PNG Specification: Color Tutorial

PNG Specification: Color Tutorial
PNG (Portable Network Graphics) Specification, Version 1.2
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14. Appendix: Color Tutorial
(This appendix is not part of the formal PNG specification.)
14.1. About chromaticity
The
cHRM
chunk is used, together with the
gAMA
chunk, to convey precise color information so that a PNG image can be
displayed or printed with better color fidelity than is possible without
this information. The preceding chapters state how this information is
encoded in a PNG image. This tutorial briefly outlines the underlying
color theory for those who might not be familiar with it.
Note that displaying an image with incorrect gamma will produce
much
larger color errors than failing to use the chromaticity
data. First be sure the monitor set-up and gamma correction are right,
then worry about chromaticity.
14.2. The problem of color
The color of an object depends not only on the precise spectrum of
light emitted or reflected from it, but also on the observer--their
species, what else they can see at the same time, even what they have
recently looked at! Furthermore, two very different spectra can produce
exactly the same color sensation. Color is not an objective property of
real-world objects; it is a subjective, biological sensation. However,
by making some simplifying assumptions (such as: we are talking about
human
vision) it is possible to produce a mathematical model of
color and thereby obtain good color accuracy.
14.3. Device-dependent color
Display the same RGB data on three different monitors, side by
side, and you will get a noticeably different color balance on each
display. This is because each monitor emits a slightly different shade
and intensity of red, green, and blue light. RGB is an example of a
device-dependent color model
--the color you get depends on
the device. This also means that a particular color--represented as
say RGB 87, 146, 116 on one monitor--might have to be specified as
RGB 98, 123, 104 on another to produce the
same
color.
14.4. Device-independent color
A full physical description of a color would require specifying the
exact spectral power distribution of the light source. Fortunately,
the human eye and brain are not so sensitive as to require exact
reproduction of a spectrum. Mathematical, device-independent color
models exist that describe fairly well how a particular color will be
seen by humans. The most important device-independent color model, to
which all others can be related, was developed by the International
Commission on Illumination (CIE, in French) and is called "CIE XYZ" or
simply "XYZ".
In XYZ, X is the sum of a weighted power distribution over the whole
visible spectrum. So are Y and Z, each with different weights. Thus
any arbitrary spectral power distribution is condensed down to just
three floating-point numbers. The weights were derived from color
matching experiments done on human subjects in the 1920s. CIE XYZ has
been an International Standard since 1931, and it has a number of useful
properties:
two colors with the same XYZ values will look the same to humans
two colors with different XYZ values will not look the same
the Y value represents all the brightness information (luminance)
the XYZ color of any object can be objectively measured
Color models based on XYZ have been used for many years by people who
need accurate control of color--lighting engineers for film and TV,
paint and dyestuffs manufacturers, and so on. They are thus proven in
industrial use. Accurate, device-independent color started to spread
from high-end, specialized areas into the mainstream during the late
1980s and early 1990s, and PNG takes notice of that trend.
14.5. Calibrated device-dependent color
Traditionally, image file formats have used uncalibrated,
device-dependent color. If the precise details of the original display
device are known, it becomes possible to convert the device-dependent
colors of a particular image to device-independent ones. Making
simplifying assumptions, such as working with CRTs (which are much
easier than printers), all we need to know are the XYZ values of each
primary color and the CRT exponent.
So why does PNG not store images in XYZ instead of RGB? Well, two
reasons. First, storing images in XYZ would require more bits of
precision, which would make the files bigger. Second, all programs
would have to convert the image data before viewing it. Whether
calibrated or not, all variants of RGB are close enough that undemanding
viewers can get by with simply displaying the data without color
correction. By storing calibrated RGB, PNG retains compatibility with
existing programs that expect RGB data, yet provides enough information
for conversion to XYZ in applications that need precise colors. Thus,
we get the best of both worlds.
