[opendtv] Re: New Chips Improve Color TV Dramatically




Hello,

I wrote:
>> 2  SMPTE RP145: R=(0.630,0.340) G=(0.310,0.595) B=(0.155,0.070)
>> 3  EBU T.3213:  R=(0.640,0.330) G=(0.290,0.600) B=(0.150,0.060)
>> 4  ITU R.709:   R=(0.640,0.330) G=(0.300,0.600) B=(0.150,0.060)
>>
>> Note that the primaries
>> vary only insignificantly between these 3 standards!

Mark Schubin wrote:
> I do not dispute that there is only minor variation between the
> primaries for 2-4, but I dispute that the chart allows us to come to
> that conclusion.  These coordinates are not on a perceptually linear
space.

Come on, Mark, the differences are small in ANY space.
I was just trying to make a couple of points here:
- there practically IS no difference between the primaries
  in the USA and those in Europe, and
- there practically IS no difference between the primaries
  of SDTV and those of HDTV.


There is however a large and unfortunate difference between
the coefficients of the Luma Function between ITU-Rec.601
(SDTV) and ITU-Rec.709 (HDTV):
  Y' = 0.299  * R' + 0.587  * G' + 0.114  * B'
versus
  Y' = 0.2126 * R' + 0.7152 * G' + 0.0722 * B'

The first formula was derived from the luminosity of NTSC
primaries, the second one was derived from that of Rec.709
primaries. However, since the derivation is only valid in the
linear-light domain but the formula is applied in the gamma
domain, it is largely irrelevant which values are chosen.
Poynton argues that it was senseless to change the values,
it only leads to confusion and errors.

So if you ever see wrong colours and you suspect that it is
due to the difference between two standards: blame the Luma
Function, not the colour primaries. Because the primaries
are really not that different.


Of course, really different primaries CAN make a real
difference. Only problem is: hardly anyone is using them yet.
There are some efforts regarding conversion from a signal
with standard primaries to a display with non-standard
primaries, witness our own Philips-Genoa LCoS displays and
various 4- and 6-primaries direct-view LCDs. There are some
efforts about new standard primaries in relatively closed
applications (digital cinema, digital photography, ... ?)
but nothing seems to be happening in (digital) television.

But note that it is not necessary to define new primaries !
Any wide-gamut signal can be "compressed" into a standard
colour gamut by attenuating the colour-difference signals
(in the linear-light domain, of course) while converting
(3x3 matrix) from the space of the wide-gamut primaries to
the space of the standard primaries. This yields valid RGB
signal values, to which the normal gamma correction can be
applied. (No need for the extended function of e-sRGB.)
Effectively this allows you to go outside the triangle.
At the receiver side, the opposite transformation can be
applied. Standard receivers (with a standard colour gamut)
would display a slightly undersaturated picture. That is
the simplest form of gamut mapping: linear de-saturation.
Advanced receivers can apply a more accurate gamut mapping,
and that is the only place where knowledge about the actual
display primaries is needed. Not earlier.

Let's write a new wide-gamut transmission standard ?

Regards,
-- Jeroen.
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| From:     Jeroen H. Stessen | E-mail:   Jeroen.Stessen@xxxxxxxxxxx  |
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