Color Management 101: Part A

Started Mar 15, 2008 | Discussions thread
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gollywop
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Color Management 101: Part A
Mar 15, 2008

At the risk of seeming presumptuous, which I probably am, and producing flames, which I probably will, I'm going to go out on a limb and attempt here some basic description of color management, since the issue in one form or another arises in a number of threads on this forum and seems to be a source of considerable confusion. I don't profess to be a "pro" in these matters, and what follows may be a bit off in some particulars, but the basic principles are, I feel, essentially correct -- so I hope you'll cut me some slack, and I hope you'll find it useful. My favorite image-processing software is CS3 (and ACR), and so this discussion uses this as a primary basis for illustration. However, I feel sure the basic principles would apply to most typical workflows with simple modifications. Anyone wishing to chime in with corrections or additions is more than welcome. Because of its length, this is being posted in five parts, A, B, C, D, and E. Please put your gripes, comments, and replies after part E in order to keep the original 5 posts together.

0. INTRODUCTION: There are five basic color spaces that are of importance to those who use dSLRs and who display to a printer or the web (namely, all of us). These are the camera's native (or RAW) data, the camera's assigned JPEG space, the processor's (say, Photoshop's) working space, the computer monitor's color space, and the output color space (either web or printer). These color spaces can be (and typically are) different. They differ in the amount of colors they contain (the gamut) and the numbers assigned to a particular color. I am going to assume here we are dealing with RGB (red-green-blue) color coding, in which each color is assigned a triple of RGB numbers. In 8-bit modes, each of these three numbers is between 0 and 256 (2^8) -- for a total of 2^8^3 = 16,777,216 colors --, while In 16-bit modes, the numbers are between 0 and 65536 (2^16) -- for a total of 2^16^3 = more than 2.8*10^14 colors. Assigned to each color space is a profile that specifies how the RGB triples are to be interpreted. These profiles have various names, like sRGB, Adobe RGB, ProPhoto RGB, LCDMonitor, Epson S04164X, Canon iP6700D PR1, and the like. Since the same image can pass among different color spaces, it is necessary that one be able to translate one space (one set of numbers) into another so as to keep the colors the same. To do this, something like the LAB space (CIE 1976) or XYZ space (CIE 1931) is used as a common reference (a Profile Connection Space) for this translation. This is what Color Management is all about.

LAB is a color space that has an incredibly wide gamut, encompassing every color we humans can see and many, many more that are not "real"; i.e., they exist only in theory. As I understand it, LAB's RGB numbers have been determined to be able to be associated with colors that people know. Thus, a LAB RGB triple, say, of (240, 20, 10) might correspond to a particular shade of red, like "fire-engine red." These LAB numbers become a "standard" against which all others can be compared; LAB can be used to translate one color space into another, and that's essentially what the profile information allows. All color mode changes, for example, in Photoshop are done via translations through LAB.

Let us say, now, that we take a picture of a lovely red fire engine, and let's follow through what happens.

1. CAMERA RAW DATA: First, the camera sensor captures the basic data in its separate R, G, and B pixels. These are the RAW data, and if you are shooting RAW, this is what gets sent on to your RAW processor, say ACR or CaptureNX or Olympus Master, or whatever. But, for the moment, let us suppose you are shooting JPEG. The color engine in your camera takes these RAW data and "develops" them through a process called demosaicing, combining the separate R, G, and B sensor sites into a triple of RGB numbers for each of your camera's 10Mb (or whatever) pixels. Next comes the

2. CAMERA JPEG SPACE: Let us suppose you selected sRGB as your camera's color space. This space has a particular set of RGB numbers for each color. They are not the same as LAB's or Adobe RGB's or the camera's native (RAW) space. But the camera's engineers have programmed into the camera's image processor (engine) the sensor's profile (RGB values) and those of the various color spaces the camera can be instructed to attach to the JPEGs (typically sRGB and Adobe RGB). So the engine takes the demosaiced RGB values and converts them to sRGB values and then indicates this by attaching (embedding, tagging) the sRGB name to the file to say that this is the way the numbers should be interpreted. In particular, the camera's native capture of RGB values for fire-engine red are now changed to the sRGB values for that same fire-engine red in the JPEG file that it develops. The result is an image with appropriate sRGB color values assigned to each pixel. If you had instead chosen Adobe RGB for your JPEG space, the values would be Adobe RGB values. In each case, you would have the same colors, but with different values of R, G, and B assigned and the appropriate profile name attached.

It should be noted here that, if you shoot RAW, no color space is assigned to the image. The choice of color space in the camera directly affects only the in-camera JPEG development of that image and the thumbnail that is displayed on the camera's LCD. Your choice of color space is, however, attached to the EXIF data. I will deal with this more below. But the important thing to realize is that the choice of color space in the camera has no effect on the RAW data.

Continued in part B
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gollywop

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