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Old May 18, 2006, 12:38 AM   #11
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I merely said I had read about Photoshop doing this. Here are two related references I just found via a web search.

The first one is completely on-topic, but looks to be anecdotal:

http://www.retouchpro.com/forums/sho...0&p=104205

Go to post #8 by Stephen Marsh.

The second one shows a ditheroption available when doing color space conversions in PS. I only have Elements, so I don't see this option.

http://www.computer-darkroom.com/ps8_colour/ps8_6.htm

Go to Figure 16.

I never said that it was necessary in 24-bit RGB. I know that in other color spaces, using dither to approximate higher bit depth canhave a dramatic visual impact. I have nothing to substantiate it, but I also suspect some dithering is done when digital camera images are converted to 24-bit (in-camera and RAW converters). If not, I think we'd see a little more banding in gradients (like skies). It could also be that the inherent noise in the sensors takes care of that even before the data gets converted. Dithering to 24-bit RGB would be extremely subtle. One wouldn't be able to see it by peeping at pixels. But, it might make some gradientsa hair smoother.

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Old May 18, 2006, 8:31 AM   #12
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I really don't think PhotoShop (or anything else) is doing any dithering when it comes to 16-8 bit per channel conversions. Dithering is really only an option when you are dealing with indexed color palettes like 256 colors. With most reasonably sized color spaces (Adobe RGB size or smaller), you'll likely never see any banding in colors because 8 bits/channel simply covers more gradients than your eye can detect anyway. It only becomes an issue (and 16 bits/channel really only becomes necessary) when you start talking about huge color spaces like Pro Photo or Wide Gamut RGB.

I've done lots of comparisons with PhotoShop doing 16 bit --> 8 bit conversions of color spaces, simple 16 --> 8 mode changes, etc. and have never found an instance where PhotoShop dithers the 24 bit result. Not that it can't be done... just that no one does it because it is almost never necessary.

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Old May 19, 2006, 11:10 AM   #13
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16 bit processing seems problematic. I have several versions of the same image, most in 16 bits and only one 16 bit image would show correctly in Qimage. The others look as if I am looking at it through a very dirty screen.

I didn't think that Qimage worked with 16 bit as this isn't the first time I've seen this dirty screen. All I need to do is convert to 8 bit first. I assumed that 16 bit didn't work, as many image processing softwares don't. So, you can imagine my surprise when I opened an image I printed yesterday and saw it was 16 bit. It opened fine in Qimage. So far, it's the only one that does. I came here to ask about it and found this thread.

So, what's happening?
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Old Jul 28, 2006, 1:42 PM   #14
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Canon's new Prograph iPF5000 has a 16-bit printer driver option, (see http://www.luminous-landscape.com/re...-ipf5000.shtml for a review) that gives expanded dynamic range and gamut over the 8-bit option. Presumably Epson will have to do something similar to catch up.

It may now be time to think about 16-bit processing. I know all that double precision math will be a pain (perhaps reducing the number of different interpolation options would help), but it may be necessary in the longer term.
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