Datacolor User to User Support Group.

In a message dated 3/15/06 2:49:06 PM, xun911@... writes:

> I disagree. The rainbow is very unforgiving I don't think what everything 
> the rainbow reveals should be taken literally but if you know how to use it, 
> it's wonderful tool to detect problems.
> 
> Well, given that Tom has posted, as he calls it, a sharp chainsaw in the 
files section, perhaps a bit of description is in order. Granger Rainbows are 
cross-color gradients   in flat 2 dimensional image files. Since all the colors 
have one or two of the RGB channels maxxed out, they represent full color 
saturations. The gradients are basicly the "skin" of the RGB color space they are 
displayed in. They show nothing about the inside of the colorspace (where 
photographic colors live) but only define the extreme exterior.

I like to describe a 3d graph of an ICC profile as being like a bronze statue 
of a tornado; yes, it shows a familiar form, but its all about the surface of 
the tornado, when the important stuff is happening way inside. Granger 
rainbows take this superficial focus on the skindeep to a new dimension, by focusing 
our attention on how things look at the surface of a profile only. Few if any 
of these surface colors will ever be reached by photographic content; many 
cannot be reached by the monitor they are shown on, or the printer they are pri
nted on. They are in the land of vector art and artificial colors. With a 
Granger rainbow as the active window, do a Custom Proof Setup command in Photoshop, 
and choose your monitor profile. Then select the gamut limit command, and 
watch the entire rainbow disappear... its all out of gamut. Do the same with a 
matte printer profile, and you'll lose nearly as much. A gloss profile has 
marginally more or the colors in gamut.

I often see composite test images, where half the page is devoted to 
synthetic gradients and Granger rainbows... odd, since the goal is to print good photo 
color, and there is almost no overlap between these synthetic colors, and 
photo color, or for that matter, printer color. So yes, there is a purpose to 
Granger rainbows, but it is nearly as abstract as the rainbows themselves, having 
mostly to do with out of gamut colors, gamut limits, relations between 
workingspace gamut lines and printer gamut lines, etc...

C. David Tobie
Product Technology Manager
ColorVision Business Unit
Datacolor Inc.
CDTobie@...
www.colorvision.com
>