Screencaps: What are they good for?

[Joe Cain]

(Posting in Off-Topic because the subject doesn't really seem to fit elsewhere; I trust the mods to relocate the thread if the subject or discussion seems useful enough to deserve wider play.)

Screencaps are being used a lot. I don't know much about the underlying technology. I've gotten the impression that the 1080 screencaps posted with the reviews here at blu-ray.com involve something proprietary---I'm not looking for that level of information.

But with last year's Dracula brouhaha, this year's redux with the Godfather trilogy, and other such discussions, there are some things I'd like to understand....things which have nothing to do with the "right"-ness of a given image, but which are more about what constitutes a quality capture, what information can fairly be deduced from such an image, and what criteria must be met for two captures to provide a fair basis for comparison and contrast.

If a full resolution capture is made of a BD or DVD's still image, is it necessarily accurate? Is the accuracy dependent on hardware or software? If a capture is down-resolved, whether as a part of the capture or afterwards as part of exporting a smaller file for posting online, what happens to the hue, the contrast, the sharpness? If, for the sake of comparing the same image from two separate releases, separate screen captures are made by different people on different hardware, does there exist a fair basis for comparison? Would that answer depend on whether or not the same capture process was employed by both parties? (And if both captures were less than full resolution, could a fair comparison be made of anything besides framing?)

Thanks in advance for useful, informed responses.