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COLOR MANAGEMENT: COLOR SPACES

2009年06月20日, 星期六 留下评论 Go to comments

http://www.cambridgeincolour.com/tutorials/color-spaces.htm

A color space relates numbers to actual colors, and is a
three-dimensional object which contains all realizable color
combinations.  When trying to reproduce color on another
device, color spaces can show whether you will be able to retain
shadow/highlight detail, color saturation, and by how much either
will be compromised.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfTYPES

Color spaces can be either dependent to or independent of a given
device.  Device-dependent spaces express color relative to some other
color space, while device-independent color spaces express color in
absolute terms.  Device-dependent color spaces can tell you valuable
information by describing the subset of colors which can be shown with
a monitor or printer, or can be captured with a camera or scanner. 
Devices with a large color space, or "wide gamut," can realize more
extreme colors, whereas the opposite is true for a device with a narrow
gamut color space.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfVISUALIZING COLOR SPACES

Each dimension in "color
space" represents some aspect of color, such as lightness,
saturation or
hue, depending on the type of space.  The two
diagrams below show the outer surface of a sample color space from
two different viewing angles; its surface
includes the most extreme colors of the space.  The vertical
dimension represents luminosity, whereas the two horizontal
dimensions represent the red-green and yellow-blue shift. 
These dimensions could also be described using other color properties.

Sample Color Space: Adobe RGB 1998 Sample Color Space: Adobe RGB 1998, view 2
  Sample Color Space (Same Space Rotated 180�)  

The above color space is intended to help you qualitatively understand
and visualize a color space, however it would not be very useful for
real-world color management.  This is because a color space almost
always needs to be compared to another space.  In order to visualize
this, color spaces are often represented by two-dimensional regions.  
These are more useful for everyday purposes since they allow you to
quickly see the entire boundary of a given cross-section.  Unless
specified otherwise, two-dimensional diagrams usually show the
cross-section containing all colors which are at 50% luminance (a
horizontal slice at the vertical midpoint for the color space shown
above).  The following diagram shows three example color spaces: sRGB,
Wide Gamut RGB, and a device-independent reference space.  sRGB and
Wide Gamut RGB are two working spaces
sometimes used for image editing.


2D Color Space Comparison

(Colors at 50% Luminance)

What can we infer from a 2D color space comparison? 
Both the black and white outlines show the subset of colors which
are reproducible by each color space, as a fraction of some
device-independent reference space.  Colors shown in the
reference color space are only for qualitative visualization, as
these depend on how your display device renders color.  In
addition, the reference space almost always contains more colors
than can be shown on a computer display.

For this particular diagram, we see that the "Wide Gamut RGB" color space contains
more extreme reds, purples, and greens, whereas the "sRGB" color
space contains slightly more blues.  Keep in mind that this
analysis only applies for colors at 50% luminance, which is what
occupies the midtones of an
image histogram
.  If we were interested in the color gamut for the
shadows or highlights, we could look at a similar 2D cross-section
of the color space at roughly 25% and 75% luminance, respectively.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfREFERENCE SPACES

What is the device-independent reference space shown above? 
Nearly all color management software today uses a
device-independent space
defined by the Commission International de l’ éclairage
(CIE) in 1931.  This space aims to describe all colors
visible to the human eye based upon the average response from a set
of people with no vision problems (termed a "standard colorimetric
observer"). 
Nearly all devices are subsets of the visible colors specified by
the CIE (including your display device), and so any representation of this space on a monitor should
be taken as qualitative and highly inaccurate.

