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Re: Converting Between R'G'B' and 4:2:2 (The ImpossiblyBlue Sky)

I'd like to address Randy Reck's desire for 160% [sic] blue.

> to use phrase "lost colors" when merely referring to this missing precision
> WITHIN the R'G'B' space confuses this problem with an even greater problem
> existing in my little world: there are colors (in whole chunks) that are
> truly lost going in the OTHER direction- Y'CbCr to R'G'B' ! ...

> ... On an RGB waveform monitor, one of these colors might show up as
> 100%red channel, 100% green channel and 160% (sic) blue channel. ...

Video systems are engineered to have predictable behavior for signals
between reference black and reference white. Digital video ( la Rec. 601)
accommodates headroom and footroom equivalent to about 7% below and 9%
above full scale. The signal is permitted to excurse for short periods of
time into the footroom region; if these values are presented to a monitor,
the monitor clips (no light is sucked from the room). The signal can
excurse for short periods of time into the headroom region; if these values
are presented to a monitor, light output beyond 100% is obtained.

Most analog monitors will display increasing light intensity for increasing
signal voltage beyond the headroom region. But predictable results are not
obtained with signal values much in excess of 110%, and certainly there is
no predictability in the region of 160% overdrive - the monitor is at best
behaving highly nonlinearly, or at worst, saturated or clipped.

If your goal is to produce art, then the unique physical characteristics of
your medium, or the unique physical characteristics of a physical device,
can be expoited. It could be an artistic statement to be unsure of what
color the viewer will see in your piece. In video and film production, part
of what we do involves art. But we wish to distribute our art to the
viewers in a manner that allows us to predict what colors the viewers will
see. We try to achieve art, but we build the art on a foundation of science
and craft. To drive a monitor to 160% blue might well be considered art,
but it is bad science and bad craft. Most scientists and craftsmen would
agree on avoiding these uncertainties.

Many of us in HDTV have the goal of increasing the color gamut and
highlight handling. But we hope this will be achieved through systematic
means, not by means that depend upon the performance of certain monitors,
or upon means that cannot be converted from one format (such as Y'CbCr) to
another (such as R'G'B'). 


Charles Poynton 
<mailto:poynton at poynton.com> [Mac Eudora, MIME, BinHex, uu, qpv]

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