[opendtv] Re: News: High Dynamic Range imaging
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 04 Aug 2006 15:36:53 -0400
Craig Birkmaier wrote:
> One of the magical aspects of film negatives is that they do not have
> linear capture characteristics. The knee and toe that i mentioned
> yesterday, are in fact a very important aspect of film response. Over
> the mid range values film tends to be linear, as is the case for
> electronic sensors. But in the blacks and whites at the ends of the
> linear range film has a logarithmic response - essentially an
> S-curve. This additional range can provide more image detail in the
> blacks and whites (based on the actual exposure), and it can be used
> during printing or digital scanning to bring out the details desired.
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 still wouldn't get around the limitations of the sensors but likely
would give more range to things like 8-bit encoding.
- Tom
> At 9:32 AM -0400 8/3/06, Tom Barry wrote:
>
>>Interesting article, and something I don't think I've ever properly
>>considered before. Can anyone explain why you couldn't solve the
>>problem simply by extra pixel bit depth, maybe in a log scale? I'm not
>>sure why you need to take multiple pictures.
>
>
> Mark and others have already provided some helpful feedback to your
> questions, so i will only expound on a few points.
>
> One of the magical aspects of film negatives is that they do not have
> linear capture characteristics. The knee and toe that i mentioned
> yesterday, are in fact a very important aspect of film response. Over
> the mid range values film tends to be linear, as is the case for
> electronic sensors. But in the blacks and whites at the ends of the
> linear range film has a logarithmic response - essentially an
> S-curve. This additional range can provide more image detail in the
> blacks and whites (based on the actual exposure), and it can be used
> during printing or digital scanning to bring out the details desired.
>
> Logarithmic coding at 10-14 bits is commonplace today for the files
> produced by film scanners. And we are now seeing digital imaging
> systems that capture RAW image data from the sensors (before gamma
> correction) at extended bit depths (10-12 bits); this added
> information can then be used in post production to extract the
> desired image details during color grading and correction.
>
> The reason to take multiple pictures is the linear response of the
> analog image sensors. By taking two or three images with different
> exposures we are trying to emulate the logarithmic response of film
> negative, providing the additional range of blacks and whites that
> are typically lost with a single exposure.
>
>
>>Also, assume you instead are Hollywood and currently starting from film
>>cameras, getting some extra dynamic range. Is there still extra range
>>by the time you transfer it to some electronic medium?
>
>
> This is certainly possible, but is limited by the weakest link in the
> chain. If the image is subjected to 8 bit processing then the range
> of possible values is limited. But the mapping of values from the
> original HDR image is typically much better than a camera with linear
> response, acquiring the images with 8-bit depth.
>
>
>>If so, why? Do
>>telecine machines just have better range than cameras? Can that extra
>>range transfer all the way to, say, DVD, or HDTV displays?
>
>
> Telecine machines are now capable of delivering 12-14 bit response,
> even higher in some cases. This is a major reason that most Hollywood
> projects are still captured with film, which is then digitized for
> post production and eventual release, either as film prints or
> digital presentation.
>
>
>>And does film have better range in theater film projectors as opposed to
>>current digital theaters?
>
>
> Theater film projectors have their own liabilities. As Mark stated,
> the quality of release prints is not the same as the original
> negatives, or a high quality print. In addition, film gate jitter
> tends to act as a low pass filter, removing some of the high
> frequency detail that may be present in the release print. Thus
> digital presentation may eventually prove to offer superior
> presentation characteristics, especially as the dynamic range of the
> projectors improves.
>
> Regards
> Craig
>
>
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