ProVideo Coalition.com: Creating Motion Graphics: Keyframes by Chris & Trish Meyer

(Note: The workflow has changed in After Effects CS3; we discuss it in detail in Chapter 25 of our book Creating Motion Graphics 4th Edition. However, if you’re not ready yet to dive head-first into a color managed workflow, the technique in this column provides a workaround that works fine in After Effects CS3.)

“601" luminance range

This issue has existed since the earliest days of digital video, but unfortunately many users still don’t fully grasp on it - a situation not made any easier by many manufacturers. First, we’ll cover some history and the underlying behavior; then we’ll get to the new fix inside After Effects 7 - and a fallback fix in Apple’s Final Cut Pro.

Internally, most digital video streams are expressed as YUV, not RGB values. The “Y” is the luminance (brightness) component of each pixel. In an 8 bit per color system with values ranging from 0 to 255, these streams define black as having a value of 16 (not 0), and white as having a value of 235 (not 255); the values for 10 or 12 bit systems scale accordingly. This allows them some extra headroom for hot spots, and enables arcane techniques such as “superblack” keying where the alpha channel is defined by luminance values below 16. This 16-235 range is known as the “601” luminance range, based on the ITU-R 601 standard which much digital video obeys.

What happens to these values is determined by the QuickTime or Video for Windows codec they employ, and occasionally by the application processing these images. In the case of After Effects, it relies on the codec to translate between the stream’s YUV values and the RGB values that After Effects prefers. During this process, some codecs convert between their internal 16-235 range and the fuller 0-255 range most users expect - while some codecs don’t. Although a few have option switches to decide whether or not this range conversion happens, most manufacturers don’t go out of their way to let users know exactly what their codecs are doing, leaving users in the dark (or a slightly washed-out light, as it so happens).

Some codecs are known quantities. For example, Blackmagic’s DeckLink codecs, as well as the DV codec in QuickTime on the Mac, do this automatic conversion for you, so any footage captured to these systems appear to have their full contrast inside After Effects. Other codecs - such as the old Media 100 codec, and many DV codecs on Windows - do not do this luminance range conversion, so footage captured to those systems can appear to be lacking in contrast once in After Effects. Avid and Aurora codecs can have hidden switches that determine their behavior; Avid footage is usually converted, while Aurora footage usually is not.

Friday, September 01, 2006

Brighter Whites; Richer Colors, Part 1: 16-235

Two new features in After Effects 7 ease color-critical format conversion.

“601" luminance range