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32bit per channel isn't out of the realm of sanity - think computer graphics.

But 64bit? That's pushing it more than a little.
http://www.anyhere.com/gward/hdrenc/hdr_encodings.html

Maybe if you wanted to capture in a single scene the darkest material ever made, in the shadow of a nuclear explosion.

So is HD photo any good or not? I've done some google searching, but it seems kind of hard to find good technical discussion of it.

The Microsoft attempt at an HD format called "scRGB", discussed in Greg Ward's nice write up on HDR encodings from a few years ago, was truly awful.

I rather hope HD photo is better (or even, dare to dream, good), but knowing MS, I have nagging feeling they just put a pretty wrapper around the same old crappy scRGB format, made a spiffy logo, and increased their lobbying budget...

Unfortunately, what google has turned up does seem to suggest that the HDR representation in HD photo is indeed just (crappy) scRGB.... :-(

Try these links-

http://en.wikipedia.org/wiki/High_dynamic_range_im aging

A very good one here - the original authority on the matter-
http://www.debevec.org/Research/HDR/

Some technical research (with good examples and clips)
http://www.anyhere.com/gward/hdrenc/hdr_encodings. html

Does that help? Probably should have included it in my earlier post.

EXR is a very specialized format used mostly in "film" (ie movie production.)

Huh? OpenEXR isn't particularly "specialized" (in fact it's pretty flexible) and it's what you want to be using if you're doing HDR photos.

[If it's currently used mostly in film production, that's more likely due to history and the fact that they're incredibly picky about image quality.]

I don't know if HDMI is using RGB or not, but it's worth noting that RGB wastes bits in places the eye won't notice leaving fewer bits for places where it wil notice. See Greg Ward's page, High Dynamic Range Image Encodings

As another poster hinted at, the RGB color model cannot encode every color the eye can see. It can encode every color today's monitors can display, or at least that's why it exists. However, when the output is something with higher color resolution or more importantly higher dynamic range such as film, more complete color models are needed, typically based on CIE XYZ. Furthermore, RGB is not a perceptual encoding; that is, it wastes bits in areas humans aren't likely to notice. Greg Ward's LogLuv format, for example, uses the L*u*v* color model (CIE XYZ based) to conserve bits and in 24bit mode encodes three orders of magnitude of dynamic range beyond RGB and just barely the smallest color difference the eye can detect. (LogLuv is typically used in 32 bit mode however, where it has more than 40 orders of magnitude of dynamic range beyond RGB's and finer colors). See Greg Ward's High Dynamic Range Image Encodings .

So, 24 bit color is as good as the eye can see on a computer monitor. Any image processing will require more bits to reduce rounding error artifacts, and any superior display will require better encodings to represent more colors and higher dynamic range.

Not to mention color depth: 24 bit color is clearly inadequate for many uses, and formats like OpenEXR (one of the current best HDR image formats; it uses 48 bits per pixel, in the form of 3 16-bit "half floats") are gaining popularity. For some uses RGB isn't sufficient either, and you want samples at many more wavelengths.

for anyone out there interested in what this HDR business is all about, there's an HDR image builder (http://www.anyhere.com/) out there for mac and linux. the basic concept is that you take a series of photographs at different exposures ranging from extremely underexposed to extremely overexposed (which means this only works really well for still-life shots unfortunately) and the program can compile them into an image that would mimick fairly well what the human eye would see. this is also a good tool for those of you who might be interested in using your digital camera as a luminance meter by using radiance (http://www.radiance-online.org/) to generate falsecolor luminance maps. if nothing else, it'll give you a good idea of how they're generating the lighting for Lost Coast.