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When it comes to greenscreen compositing, most people know that a clean and natural-looking matte is crucial to getting good results. Hours are spent tweaking the various controls, and many cups of coffee are drained trying to get that "perfect" key. The truth is that our elusive "perfect composite" is rarely achieved, no matter how much you tweak the controls in your favorite keying application, and the reason is very simple. Without effective and targeted color correction, no composite will look proper, much less perfect.
This tutorial focuses on color correction techniques for greenscreen compositing, including the construction of artificial highlights to add an extra layer of realism to your shots. But before we leap headlong into the fun stuff, a little bit of learning is required.
Out of the dozens of color correction tools available, I have come to rely on only three of them - Levels, Exposure and Hue/Saturation. I assure you that, in the right hands, these simple tools work together to offer all of the control and quality you could ever want or need for color correction. I first came to accept this while working at The Orphanage under the guidance of After Effects guru Stu Maschwitz, where these three tools are used extensively for effects work on many of your favorite feature films.
In order to use them effectively you need to understand how they work, so here's a basic breakdown of the Levels effect and what each slider does:
For the sake of simplicity you can think of it like this - the upper "Input" sliders are used to increase values while the lower "Output" sliders are used to decrease values. For example, "Input Black" increases or darkens shadows, and "Output Black" decreases or lightens shadows. While this isn't an accurate description of what these sliders really do, it's a useful way of thinking until you become more comfortable with them. The Channel dropdown menu at the top allows you to select whether to modify the overall image (RGB) or each individual color channel (red, blue, green or alpha).
Here's the part which tends to trip up a lot of folks, and understanding it is critical to using Levels properly. When using Levels on individual color channels, you are increasing or decreasing that specific color in your image, rather than affecting overall darkness and brightness. For example, if you select the Red channel in Levels and tweak the Input Black slider, you are only altering the amount of red that appears in the shadows. While it takes a little getting used to, it is this discrete control over color that makes Levels so powerful.
So moving forward, here are the plates I'm using for this tutorial:
The greenscreen footage is a shot of Isaac Brock, frontman of Modest Mouse, playing his guitar. The footage was provided as part of their recent music video contest, and is freely available at http://edcommunity.apple.com/epic/contest.php. The background shot was chosen primarily because the lighting is so different from the greenscreen footage, which will really help to illustrate the power of color correction.
After setting my project to be 16-bit (File>Project Settings) to ensure I'm getting the best quality possible out of my adjustments, and pulling the initial key, here's what the composite looks like:
As a first step in color work, you should always take some time to analyze your composite and make a few notes on what needs repair. I like to break this up into two basic groups - lighting and color. While the light falling on the actor is from the proper direction, his overall illumination is far too dim when compared to the sunlit background. His mid-tones are much denser than the mids in the background plate, and he lacks the strong, sunny highlights seen on the building behind him. With regard to color, the actor has an overall magenta color cast while the background has more of an overall yellow palette. This information gives us a clear idea of what needs to be done, and was time well spent. Planning, after all, is your friend.
So let's start repairing this composite one step at a time. The first step I took was to subcomp my keyed greenscreen footage and applied Levels to the subcomp, rather than applying the effect directly to my footage. Using a subcomp in this manner will give us some much needed flexibility later.
Matching the colors in the actor plate to the environment will require Levels adjustments to the individual R, G and B color channels only. Understanding how to adjust your levels to get the desired color shifts can be a bit tricky, but a good way of understanding how colors are interrelated is to look at the Color Balance filter in Photoshop:
Notice how certain colors like Magenta and Green share a slider? This is because these two colors are opposite each other on the color wheel, otherwise known as complimentary colors. Adding Magenta removes Green, and there is no way around that fact.
The Levels control works in the exact same fashion when used on individual color channels. If you want to remove some Magenta from the image, you would make those changes in the Green channel of the Levels Effect. Here are the basic color changes I made in Levels:
- Reduced overall Magenta by adjusting the Green channel's mid-range slider, and the Output Black slider.
- Increased the warmth using the Red channel's mid-tone slider, and the Blue channel's mid-tone slider.
