Message
Re: [colorvision_group] Using Colorimeter to evaluate and edit profiles??
2008-05-11 by CDTobie@aol.com
In a message dated 5/11/08 9:56:08 AM, cgdery@... writes:
..........Learn to speak Lab. It makes clear statements about neutrality without an RGB conversion: zero values in a* and b* means neutral; positive a is red, and so on around the color wheel.........interesting. Is there something somewhere that tells me what the shift is with various A8 B8 results? (green, red blue etc)
Well, if you look at any standard color diagram, red is on the right (positive a value , if you remember your geometry), with green up (positive b ) blue to the left (negative a) and purples being down (negative b). The amount the value is from zero indicates how far from neutral. An ideal color profile for a very good, recent, two gray ink printer, on a media that works well with that printer and ink, at an appropriate media setting, can produce an extended grays profile where any neutral gray patch from 50,50,50, on up to 2000,200,200 prints with fractional a & b values, meaning no a*/b* values beyond one or negative one when you measure a gray ramp. In the real world we're pretty happy is most values are that tight, and don't necessarily expect all of them to be (there can be some glitches at ink transition zones).
But why did I start at 50 and end at 200, instead of going all the way from 0 to 255? Thats because the profile can't change the color of ink black or paper white, and the ends will be influenced by these. In fact, the "absolute blacks/relative blacks" checkbox mostly has to do with how far you want paper white's tone to run into your gray ramp: do you want it only to effect the highlights, or do you want a longer, more graceful transition into the midtones? While its easy to talk about "perfect grays" (which I would be tempted to call "literal grays") its much more difficult to decide what "ideal grays" are... the ones that make the image look neutral and harmonious with the paper its on, and the black ink it uses.
C. David Tobie
WW Product Technology Manager
Digital Imaging & Home Theater
Datacolor
CDTobie@...
www.datacolor.com/Spyder3
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