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John
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 Posted: Fri Aug 31, 2007 11:10 am Post subject: Color conversion for 6 or 7 inks |
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Hello,
I have found a formula for converting RGB->CMYK
Some printers work with 6 or 7 colors, where can I found formulas for
converting to these CMYK+XXX ?
Thanks
John |
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| Posted: Fri Aug 31, 2007 5:35 pm Post subject: Re: Color conversion for 6 or 7 inks |
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John wrote:
| Quote: |
Hello,
I have found a formula for converting RGB->CMYK
Some printers work with 6 or 7 colors, where can I found formulas for
converting to these CMYK+XXX ?
Thanks
John
|
If you want to get *any* kind of precision and interoperability forget
the formula -- use ICC profiles instead of.
You can build your own profiles, e.g. with ArgyllCMS
<http://argyllcms.com> (open source) or with the colorxact online
profiler
<http://www.colorxact.net/component/option,com_wrapper/Itemid,276/>
(public beta), but you need a spectrophotometer to measure the device
response.
Consumer CMYKxxx-Printers are controlled with RGB data usually (the
printer driver handles the RGB to device conversion).
CMYKxxx-Printers with "light" inks (e.g. CMYK + light cyan + light
magenta + gray) controlled by professional PostScript RIPs are fed with
CMYK data and profiled as CMYK devices. The split-up between normal and
light ink is controlled by the RIP. Typically the curve for the light
ink has a maximum somewhere in the mid-tones and decreases to nearly
zero at 100% while the normal ink starts roughly in the quarter tones
and increases monotonically.
Only devices with additional primary colors (e.g. "Hexacrome" Printers
with CMYK + orange + green + blue) benefit from profiles with more than
four channels. Creating such profiles is costly and requires expensive
software like X-Rite ProfileMaker Publish
<http://www.xrite.com/product_overview.aspx?ID=793> ProfileMaker
Packaging <http://www.xrite.com/product_overview.aspx?ID=795> or
MonacoPROFILER Platinum <http://www.xrite.com/product_overview.aspx?ID=583>
Klaus
--
echo '4b6c617573204b6172636865722c206d61696c746f3a6c6973
7473406469676974616c70726f6f662e696e666f0a' | xxd -r -ps |
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John
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| Posted: Fri Aug 31, 2007 8:30 pm Post subject: Re: Color conversion for 6 or 7 inks |
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Thanks Klaus for your reply.
In fact I don't need something precise like ICC profile, I just need a
"standard" formula like it exists for RGB<->CMYK
Thanks |
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Graeme Gill
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| Posted: Sat Sep 01, 2007 7:23 am Post subject: Re: Color conversion for 6 or 7 inks |
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John wrote:
| Quote: |
In fact I don't need something precise like ICC profile, I just need a
"standard" formula like it exists for RGB<->CMYK
|
If you want to use the extremely crude approximations along
the lines of C = 1 - R, then simply treat the printing device
as set of block RGB filters. Assume the mixing model
is filters in series with the RGB emanating from the
substrate (ie. the C filter reduces R, M reduces G, Y reduces B).
Decompose the non CMY inks into CMY ink equivalents, apply the
them (multiply R by 1-C etc.). Set the non CMY inks to
zero on conversion the other way.
Graeme Gill. |
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John Venture
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| Posted: Mon Sep 03, 2007 11:11 am Post subject: Re: Color conversion for 6 or 7 inks |
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| Thanks Graeme |
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