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Color Grading in After Effects: Basic Color Control
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This series is discussing a couple of approaches towards color grading using After Effects 5.5. Barend Onneweer starts out with a rather mathematical approach to match the overal color balance between shots. This procedure can also be put to good use to correct badly white-balanced video.
What is 'Color Grading'?
A very important stage in finishing a project on film or video is color grading or color correction (both terms are used for the same process). There are basically two aspects to the colorists job. He will provide a sense of color continuity throughout a scene by individually adjusting the colors and luminance to match the different shots. And, often working with the cinematographer, he can create an overall look and visual atmosphere for the film by adjusting the color palette and contrast. Apart from this it's also often the colorist that controls the final image sharpness and grain.
In the old days of film (...) the colorist was also called timer, and he used red, green and blue lights to adjust the colors and luminance of a film transfer. Apart from the lights the look of the images could also be affected by chemical processes like 'skip bleach' or 'bleach bypass', to suppress certain colors, or increase the contrast.
Nowadays, even material that was shot on film is often transferred to a digital format to allow for digital color grading, which allows for much more interactivity between the image and the colorist, and the possibilities are near endless, as long as the image quality allows.
This article will focus on the tools and possibilities for digital color grading using Adobe After Effects. I will try to show some different approaches towards using these tools, for a dry explanation of the different tools, use the on-line help files or the manual.
General Tips
It is common procedure to start your work on a wide mastershot of a given scene to develop the look and palette for the scene. From there on all the other shots will be processed to match this look.
In case there are very difficult shots or dramatically under-exposed shots you need to work on, it may be necessary to start working on those shots first, to establish the best result possible, and then possibly degrade the other material to match the look of those worst shots.
The Scene
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For this example I'll use a scene that is not too problematic. The shots are slightly under-exposed, and the white-balance seems a bit off, casting a yellowish tint over the images. Also, there isn't much depth in the image and I'd really like to separate the actors from the background a bit more.
SHOT 1 - ORIGINAL SHOT 2- ORIGINAL SHOT 3 - ORIGINAL SHOT 4- ORIGINAL
Basic Color Control
All four shots suffer from a yellowish color cast, caused by the fluorescent lighting. The first thing I want to do here is to create a more natural color balance. To do this, I can individually adjust each color channel (Red, Green and Blue) untill I like what I see.
After Effects has a couple of different tools that allow the user to work on the color balance, like Color Balance, Curves, and a couple of others. The Curves filter offers the most in flexibility and detail, but the Levels filter is often a bit easier to use. Curves allows you to set as many control points as you need, whereas Levels has to get by with five, of which we'll only use one this time: the Input White value. Lowering this value from 255 down, lifts the level of the RGB (or individual channel), in effect making the image brighter, while leaving the blacks untouched.
Back to work: I could start adding blue or removing red or simply trying out all the sliders untill I accidentally stumble upon a look that I like. That could take a while though, so here's an approach that gets me to a good starting point without too much guesswork.
Step 1: Decide on a 'Grey-point'
This procedure is based on the concept that you as a user pick an area in the image that you want to be neutral grey. It can be any luminance, although somewhere in the middle will be best. In the brightest areas one of the channels could be clipped (the value is 255) and then the procedure isn't as reliable anymore.
So in this particular scene I want to plastic backdrop to be grey-ish instead of the yellow tint it is now. I move my cursor over the area and take a look at the Info palette wich for this occasion is set to show the RGBA values in 8-bit per channel mode, producing values between 0 and 255. The Info Palette shows a value of 136 for the red channel, 119 for green and 82 for blue.
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Step 2: Calculate the Multiplier
To make this pixel a neutral grey, I'll have adjust the individual channel levels. To be precise I want to lift the luminance of the Green and Blue channels until in this specific image area, they match the Red channel (Red = 136, Green = 136 and Blue = 136) . This can be done using the Levels (Individual Controls) filter. To raise the luminance of the Green and Blue channel (effectively adding green and blue to the mix) I can lower the Green Input White and Blue Input White.
To find out how much to lower the Green Input White and the Blue Input White, I'll make a simple calculation: divide the maximum channel value (255) by the highest channel value from our Grey Point.
255 (the maximum channel value), divided by the highest channel value in our Grey Point (in this case the red channel) makes the Multiplier (in this case 1.875)
Now I'll multiply the other original channel values (119 and 82) by the same amount: resulting in the end values in the right column. CHANNEL Red Green Blue ORIGINAL VALUE MULTIPLIER 136 119 82 255 / 136 = 1.875 END VALUE 255 223 154
Step 3: Adjust the Channel Levels
These 'end values' can be used to lift the green and blue channels. We can use the Levels filter or the Levels (Individual Controls), since both do the same, only the latter has the controls for each channel directly accessible, while the regular Levels filter has a drop-down menu for this. So I apply the Levels (Individual Controls) filter, and in the green and blue channels I input the calculated values in the Green Input White and Blue Input White,
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I'll repeat the same procedure for the other two shots, each time calculating the amount that the levels need to be tweaked. I could just apply the same settings to all the shots and they would be roughly okay, but in this case I'll be thorough. For shot 2 I actually read the values from a highlight on the glove. This is a bit tricky since the glove was originally a bit more yellow-ish but since it looks okay... The best way to achieve color continuity throughout the scene would be to use the same object as a reference, if possible.
Below you can see how the 4 shots came out...
ROLL THE CURSOR OVER THE IMAGES TO SEE THE ORIGINAL IMAGES
SHOT1 COLOR LEVELS SHOT2 COLOR LEVELS SHOT3 COLOR LEVELS SHOT4 COLOR LEVELS
Step 4: Overal brightness
From here it is not too difficult to tweak the RGB Input White for each shot to match the brightness of the separate shots. A trick I used is to check the Red channel value in the Info Palette for instance on the forehead of the actor on the right side of the frame. Then I lower the RGB Input White for every shot a bit to match this value, going back and forth.
ROLL THE CURSOR OVER THE IMAGES TO SEE THE IMAGES FROM THE PREVIOUS STAGE
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SHOT 1 - ORIGINAL SHOT 2- ORIGINAL SHOT 3 - ORIGINAL SHOT 4- ORIGINAL
Notes:
Now all the shots have been colorbalanced to a more neutral palette. With some experience you'll probably find that in most cases you will be able to judge the color balance by eye, but this approach can be used in many variations as a starting point to match the general color balance between shots.
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Basic Color Control(level进行精确的校色)