The CIE space of visible color is expressed in several common forms:
CIE xyz (1931), CIE L*a*b*, and CIE L u’v’ (1976). 
Each contains the same colors, however they differ in how they
distribute color onto a two-dimensional space:

CIE xyz Color Space CIE L*a*b Color Space CIE u'v' Color Space
CIE xy CIE a*b* CIE u’v’

(All color spaces shown are 2D cross-sections at 50% Luminance)

CIE xyz is based on a direct graph of the original X, Y and Z
tristimulus functions created in 1931. 
The problem with this representation is that it allocates too much
area to the greens.  CIE L u’v’ was created to correct for this
distortion by distributing colors roughly proportional to their
perceived color difference.  Finally, CIE L*a*b* transforms the
CIE colors so that they extend equally on two axes– conveniently
filling a square.  Furthermore, each axis in L*a*b* color space
represents an easily recognizable property of color, such as the
red-green and blue-yellow shifts used in the 3D visualization above.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfWORKING SPACES

A working space is used in image editing programs (such as Adobe
Photoshop), and defines the set of colors available to work
with when performing any image editing.  Two of the most
commonly used working spaces in digital photography are Adobe RGB
1998 and sRGB IEC61966-2.1.  For an in-depth comparison for
each of these color spaces, please see
sRGB vs. Adobe RGB 1998.

Why not use a working space with the widest gamut possible?  It
is generally best to use a color space which contains all colors
which your final output device can render (usually the printer), but
no more.  Using a color space with an excessively wide gamut can increase the
susceptibility of your image to posterization
This is because the bit depth
is stretched over a greater area of colors, and so fewer bits are available to
encode a given color gradation.

sRGB vs. ADOBE RGB 1998:

Adobe RGB 1998 and sRGB IEC61966-2.1 (sRGB) are two of the most common
working spaces used in digital photography.  This section aims
to clear up some of the confusion associated with sRGB and Adobe RGB
1998, and to provide guidance on when to use each working space.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfBACKGROUND

sRGB is a RGB color space proposed by HP and Microsoft because it approximates the color
gamut of the most common computer display devices.  Since sRGB
serves as a "best guess" for how another person’s monitor produces
color, it has become the standard color space for displaying images
on the internet.  sRGB’s color gamut encompasses just 35% of
the visible colors specified by CIE (see section on
color spaces).  Although sRGB results in one
of the narrowest gamuts of any working space, sRGB’s gamut is still
considered broad enough for most color applications.

Adobe RGB 1998
was designed (by Adobe Systems, Inc.) to encompass most of the colors
achievable on CMYK printers, but by using only RGB primary colors on a
device such as your computer display.  The Adobe RGB 1998 working space
encompasses roughly 50% of the visible colors specified by CIE–
improving upon sRGB’s gamut primarily in cyan-greens.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfGAMUT COMPARISON

The following color gamut comparison aims to give you a better qualitative
understanding of where the gamut of Adobe RGB 1998 extends beyond sRGB for shadow
(~25%), midtone  (~50%), and highlight colors (~75%).

sRGB IEC61966-2.1 Adobe RGB 1998
25% Luminance 50% Luminance 75% Luminance

Comparison uses CIE L*a*b* reference space; colors are only qualitative to aid in visualization.

Note how Adobe RGB 1998 extends into richer cyans and greens than does
sRGB– for all tonal levels.  The 50% luminance
diagram is often used to compare these two working spaces, however
the shadow and highlight diagrams also deserve attention. 
Adobe RGB 1998 extends its advantage in the cyan-greens for the
highlights, but now has advantages with intense magentas, oranges,
and yellows– colors
which can add to the drama of a bright sunset.  Adobe RGB 1998
does not extend as far beyond sRGB in the shadows, however it still
shows advantages in the dark greens (often encountered with dark
foliage).

http://www.cambridgeincolour.com/fonts/MathSoftText.swfIN PRINT

All of these extra colors in Adobe RGB 1998 are great to have for viewing on a computer
monitor, but can we actually reproduce them in a print?  It
would be a shame to edit using these extra colors, only to
later retract their intensity due to printer limitations.  The
following diagrams compare sRGB and Adobe RGB 1998 with two common
printers: a Fuji Frontier (390) and a high-end inkjet printer with 8
inks (Canon iP9900 on Photo Paper Pro).  A Fuji Frontier printer is what large companies
such as Walmart use for making their prints.

sRGB IEC61966-2.1 Adobe RGB 1998
25% Luminance 50% Luminance 75% Luminance
Select Printer Type: Fuji Frontier High-End Inkjet

Comparison uses CIE L*a*b* reference space; colors are only qualitative to aid in visualization.

