, it's defaulting to the blending mode of Normal.

Node vs. Layer-based compositing

Up until this point, we’ve been looking at the compositing process from a software agnostic standpoint, which is best if you still haven’t chosen what software to learn. However, when you do choose, you’ll have two main types of categories to choose from: node-based software and layer-based. Layer-based software rely on information being placed in a stack. If you want an image to go on top of another image then you would place it on top of the stack. When it comes to node-based software, you rely on input pipes to choose which image you want to be on top. If you have an “A” and a “B” pipe, you would hook your “B” pipe up for the background and the “A” pipe up for the foreground. Layer Stack vs. Node Tree There are so many different ways which you can bring images together through the process of compositing, and this is just the beginning of your learning process. Be sure to check back for more articles in the mastering VFX terminology series as they're released, or check out the CG101 series to learn about more VFX terms.