Hasta La Motion!

One of the rules is to match the black point of your CG to the black point of your film plate. You would never have a black point that is lower in the background than it is in the foreground. You can sample the color values of the darkest dark in your film plate and compare it to your darkest dark in your CGI. Do they match? Is one darker than the other? The color values may not be visible to you, but bump up that gamma on your monitor or TV. You’re sure to see some discrepancies.

What’s the difference? Grain is an artifact of shooting in film, while noise is an artifact of shooting on video. They look completely different! Some people use them interchangeably, but it can get a little confusing if someone says add a little noise to your grain (which can mean something else entirely!), so I wouldn’t recommend it.

When you’re compositing 3D images into a film or video plate, it will rarely match up. Your job as a compositor is to dial in the color values, match the blacks, flesh tones, and so forth, until what was added and what is original blend together. Adding grain onto your final 3D is a necessary step to make the shot feel as if it was shot in one take.
Grain varies from film stock to film stock, and since it is a result of the film process, certain stocks are better at filming certain things. Kodak has a special stock for bluescreen and greenscreen, which cuts the amount of grain in those channels, so you can pull an easier key. Usually you can set up a grain node that contains the right information as your original plate, and it will match the rest of the plates from that film reel. Sometimes it doesn’t or it’s slightly off, and that involves adjusting it to match.

In order, RGB layer, red, green, blue.

The amount of grain in a film plate also changes depending on the luminosity of the scene.. Bright lights, clothing, etcetera, will have less apparent grain that the darker areas of the frame. Most grain packages can compensate for this. When matching grain, the easiest way is to zoom in fairly close and look at the individual channels. Match the grain for each channel, and then look at all the color channels together. Does the grain look correct? Do you need to add a little bit more in the blue channel? There are different types of grain; chunky, soft, patterned, biased to the red, green, or blue channels, and so forth. If you have the capability, shoot some frames of gray near the end of the roll, and when it’s developed, you should have some nice grain on gray that you can use as reference or even, as an application over your CG!

Once grained, render and play out your shot.. If the grain is still not matching, you may have to increase it. Sometimes watching it in motion will give a sense that it hasn’t been grained enough. Don’t push it though, and make sure the rest of your CG is integrated correctly in the first place!

Applying noise to your CG renders in video uses the same methodology. Analyze the grain in each channel, match, render, and play back. Fine tune.