[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.
TTFN,
Mark
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