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 Author Topic: Printers and HDR images...?  (Read 2295 times)
aryko
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 « on: April 04, 2006, 10:59:34 AM » Reply

Hi all,
Long-time lurker, first-time poster.
Since Photoshop CS2 including the automated script, high-dynamic range images seem to be coming out all over the place.  I'm still getting my head around the concept of creating a 32-bit image and displaying it on a 16-bit device (ie., a computer monitor).  It seems to make sense that you'd take such an image to capture as much range as possible in an image, but in order to create a JPEG that one can use to display the image on the web, you have to compress all that information into 8 bits.

I'm thinking that the ultimate aim of an HDR image would be to have it produced on fine art paper, but the question I would like to ask is, what is the "bit quality" of paper?  Is there a printer which can faithfully produce a 32-bit image?

Pardon my ignorance, and perhaps someone can weave my thoughts into a better question; I have no idea where to even start looking into answers here.  Any comments appreciated.

Thanks!
AA.
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Dale_Cotton
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 « Reply #1 on: April 04, 2006, 11:53:50 AM » Reply

From what I've read, the primary reason for 32-bit is to minimize rounding errors during the calculations that Photoshop performs to create an HDR image. As an exaggerated example, if you use 3.1 as your value for pi and some calculation requires you to square that, you get 9.61. But if you use the more precise 3.1415962 that gives you a square of 9.8696267. That 0.26 difference when multiplied repeatedly could accumulate into a visible discrepancy. For most radical transforms, such as a fairly extreme curve, 16-bit precision does not introduce visible problems, but apparently HDR pushes the limits of the envelope to such a degree that even 16-bit precision can be insufficient.

However, both monitors and printers are 8-bit devices. Having performed HDR at 32-bits, Photoshop now has as nearly distortion-free a result as possible. When you send that to the printer Photoshop first derives the optimum 8-bit rendition of the colour of each pixel in your image. Remember that when we say 8-bits we actually mean 8-bits per each of the three RGB colour channels, which means 24-bits per pixel, which means over a million different hues, if I remember correctly. Apparently, under ideal conditions the human eye can distinguish about 12-bit/channel hue differences*, but of course 8-bit/channel gets the job done.

Quote
what is the "bit quality" of paper? Is there a printer which can faithfully produce a 32-bit image?
Got me; but the answer has to do with the dot gain (ink "bleed" or spread) of a given paper together with the smallest dot (ink droplet size) a given printer can deposit. I assume 8-bits/channel is still a good match to what current papers and inks can do, given that printer drivers are still 8-bit.
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 « Reply #2 on: April 05, 2006, 08:24:55 AM » Reply

The process of paper absorbing ink is an analog process and as such doesn't have an associated number of bits, as digital processes do.

Nonetheless, what I think you are getting at is the contrast range (or dynamic range) of a print compared to that of a captured image.  The contrast range of a print can get up to roughly 250 to 1 (not easy to achieve, I should add).

Eric
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aryko
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 « Reply #3 on: April 05, 2006, 10:42:18 AM » Reply

Thanks guys, you definitely helped to clear up my misconceptions in this area.

As a follow-up I should ask, if printing on paper can achieve (if not easily, but possibly) a contrast ratio of 250:1, what contrast range am I looking at on a typical LCD computer monitor?
 « Last Edit: April 05, 2006, 10:43:25 AM by aryko » Logged
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