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Lets say we have image of CD (from scanner). Can we technically read it? What DPI is needed?

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Why did you even think this would work. Unless you're talking about a writable CD you to reflect the laser off the pits in the CD to determine whether it is pressed in (1) or not (0). Plain and simple this will not work. –  Daisetsu May 13 '10 at 21:59
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up vote 14 down vote accepted
(700 MB) / ((10 cm) * 10 cm) = 378 839 604 bits / (inch * inch)

(from google calculator).

You'd have to get a scan with at least 378 839 604 dots per square inch, or (roughly) 20000 DPI. Good home scanners have 1200 or 2400 DPI at most.

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Plus enough contrast to actually tell a 1 from a 0. Theoretically possible, but basically. No –  Earlz May 13 '10 at 19:51
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It's actually even higher than that, since CDs are round and the above calculation is for its containing square. Never mind the hole in the middle. Beyond this I'm out of my depth, but I think you'd need an even higher imaging resolution to be able to resolve all the transitions. Wouldn't the Nyquist-Shannon Sampling Theorem come into play here? –  afrazier May 13 '10 at 19:55
    
Yes, it would. But why bother? It already is difficult enough ;-) –  liori May 13 '10 at 19:59
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I suspect you'd need some kind of laser scanner of some description, maybe we could dismantle a cd-rom drive for something that might work... –  Mokubai May 13 '10 at 20:01
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@Mokubai isn't a cd-rom drive already a kind of "laser scanner"? –  Blorgbeard May 13 '10 at 20:12
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My guess is that old-style CDs (not newer DVD/BlueRay formats) might be able to be. I'd also have to guess (again) that it would require at least double the resolution in each (2D) direction as the amount of data stored.

If those guesses have anything at all to do with reality, it would be a VERY data intensive operation. Would be much easier to just pack up the bits with a CD reader, rather than store/transmit a scan. :-)

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+1 for the insight. I believe DVDs can work, but BlueRay I believe uses a crystalline structure which must be read with a laser to figure out if it's 1 or 0 –  Earlz May 13 '10 at 19:53
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Additionally you'd have to use the right wavelength of light to emit and record... those pits are so tiny that you can only fit light of a certain frequency there- so a conventional scanner wouldn't work!

Bottom line being that you can consider "DVD drive" == "scanner" (it has the right frequency laser and reader) and "ripping software" == "software to create scan image"

So the technology exists already. I guess if you were really keen you could reconstruct that DVD reader data into an graphical format of some kind such as png or bmp. However it is much easier to use conventional "image" formats to encode and transmit the data.

Answer is "yes, sort of, but not the way you described it"

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In theory a lot of things are possible. In practice - no.

Theoretically you could build a scanner (maybe) which would be able to do such an operation, only building it would cost so much more than just taking a CD player, that noone in their right mind would do a thing like that. Which leaves us with a conclusion again, that practically, it's not possible.

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