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Adobe After Effects CS5 : Dynamic Range: Bit Depth and Film (part 1) - 16-Bit-Per-Channel Composites, Film and Cineon Files

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11/12/2013 2:38:54 AM

It may still be the case that the majority of After Effects artists spend the majority of time working in 8 bits per channel, also known as monitor color. This section details the many ways in which you can do better. The simplest and least costly of these is to move from 8-bpc to 16-bpc mode.

Notes

All but the oldest and most outdated effects and plug-ins support 16-bpc color. To discern which ones do, with the project set to 16 bpc, choose Show 16 bpc-Capable Effects Only from the Effects & Presets panel menu. Effects that are only 8 bpc aren’t off-limits, but it may be helpful to place them at the beginning (or end) of the image pipeline, where they are least likely to cause quantization by mixing with higher-bit-depth effects.


1. 16-Bit-Per-Channel Composites

After Effects 5.0 added support for 16-bpc color for one basic reason: to eliminate color quantization, most commonly seen as banding, where subtle gradients and other threshold regions appear in an image. 16-bpc mode adds 128 extra gradations between each R, G, B, and A value of the familiar 8-bpc mode.

Those increments are typically too fine for your eye to distinguish (or your monitor to display), but the eye easily notices banding, and multiple adjustments to 8-bpc images will cause banding to appear in areas of subtle shading, such as edge thresholds and shadows, making the image look bad. To raise project color depth, either Alt-click (Opt-click) the color depth setting at the bottom of the Project panel or use the Depth menu in File > Project Settings.

There are really only a couple of downsides to working in 16 bpc instead of 8. There is a performance hit from the increased memory and processing bandwidth, but on contemporary systems it is typically negligible.

Tip

The Info panel menu color value settings determine color values everywhere in the application, including the Adobe Color Picker.


The real resistance tends to come from the unfamiliarity of 16-bit color values, but switching to 16-bpc mode doesn’t mean you’re stuck with incomprehensible pixel values such as 32768, 0, 0 for pure red or 16384, 16384, 16384 for middle gray. The panel menu of the Info panel allows you to choose whichever numerical color representation works for you, including familiar 8-bpc values when working in 16 bpc (Figure 1). The following sections use the 8-bpc values of your monitor despite referring to 16-bpc projects.

Figure 1. Love working in 16 bpc but hate analyzing 16-bit values that go up to 32768? Choose 8 bpc in the Info panel menu to display familiar 0 to 255 values. Or better yet, use Decimal values in all bit depths.

Even if your output is 8 bpc, the higher precision of 16 bpc will eliminate quantization and banding. However, there is more to color flexibility than toggling 16 bpc in order to avoid banding. You may even have source images with values beyond standard 8-bit color.

2. Film and Cineon Files

Although film as a recording medium is on the wane, the standards and formats of film remain common in the pipelines of studios working on digital “films” for the big screen. 10-bit Cineon .dpx files remain a common format for storing feature film images. The process of working with film can teach plenty about how to handle higher dynamic ranges in general, and even newer formats can output film-style .dpx sequences, so here’s a brief description of the process.

After 35mm or 16mm film has been shot, the negative is developed, and shots destined for digital effects work are scanned frame by frame. During this Telecine process, some initial color decisions are made before the frames are output as a numbered sequence of Cineon files, named after Kodak’s now-defunct film compositing system. Both Cineon files and the related format, DPX, store pixels uncompressed at 10 bits per channel. Scanners are usually capable of scanning 4K plates, and these have become more popular for visual effects usage, although many still elect to scan at half resolution, creating 2K frames around 2048 by 1536 pixels and weighing in at almost 13 MB.

The world’s most famous Cineon file is Kodak’s original test image, affectionately referred to as Marcie (Figure 2) and available from Kodak’s web site (www.kodak.com/US/en/motion/-support/dlad/) . To get a feel for working with film, drop the file called dlad_2048X1556.cin from the 11_output_simulation folder into After Effects, which imports Cineon files just fine.

Figure 2. This universal sample image has been converted from film of a bygone era to Cineon format .


The first thing you’ll notice about Marcie is that she looks funny, and not just because this photo dates back to the ’80s. Cineon files are encoded using a logarithmic (log) tone response curve. To make Marcie look more natural, open the Interpret Footage dialog, select the Color Management tab, click Cineon Settings, and choose the Over Range preset (instead of the default Full Range). Aah, that looks better; the log image is now converted to the monitor’s color space.

It would seem natural to convert Cineon files to the monitor’s color space, work normally, and then convert the end result back to log, but to do so would be to throw away valuable data. Try this: Apply the Cineon Converter effect and switch the Conversion Type from Linear to Log. This is a preview of how the file would be written on output back to a Cineon log file. Upon further examination of this conversion, you see a problem: In an 8-bpc (or even 16-bpc) project, the bright details in Marcie’s hair don’t survive the trip (Figure 3).
Figure 3. When you convert an image from log space (left) to linear (center) and then back to log (right), the brightest details are lost.


What’s going on with this mystical Cineon file and its log color space that makes it so hard to deal with? And more importantly, why? Well, it turns out that the engineers at Kodak know a thing or two about film and have made no decisions lightly. But to properly answer the question, it’s necessary to discuss some basic principles of photography and light.
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