This tutorial (and enclosed project file -- download from the green bar above) was created using Adobe After Effects Production Bundle version 6.5 although one should be able to easily follow along with another Production Bundle version or even another software that has the features we require. The following built-in plugins are required to complete this tutorial:
- Leave Color
- Shift Channels
- Alpha Levels
The one-color effect is easily achieved using Adobe After Effects' built-in Leave Color plugin. You can find it in the Stylize effects sub-menu.
A photograph has been used as our source. To re-create such an effect under real-world circumstances with live video, a lot of preparation and care have to be taken to ensure that subjects/objects with colors to be "left behind" are placed such that they do not intrude into similarly colored subjects/objects that are to be turned into grayscale. Failure to do so will result in painful, time-consuming rotoscoping.
To begin, drop the source image (See Fig1A) into a newly created composition of the same size as the source image. Apply the Leave Color effect to our nested composition. Set the Match Colors parameter to Using Hue and set Tolerance to 0%. For the Color To Leave parameter, click on the Eyedropper icon and select a color within the Composition Window that you want to leave as colored - in essence this is the first of three steps required to change all other colors to gray. For our exercise, pick the red at the front of the car.
 Fig.1A
Now increment the Tolerance and Softness parameters. To do so, depress the Control key as you scrub their respective values to achieve optimal control over the settings. See Fig1B for the result.
 Fig.1B
We'll now apply Shift Channels to create an Alpha Channel where the red areas of the image are made opaque and all other areas are made transparent. Upon applying Shift Channels, set the Take Alpha From parameter to Saturation. This creates an Alpha Channel of the areas with red. However, upon closer inspection you should notice that there are areas outside of the front of the car that are also partially opaque. Additionally, the white areas in the Alpha Channel are not fully a opaque. You should view the composition's Alpha Channel and move your cursor about to see the Alpha Channel's values.
We'll fix the Alpha Channel issue first. Apply Alpha Levels and set the Input White Level to a value of about 60. This should be sufficient to change the light gray areas of the Alpha Channel into white. Look at Fig 1C1 (before applying Alpha Levels) and Fig 1C2 (after Applying Alpha Levels).
 Fig 1C1 (before applying Alpha Levels)
 Fig 1C2 (after Applying Alpha Levels)
Don't be overly concerned with the white areas in the Alpha Channel that are outside of the car's front area. Upon Nesting this composition in the next step, we'll apply a number of Garbage Masks to subtract out these areas.
Now create a similarly-sized composition and Nest this composition into it. First off, I'd like to mask off the orangy bit at the top of the car and other outlying areas. To do so, draw a garbage matte using the Pen Tool around the areas and set each mask's Transfer Mode to Subtract. See Fig1D for the result and the masks.
 Fig1D
Now drop the source image into this composition and ensure that it is below the nested composition in the Timeline and apply the Leave Color plugin to this image. Apply the same settings as previously defined but instead of the red, select the blue printed text in the image.
You should now end up with an image in similar to Fig1E. To enhance the saturation of the red and blue colors, create an Adjustment Layer at the top of the Timeline and apply Hue/Saturation and increment the saturation values to taste.
 Fig1E
That's all there is to it. Just remember that in practice your best results will be due to diligent and precise planning and perfect execution. Once all those areas have been taken care of, post production will be relatively painless. As always, I hope that you've learned something useful from this tutorial. Cheers and God Bless.
--Roland R. Kahlenberg
Please feel free to discuss this technique in the After Effects forum at Creativecow.net
Roland Kahlenberg is one of the COW's AE Forum Leaders and Founder of broadcastGEMs
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