COLOR MANAGEMENT: COLOR SPACES

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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.

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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	(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.

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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 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.

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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.

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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).

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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.

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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.

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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.

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