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Old May 1, 2005, 11:58 PM   #1
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Dear all,

This is my first post. Until now I am using ps cs to send printing jobs to my Epson R800. Since nearly everybody says Qimage is a better printing solution, I would like to give it a go. But before that, I want to knowif Qimage support 16 bit aRGBfiles 'cause I always work in 16 bit. Also, I don't knowif R800's driversupport 16 bit aRGB?My printing results seems OK. Does Qimage bypass the printer driver's conversion function and convert aRGB to CMYK or whatever colour systemand send the job directly to print? I really don't know how it works, I mean how printing works and I want to get some ideas. If I cannot gain anything by using Qimage, I would stick to ps to epson workflow. Any ideas?

James :?
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Old May 2, 2005, 12:58 AM   #2
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Hi, I find the answer from Paul Caldwell's post where mchany says "Qimage supports reading of 16 bit/channel TIFF's but all internal calculations and edits are done in 8 bits/channel."

Now my question will be: if it is better to convert 16 bit file to 8 bit in ps, or just send 16 bit file and let Qimage to do the conversion. Some say ps does not do much for printing, but what benefit can we have from Qimage, as far as bit conversion is concerned? By the way, if I use Qimage and send it pro photo colour, could I get a better and wider colour range in my print from R800 than say just use ps and aRGB to print?

I guess mchany knows Qimage a lot. Hope he could respond.

Thanks for any respond.

James
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Old May 3, 2005, 7:49 AM   #3
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It wouldn't matter whether you save as a 16 bit or 8 bit file. Qimage can read both and will read both as an identical image. When reading a 16 bit file, 8 bits are discarded so that you get an 8 bit read. It also won't matter when printing because all print drivers are 8 bits/channel. I have an article here at Steve's on the subject of higher bit depth images:

http://www.steves-digicams.com/techc...pril_2005.html

Mike
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Old May 4, 2005, 10:45 PM   #4
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Thank you, Mike for your reply and the eye-opening article. Now I know when to use or not to use 48 bit.

James
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Old Jun 30, 2005, 2:44 PM   #5
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As I understand things, Photoshop 16 bit isn't really 16 bit but rather 15+1 bits (whatever that is). According to MArtin Evening there are only 32767 levels not the 65535 we would expect.

Does Qimage take this into account when converting a Photoshop 16 bit image?
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Old May 9, 2006, 9:12 PM   #6
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In comming from the audio side (of the senses), I'm also curious whetherCimage applies dither when down converting from 48 bit to 24 bit. This tends to increase the effective bit depth somewhat and is quite noticable in audio.

Joe
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Old May 12, 2006, 11:43 AM   #7
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No. Dither is only used when you have pallete driven color like 256 color images. When converting from 16 bits/channel to 8 bits/channel, the information is simply remapped to 0-255 instead of 0-65535. All image programs do the same thing when converting from 48-24 or 24-48. None of them use dithering because it isn't necessary.

Mike
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Old May 12, 2006, 1:31 PM   #8
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Thanks Mike, I was just curious. By the way, I received Qimage and Prism a couple of days ago and have been buried deep in the learning curve (on my old beat up Lexmark Z52 - inks much cheaper). So far they both look great.

Joe
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Old May 16, 2006, 1:22 PM   #9
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I don't know where, but I read that recent versions of Photoshop do indeed dither when doing the 16->8 bit conversion.

While I'd agree that dithering to 8-bit RGB isn't _necessary_, it is one way to preserve some of the additional color resolution in the original. It could also improve accuracy when resampling down to a lower resolution. I suspect it could actually introduce some artifacts when resampling up, though.

Dithering like this makes the most sense when 8 bits simply isn't enough resolution in all areas of the color space you're working in. It's most noticeable in color spaces like YUV and YIQ, which is why you'll see 10-bit video processing advertised in product liturature for professional (and now consumer) video gear.

I could also see wider-gamut color spaces like AdobeRGB and others benefitting from this, since the "normal" color ranges are squeezed into fewer bits.

- LoopinFool
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Old May 17, 2006, 8:05 AM   #10
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Do you have anything to document/substantiate this or some workflow that can reproduce this dithering? I've taken 16 bit/channel images and just lopped off the low order 8 bits (basically just divide by 256) and have gotten the exact same image as PhotoShop doing "Image", "Mode", "8 bits/channel". It looks to me like all they are doing is reranging and not dithering!

Mike
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LoopinFool wrote:
Quote:
I don't know where, but I read that recent versions of Photoshop do indeed dither when doing the 16->8 bit conversion.

While I'd agree that dithering to 8-bit RGB isn't _necessary_, it is one way to preserve some of the additional color resolution in the original. It could also improve accuracy when resampling down to a lower resolution. I suspect it could actually introduce some artifacts when resampling up, though.

Dithering like this makes the most sense when 8 bits simply isn't enough resolution in all areas of the color space you're working in. It's most noticeable in color spaces like YUV and YIQ, which is why you'll see 10-bit video processing advertised in product liturature for professional (and now consumer) video gear.

I could also see wider-gamut color spaces like AdobeRGB and others benefitting from this, since the "normal" color ranges are squeezed into fewer bits.

- LoopinFool
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