[opendtv] Re: RGB mania ('translated' for Prin)
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 15 Jan 2005 09:38:44 -0500
At 6:31 PM -0500 1/14/05, Manfredi, Albert E wrote:
>This is a fascinating thread.
Good observations Bert!
It should come as no surprise that a digital TV standard, which was
designed to deal with legacy analog/digital signal processing and CRT
problems, would have some quirky attributes.
It is ALSO not unusual for video types to turn a defect into a feature.
The fact that the small area response to an impulse is different than
a large area response to the identical stimulus is a defect. One ,
which happens to work in the defined situation.
The fact that the specular highlights are being clipped, rather than
given values that are appropriate for these highlights is also a
defect. In fact it is one of the major defects of "video" that keep
the folks in Hollywood shooting film.
The correct approach is to encode luminance values properly. This
implies two important things:
1. That there be adequate headroom in the encoding system to handle
both specular highlights and details in the dark areas of the image;
2. That the luminance encoding ranges be extended in the dark and
highlight areas of the image such that it is possible to provide more
detail in these ares (this by default makes it possible to
differentiate between an are that is at full brightness, versus a
specular highlight that is above this level.
One of the tricks that we have used for years to deal with this
shortcoming is to set the nominal while level for a display or an
image processing system at approximately 90% of the peak luma value.
This leaves the last 10% to deal with highlights. By the way, NTSC
encodes do allow this kind of information to be encoded.
>Seems to me that the newer types of display
>could incorporate whatever smarts are
>needed to post-process the incoming data
>to emulate the CRT response. If they need
>scalers and deinterlacers anyway, they
>already have built-in smarts.
As a band-aid to deal with legacy content Bert may have a point,
although it is relatively easy to adjust a display to deal with this
type of source imagery properly.
But there is NO reason to continue using video encoding techniques
that do not improve the ability to delive more detail at the ends of
the luminance ranges. Your LCD panel does just fine in reproducing
all of these values. If you doubt this, just hook up a graphics card
that supports 24 bit RGB and try it out. You now have 256 steps for
luminance instead of 219, and a much larger color gamut than can be
encoded with either NTSC or MPEG.
Regards
Craig
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