14.6. Chromaticity and luminance
Chromaticity is an objective measurement of the color of an object,
leaving aside the brightness information. Chromaticity uses two
parameters
and
, which are readily calculated
from XYZ:
x = X / (X + Y + Z)
y = Y / (X + Y + Z)
XYZ colors having the same chromaticity values will appear to
have the same hue but can vary in absolute brightness. Notice that
x,y
are dimensionless ratios, so they have the same values no
matter what units we've used for
X,Y,Z
The
value of an XYZ color is directly proportional to its
absolute
brightness and is called the luminance of the color. We can describe a
color either by XYZ coordinates or by chromaticity
x,y
plus
luminance
. The XYZ form has the advantage that it is linearly
related to RGB intensities.
14.7. Characterizing computer monitors
The "white point" of a monitor is the chromaticity
x,y
of the monitor's nominal white, that is, the color produced when
R=G=B=maximum
It's customary to specify monitor colors by giving the chromaticities
of the individual phosphors R, G, and B, plus the white point. The
white point allows one to infer the relative brightnesses of the three
phosphors, which isn't determined by their chromaticities alone.
Note that the absolute brightness of the monitor is not specified.
For computer graphics work, we generally don't care very much about
absolute brightness levels. Instead of dealing with absolute XYZ
values (in which
X,Y,Z
are expressed in physical units of radiated
power, such as candelas per square meter), it is convenient to work in
"relative XYZ" units, where the monitor's nominal white is taken to have
a luminance (
) of 1.0. Given this assumption, it's simple to compute
XYZ coordinates for the monitor's white, red, green, and blue from their
chromaticity values.
Why does
cHRM
use
x,y
rather than XYZ? Simply
because that is how manufacturers print the information in their spec
sheets! Usually, the first thing a program will do is convert the
cHRM
chromaticities into relative XYZ space.
14.8. Uses for XYZ
If a PNG file has the
gAMA
and
cHRM
chunks, the
source RGB values can be converted to XYZ. This lets you:
do accurate grayscale conversion (just use the
component)
convert to RGB for your own monitor (to see the original colors)
print the image in Level 2 PostScript with better color fidelity
than a simple RGB to CMYK conversion could provide
calculate an optimal color palette
pass the image data to a color management system
etc.
14.9. Converting between RGB and XYZ
Make a few simplifying assumptions first, like the monitor really
is jet black with no input and the guns don't interfere with one
another. Then, given that you know the CIE XYZ values for each of red,
green, and blue for a particular monitor, you put them into a matrix
Xr Xg Xb
M = Yr Yg Yb
Zr Zg Zb
RGB intensity samples normalized to the range 0 to 1 can be converted
to XYZ by matrix multiplication. (If you have gamma-encoded RGB
samples, first undo the gamma encoding.)
X R
Y = M G
Z B
In other words,
X = Xr*R + Xg*G + Xb*B
, and similarly
for
and
. You can go the other way too:
R X
G = invM Y
B Z
where
invM
is the inverse of the matrix
14.10. Device gamut
The gamut of a device is the subset of visible colors that the
device can display. (It has nothing to do with
gamma
.) The
gamut of an RGB device can be visualized as a polyhedron in XYZ space;
the vertices correspond to the device's black, blue, red, green,
magenta, cyan, yellow, and white.
Different devices have different gamuts, in other words one device
will be able to display certain colors (usually highly saturated ones)
that another device cannot. The gamut of a particular RGB device can be
determined from its R, G, and B chromaticities and white point (the same
values given in the
cHRM
chunk). The gamut of a color printer
is more complex and can be determined only by measurement. However,
printer gamuts are typically smaller than monitor gamuts, meaning that
there can be many colors in a displayable image that cannot physically
be printed.
Converting image data from one device to another generally results in
gamut mismatches--colors that cannot be represented exactly on the
destination device. The process of making the colors fit, which can
range from a simple clip to elaborate nonlinear scaling transformations,
is termed gamut mapping. The aim is to produce a reasonable visual
representation of the original image.
Further reading
14.11. Further reading
References
COLOR-1
through
COLOR-5
provide more detail about
color theory.
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