Rendering intents: Perceptual vs. RelCol
Rendering intents: Perceptual vs. RelCol
- Subject: Rendering intents: Perceptual vs. RelCol
- From: Terry Wyse <email@hidden>
- Date: Fri, 26 Jan 2001 09:53:14 -0500
At the risk of sounding stupid (yea, I know, no question is stupid), I have
a question specific to the Perceptual vs. Relative Colorimetric rendering
intents.
For the sake of argument, let's assume I'm using Adobe98 as my RGB WS and
converting to some "PressCMYK" WS that is, of course, quite a bit smaller
than my RGB WS.
Let's also assume I have two RGB images, one that is very "out-of-gamut"
relative to my PressCMYK WS ("Image A") and one that has colors that are
totally in-gamut relative to my PressCMYK WS ("Image B").
This is what I understand to be happening with my "Image A":
RelCol: RGB colors that fall *within* my CMYK WS get mapped more-or-less
one-to-one and colors *outside* my CMYK WS get clipped and mapped to colors
at the very edge of my CMYK WS, possibly overlapping some in-gamut colors at
the edge of my CMYK gamut.
Perceptual: My out-of-gamut colors get brought into my CMYK WS and all the
in-gamut colors get compressed or "scaled" in some manner that attempts to
maintain the "perception" of the original relationship between the out-of
and in-gamut colors. So far, so good.
What happens to "Image B", my totally in-gamut image:
RelCol: Nothing much happens here. Since all colors are in-gamut, everything
gets mapped one-to-one with no clipping and overlapping of colors.
(the question:)
Perceptual: What happens here? Does A) all the colors *still* get compressed
because there's no mechanism for it deduce that all the colors are in-gamut
so no compression is necessary or B) all the colors get compressed anyway
because it has to *assume* that there are colors stretching out to the edges
of the RGB WS? In other words, if you have RGB WS "A" and CMYK WS "B" and
CMM "C", every image, regardless of content or gamut, gets "compressed"
exactly the same way?
Question 2: in a RelCol gamut compression, I *assume* that as colors get
closer to the outer edge of the color gamut, the amount of compression or
"scaling" that takes place actually *increases* relative to colors that are
farther away from the perimeter of the color gamut. True?
My motivation for a better understanding here is trying to decide what is a
"better" rendering intent to use as a DEFAULT intent in Photoshop for
RGB2CMYK conversions. I've always assumed that Perceptual was preferable but
I'm now wondering if RelCol isn't a better default (more "accurate"?) and
from there just use the Gamut Warning as way to decide whether Perceptual
should be considered. So RelCol is the default and Perceptual is used on an
as needed basis. Make sense or am I nuts?
Cheers,
Terry
_____________________________
Terence (Terry) Wyse
PrePress Specialist
All Systems Integration, Inc.
781.935.3322 voice
781.935.6622 fax
http://www.allsystems.com
email@hidden
_____________________________