We see a big difference in how each printer uses the
additional colors provided by Adobe RGB 1998: The Fuji Frontier only
uses a small patch of yellow in the highlights, whereas the high-end
inkjet printer exceeds sRGB for colors in shadows, midtones,
and highlights.  The high-end inkjet even exceeds the gamut of
Adobe RGB 1998 for cyan-green midtones and yellow highlights.

The printer should also be considered when choosing
a color space, as this can have a big influence on whether the extra
colors are utilized.  Most mid-range printer companies provide a downloadable color profile for their printer.  This
color profile can help you achieve similar conclusions to those
visible
in the above analysis.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfINFLUENCE ON BIT DEPTH DISTRIBUTION

Since the Adobe RGB 1998 working space clearly provides more colors to
work with, why not just use it in every situation?  Another
factor to consider is how each working space influences the
distribution of your image’s
bit depth
Color spaces with larger gamuts "stretch" the bits over a broader
region of colors, whereas smaller gamuts concentrate these bits
within a narrow region.  Consider the following green "color
spaces" on a line:

Large Gamut
 
Small Gamut

If our image contained only shades of green in the small gamut color
space, then we would be wasting bits by allocating them to
encode colors outside the small gamut:

  For a limited bit depth which encodes all colors within the large gamut:
Large Gamut
 
Small Gamut
Wasted Bits
  If all bits were concentrated within the smaller gamut:

A similar concentration of bit depth occurs with sRGB versus Adobe RGB
1998, except in three dimensions, and not quite as dramatic as
demonstrated above.  Adobe RGB 1998 occupies roughly 40% more
volume than sRGB, so you are only utilizing 70% of your bit depth if
the colors in Adobe RGB 1998 are unnecessary (for evenly spaced
bits).  On the other hand, you may have plenty of "spare" bits
if you are using a 16-bit image, and so any reduction due to your
choice of working space might be negligible.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfSUMMARY

My advice is to know which colors your image uses, and whether these
can benefit from the additional colors afforded by Adobe RGB 1998.  Ask
yourself: do you really need the richer cyan-green midtones,
orange-magenta highlights, or green shadows?  Will these colors also be
visible in the final print?  Will these differences even be
noticeable?  If you’ve answered "no" to any of these questions, then
you would be better served using sRGB.  sRGB will make the most of your
bit depth because it allocates more bits to encoding the colors present
in your image.  In addition, sRGB can simplify your workflow since this
color space is also used for displaying images on the internet.

What if you desire a speedy workflow, and do not wish to decide on
your working space using a case-by-case method?  My advice is
to use Adobe RGB 1998 if you normally work with 16-bit images, and
sRGB if you normally work with 8-bit images.  Even if you may
not always use the extra colors, you never want to eliminate them as
a possibility for those images which require them.

http://www.cambridgeincolour.com/fonts/MathSoftText.swfOTHER NOTES

It is apparent that Adobe RGB 1998 has a larger gamut than sRGB, but
by how much?  Adobe RGB is often depicted has having a superior
gamut in greens, however this can be misleading and results mainly
from the use of the CIE xyz reference space.  Consider the
following comparison:

sRGB IEC61966-2.1 Adobe RGB 1998
CIE xy
Exaggerates difference in greens
CIE u’v’
Closer to the eye’s perceived difference

When the two are compared using the CIE u’v’ reference space, the
advantage in greens becomes less apparent.  In addition, the
diagram on the right now shows Adobe RGB 1998 having similar
advantages in both the cyans and greens– better representing the
relative advantage we might perceive with our eyes.  Care
should be taken to also consider the influence of a reference space
when drawing conclusions from any color space comparison diagram.

Wanna see more detail?
go to :http://www.cambridgeincolour.com/tutorials/sRGB-AdobeRGB1998.htm

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