[opendtv] Re: News: High Dynamic Range imaging

  • From: Jeroen Stessen <jeroen.stessen@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Mon, 7 Aug 2006 10:51:17 +0200

Hello ! 
Tom Barry wrote:
>> I've wondered for some time now if it would be better to
>> use some non-fixed color space where that S-curve, log
>> curve, or whatever was parametrically defined for each
>> frame, based upon the probability distribution of the actual
>> values needed.  The actual values used for each frame
>> would be determined at the time of down converting from
>> some higher bit space, say in the camera or telecine
>> machine.

That's exactly what I once proposed to Prin, after his 
presentation on Digital Cinema. My exact words: At the end of 
the day a gamma function is just a quantisation table. 
There is no reason to choose a fixed function and then waste 
many quantisation codes on values that are never used. You need 
the low values for the dark scenes and the high values for the 
bright scenes, but not at the same time. 

I hope that nobody is suggesting that we use HDR displays 
(like BrightSide's) for entertainment purposes ? 
See: http://www.brightsidetech.com/

Because my non-HDR eyes will protest ! Dust in my eyeballs, dust on 
the projection optics, dust in the theater, and any other causes of 
flare, they will all cause the black level to be raised to dark grey 
and all details to be lost. There is no point in trying to render deep 
blacks if they never reach your retina. Better to compress the dynamic 
range to between 1000:1 and 100:1 and preserve at least the details 
in the blacks. For example with an algorithm like Nasa's Retinex: 
  http://dragon.larc.nasa.gov/

It is okay to capture data in HDR, but it's not okay to present it 
in that form to the human eyes. It will be like driving with the 
sun in your face and a dirty windshield. Not a pleasure ! 


Bert Manfredi wrote: 
> What seems to be missing from this thread is that the 8 or 12 or 
whatever 
> bits used to quantize the light samples are not linear with intensity, 
> right? I mean, in practice (although they could be made linear, I 
suppose.) 
> As intensity increases, the coarseness of quantization increases by some 

> power factor -- Gamma correction. So things aren't quite as bad as 
linear 
> coding.

Exactly. Gamma functions are used for video, and log curves sometimes 
for professional applications. This is necessary for not wasting too 
many codes on shades of white that can not be distinguised anyway. 

But it is still wasteful... You need more bits for coding variations 
between scenes than for variations within a scene. Going to 12 or 
more bits just because in a dark cinema you can better distinguish 
details in the dark scenes is wasteful. If there were a bright object 
in the same scene then you could not look in the dark anymore, so you 
could give some coding values to white and take them away from the 
blacks. 

So this obviously calls for a dynamic coding, a variable range. 
I would like to take the concept of "perceptual coding" one step 
further, and include the dynamic adaptation of the eye... 

> Doesn't ATSC use some gamma correction defined in ITU-R BT 709? 

Officially yes. Unofficially they will use whatever correction is 
necessary to make the picture look good on the studio monitor... 

> Charles Poynton sez that if you do gamma correction before quantizing, 
> even 8 bits per sample are actually okay. 

That is only very conditionally true ! Three major conditions: 
- you need a typical viewing ambient, not too dark, and 
- you need some analog noise on the signal before quantisation, and 
- you can't do any cascaded operations on the 8-bits signals ! 
(Operations of the type that introduce more quantisation noise.) 

If you had a digitally rendered signal and quantize it to 8-bits 
on a Rec.709 gamma function and then view it in a dark cinema, I 
am certain that the quantisation of the blacks will look terrible. 
The toe gain of 4.5 is way way too low for preserving the blacks. 

8-bits D1 was good for taping noisy analog video signals, it is 
not enough for a complete digital video chain. 

> That whole presentation is pretty interesting. He says that linear 
> 8 bit quantization is not nearly enough.

That is very true, however, his "nemesis" Timo Autiokari also 
makes a good point of why the gamma domain should not be used for 
some (many ?) types of signal processing: 
http://www.aim-dtp.net/aim/evaluation/gamma_error/index.htm

Did you know that Charles is fully employed these days ? 

Best, 
-- Jeroen

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