[opendtv] Re: News: High Dynamic Range imaging
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 7 Aug 2006 11:21:50 -0400
Jeroen Stessen wrote:
> 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.
I can see the point. You don't need the full dynamic range for any given
scene, so it's best to shift the quantization as a function of average
image brightness (or some other similar strategy). Equivalent to dynamic
compression/expansion for audio.
In audio, sometimes this creates audible artifacts, as the playback
system constantly readjusts to the sound levels. I think they call it
"pumping." It's basically a control system with a feedback loop, which
can become less than linear and stable if the feedback is shifted in
phase wrt the input. I wonder if there wouldn't be similar risks with a
video equivalent?
You don't want to react too fast, in scenes where light levels change
often, e.g. driving through a dark tunnel with bright lights
periodically appearing, or you might perceive faster readjustment to
ambient light levels than the eye would naturally perceive. And you
don't want to react too slowly, or going from a light to a dark scene
will result in a long period of annoying darkness. And vice versa. Like,
emerging out of a dark tunnel.
Perhaps, the idea of *not* doing what you and Tom suggest is that, given
a good display of course, this allows any individual viewer's eyes to
react naturally, without making assumptions on how fast or slow that
individual's vision adjustment is?
Me, I wish my LCD with not-very-adequate 600:1 contrast ratio (dynamic
range) would amplify the dark scenes. Seems to be a problem with LCDs,
at least of the vintage of mine. I'd be satisfied with a static dynamic
compression, actually.
Bert
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