the basic idea of color profiles
the basic idea of color profiles
- Subject: the basic idea of color profiles
- From: John Gnaegy <email@hidden>
- Date: Mon, 25 Mar 2002 12:27:26 -0800
Here's the basic idea of color profiles, in case you're just getting
into this. If you already know this forgive the simplification.
Let's say you scan an image on a scanner, and you end up with a file
with a bunch of data, just bits, numbers. The numbers for a medium red
might be 880000, or 50% red, however you want to think of it. The data
in the file says "this red's appearance stimulated the scanner half the
amount of the maximum red".
Question is...half of what? On what device? You scanned it so you know
it's a scanner, but what kind of scanner? Take the same piece of slide
film and scan it on a professional scanner vs a consumer scanner and the
data you get, the numbers for the red, will be different.
So, you need to know what kind of device goes with these numbers, some
description of the device. That description is an ICC profile, also
known as a ColorSync profile. Usually people "embed a profile" into the
image file, so the profile is sort of appended to the original image
file. That way when you send the file to someone else, or open the file
in an application, the profile is there and can be used to give the data
a meaningful frame of reference.
Say you want to view this scanned image file on a monitor. According to
the numbers in the image file, this red "stimulated the scanner half the
amount of maximum red"...but that only describes how the scanner saw
it. It doesn't say anything about how a monitor might represent it, how
much to drive the electron guns. This is important to note, because
those are two different things.
You have the data describing how the scanner saw it, but what you need
now is data describing how to tell a monitor to show it. You need to
mathematically transform that data, you need to "match" the image file
from one device to another, from your scanner to your monitor.
That's why you need a profile of your monitor. One profile by itself is
meaningless, you have to have at least two, a source and a destination.
Color management is about transforming data from one device into data
for another device. Once you have a profile of the scanner and a
profile of your monitor, an application can use ColorSync to
mathematically transform the description of what the scanner saw into
meaningful numbers you can feed your monitor so that, ideally, the
monitor can display the same red the scanner saw.
You start with an image file generated by the scanner, then match it
from the scanner's color space to the monitor's color space. This
matched image data could be sent directly to the video card for the
monitor, or could be saved to disk into a second file and embedded with
the monitor profile. Since the monitor profile is embedded in this
second image file, any ColorSync aware app will know that the image data
is valid for that specific monitor device. The next time you want to
view the image you'll be able to skip the transformation step and send
the numbers right to the display.
This is a simple two color space example, input and output. You'll
notice Photoshop has the concept of a working space. It's an
intermediate space between input and output, for the sake of editing.
You'd match your scanner file from the scanner profile to the working
space profile, then edit however you want. The working space also has a
profile, and gets embedded into saved files. When you print, the image
data gets matched from your working space profile to your printer
profile.
It's easy to get confused with so many options and so many places in the
chain to select a profile. It helps to think about each step
separately, try to imagine what's happening when you open a file in
Photoshop (input to working space) or print from Photoshop (working to
output). In some apps there isn't a working space, just the simple
input and output like the scanner / monitor example. In all cases it's
important that the image data has a profile that accurately describes
the device.
---
John Gnaegy
email@hidden
colorsync
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