If you look at the Color Balance reference image above as you review the changes I made, you will begin to understand what is happening. To get a warmer image I needed to remove some Blue (pushing the image toward a Yellow hue) and increase the Reds (which removed some Cyan). Note that the shifts I made were small and incremental - being heavy-handed with color correction can degrade the quality of your footage, looks fake, and is a very common mistake.
The result of these initial corrections is far from perfect, but the colors are starting to match the background image much more:
Here's a very cool trick for color correcting that makes it much easier to eyeball values and match plates. At the top of your main composition, add a new Adjustment Layer with the Levels Effect applied and push the Input White slider (the top right-most slider) to the left so that your image almost blows out. This layer will allow you to temporarily blow out your image by simply turning it on and off:
Now, instead of adjusting each color channel while viewing the result in RGB mode, as seen above, set your composition viewer mode to the color channel you are currently correcting. While correcting the Red channel, it would look like this:
With the levels "slammed" like this, we can clearly see how much heavier the Red mid-tones of the actor are compared to the background. The goal now is to correct the red values for the actor so that these grayscale tones match those seen in the background plate. Using the Levels Effect on the actor plate (NOT the adjustment layer you just created) push the mids to the left:
Notice how the actor now matches the background plate much better? Now switch your Levels effect to the Green channel, set the composition viewer to view the green channel, and repeat the above process, correcting the shadows, mids and highlights as needed. After you've done this for each channel, toggle your composition viewer back to full RGB mode and hide the "slammed" adjustment layer. The actor's color should now match near perfectly into the background plate. This is a much more visual and intuitive way of color correcting, since you are merely comparing grayscale values rather than needing a fine eye for subtle color differences.
Now I will focus on the overall lighting differences between the foreground and background. The dark, dense shadows on the actor are really making him look out of place, so let's fix that first. In RGB mode of the Levels effect, I pushed the mid-range slider to the left slightly, which increases the actor's overall lightness, and then moved the Output Black slider to the right which lightens the shadows, giving them more of a milky appearance instead of their current jet black values. Here's the result:
The actor is starting to feel more like he belongs in the background environment, but he still lacks the bright, sun-drenched highlights seen in the background. This requires a whole new technique, but it's quite easy. Moving into your actor subcomp, add an Adjustment Layer above the actor clip. Loosely draw a mask on the Adjustment Layer that approximates where the highlights might fall on your actor. You may find yourself needing to create multiple masks to get the right effect, such as simulating both foreground and background light sources. In these situations, it's usually better to use a separate adjustment layer for each simulated light source, so that you can adjust their values independently. Here's what my mask looked like:
It's okay to be loose and sloppy with the mask, because the next step is to feather its edge heavily, which will hide any imperfections and smooth over everything nicely. I used a mask feather value of 12:
Now apply the Exposure Effect (Effect>Color Correction>Exposure) to this masked Adjustment Layer. Increase the exposure value until your highlight approaches the brightness needed to match your background plate. For my clip, my Exposure control was set to 1.4:
This step alone would dramatically improve most of the bad compositions I've seen, but it does come at a price. If you use this technique on a moving footage clip as I have done, you will need to animate the mask so that these artificial highlights "stick" to your actor. Fear not, however - you won't always need to be super precise with your animations since the mask is pretty loosely constructed and feathered, and the time spent will add immense production value to your composites. Note that because we are working within our actor subcomposition, this exposure adjustment will not affect our background image in the main comp, no matter how loose and sloppy our mask is. I told you it would be worth it to subcomp! Back in our main composition, things are starting to come together quite nicely:
Our actor is certainly looking more embedded into the background plate, but as a consequence of our various tweaks his color values have become a little oversaturated. Simply apply Hue/Saturation to the Actor subcomp and bring things down a bit. Here's the final result of all our color correction efforts:
Before:
After:
Additional tweaks to the color and brightness would make this composite much nicer, but it has come a very long way overall and only took a few minutes to achieve.
Please feel free to discuss this technique in the After Effects forum at Creativecow.net
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