[opendtv] Re: News: High Dynamic Range imaging

  • From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Thu, 03 Aug 2006 12:58:40 -0400

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?
The sensor has a fixed range between noise and overload.  It may not be 
sufficient for the desired range.  Even eyes have irises (and chemical 
range controls).

> 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?  If so, why?
Film has a different transfer characteristic than do electronic cameras, 
so more range can be compressed into the electronic medium from the film.

> And does film have better range in theater film projectors as opposed to 
> current digital theaters?
That's a very interesting question.  The film, itself, has terrific 
range, but it has been copied from a copy (and maybe more generations 
before that), each copying stage reducing the range.

> And how the heck does the human eye do it?
Mostly with chemicals.  That's why you can't see much when you emerge 
from a cave into bright sunlight but can see fine a little later; it 
takes some time for the chemical reactions to take place.

> My apologies as I see my above post looks something like a freshman exam 
> question that I guess I should research a bit first.  But I think it's 
> an interesting topic with a likely huge payoff for Hollywood once a 
> solution is (economically) available.
Yes.  But consider some more features of film.  Suppose you want to 
shoot the brightest thing imaginable -- something like a thermonuclear 
explosion.  Aim a video camera at it, and either you burn out its sensor 
or you use so much filtering that you can't see anything after the 
explosion.  Use a film camera, and you might burn out a frame, but the 
next frame is completely fresh.

Now consider resolution.  Suppose you shoot something electronically 
that corresponds to just one pixel on the sensor grid (ignore optical 
low-pass filtering for the moment).  Now shift it or the camera by 1/2 
pixel diagonally.  What was previously 100% in one pixel is now 25% each 
in four pixels.  That shift can cause an awkward effect.  In film, grain 
is completely randomly distributed, both within a frame and from frame 
to frame.  No fixed-pixel-grid effects are possible.

There are many more differences.

But electronic cameras are certainly catching up.  Go see "Click."  It 
was shot on Panavision's Genesis digital-cinematography camera.


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