Technology 3B - Computers

1. What is the purpose of colour management ? (http://www.normankoren.com/color_management.html) viewed 21 March 2012.

A1. "Obtaining predictable color reproduction in the digital darkroom can be a challenge because each device-- digital camera, scanner, monitor, or printer-- responds to or produces color differently."

2. What problem makes colour management necessary ? (http://www.luminous-landscape.com/tutorials/whats-the-problem.shtml) viewed 21 March 2012.

A2. Colour exists in the human brain and people don't see colour matches. There is a large variation in human perception among people. Without colour management standards reproduction of colours is not accurate. You must have calibration standards to ensure colour calibration is true across various devices with different colour gamut.

3. What are the components of a device profile (ie what information do they contain)? (http://www.adobepress.com/articles/article.asp?p=1315593&seqNum=2) viewed 22 March 2012.

A3. Colour-Matching Engine AKA Colour-matching method (CMM) software that converts color meanings between different device-specific colour spaces. Adobe had the Adobe Color Engine (ACE).

The reference color space (also known as the profile connection space, or PCS) is a device-independent, perceptually based color space. Most current CMSs use a CIE-defined color space, such as CIE Lab or CIE XYZ. You never have to work directly with the reference color space; it's the theory behind how the software works.

A profile describes the behavior of a device like a scanner, monitor, or printer. For instance, a profile can tell the CMS, "This is the reddest red that this device can output." A profile can also define a virtual color space that's unrelated to any particular device (the Adobe RGB space is an example of this; we'll see how it's useful later on). Profiles are the key to color management. Without a profile, 100 percent red has no specific meaning; with a profile, the color management system can say, "Oh, this color is supposed to be red in the specific way that red appears on that printer." Profiles conform to the standard ICC (International Color Consortium) specification that lets them work with all CMSs on all platforms. ColorSync profiles on the Mac and .icm or .icc profiles in Windows both follow the ICC spec and work on both platforms.

Fortunately, you have to work only with the last of the three components: profiles. You'll run into profiles if your images come from many sources or go to many different types of output media, while the CMM and reference color space may never need to be changed and are usually invisible to you.

... the only thing CMSs do. They convert color data from one color space (one "language") to another, using profiles to preserve the intended appearance of the colors throughout the workflow.

4. What is the difference between a device profile and a working space ? (http://en.wikipedia.org/wiki/Color_management) viewed 22 March 2012. and/or (http://www.adobepress.com/articles/article.asp?p=1315593&seqNum=2) viewed 22 March 2012.

A4. What's the Difference Between Document, Device, and Working Spaces? These are all color spaces that are simply used in different ways. One way to think about this is that they go from general to specific:

The working space that you set in the Edit > Color Settings dialog is the default color space you set in Photoshop. If you start a new document or open a document that doesn't have a profile, the working space is the profile that will be associated with the image. It also means that if you're always opening images that already contain the right profile, the working space never comes into play. You'll notice that there are four working spaces in the Color Settings dialog—that's because each color mode gets its own default. RGB has its own working space, CMYK has its own, and so on. When picking an RGB working space, it's usually best to choose one that's built into Photoshop; for more information see "About the Built-In RGB Working Spaces" later in this chapter.

The document color space is just another way of saying "the profile that's embedded inside an image." If there is no profile embedded in an image, you can either let your Photoshop default working space take over, manually assign a profile to it, or tell Photoshop to leave it untagged (that is, don't color-manage the document). Photoshop handles document profiles very intelligently: If you have five documents open and each has its own correct but very different profile, there won't be any need to apply the working space to any of them, and in addition, Photoshop will maintain each document's profile separately. Photoshop won't let one document's profile affect another document.

A device color space represents the range of color produced by a device you use to create or output images. On the creation side, it could be a digital camera or scanner. On the output side, it could be a printer. As we've discussed, device color spaces are valuable for precisely describing the colors of the device that an image came from or is going to, but they are not good for editing, so you'll typically run into device color spaces (device profiles) when you first create or finally output an image.

In a typical image-processing workflow among color spaces, an image begins its life containing a device-specific source profile and gets converted to a standard, perceptually uniform workspace (such as Adobe RGB or sRGB) for editing and archival storage. Media-specific copies of the image are then converted to the color spaces for the media where they'll be used (the Web, print, video, and so on).

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5. What is a ‘reference colour space’ and how are they used ? Give an example of one. (http://www.cambridgeincolour.com/tutorials/color-spaces.htm) viewed 22 March 2012.

A5. Reference colour space is also known as Device-independent space and defined by the above reference – Device-independent spaces express color in absolute terms. These often serve as universal reference colors, so they're useful as a backdrop for comparing other devices. Otherwise these are usually an unseen color space, since they're knowingly interacted with during the photo editing process only rarely. ... Device-independent spaces express color in absolute terms. These often serve as universal reference colors, so they're useful as a backdrop for comparing other devices. Otherwise these are usually an unseen color space, since they're knowingly interacted with during the photo editing process only rarely.

Examples are Adobe RGB 1988 and sRGB IEC61966-2.1

Other sources of info on reference space below.

2. Profile connection space (PCS)

A key component of the specification is a well-defined profile connection space. This standard color space is the interface which provides an unambiguous connection between the input and output profiles as illustrated in the diagram below. It allows the profile transforms for input, display, and output devices to be decoupled so that they can be produced independently. A well-defined PCS provides the common interface for the individual device profiles. It is the virtual destination for input transforms and the virtual source for output transforms. If the input and output transforms are based on the same PCS definition, even though they are created independently, they can be paired arbitrarily at run time by the color-management engine (CMM) and will yield consistent and predictable results when applied to color values.

The profile connection space is based on the CIE 1931 standard colorimetric observer. This experimentally derived standard observer provides a very good representation of the human visual system color matching capabilities. Unlike device dependent color spaces, if two colors have the same CIE colorimetry they will match if viewed under the same conditions as those defined for the colorimetry.

Because images are typically produced for a wide variety of viewing environments, it is necessary to go beyond simple application of the CIE system. The profile connection space is defined as the CIE colorimetry which, in the case of the perceptual rendering intent (defined later), will produce the desired color appearance if rendered on a reference imaging media and viewed in a reference viewing environment. This reference corresponds to an ideal reflection print viewed in a standard viewing booth conforming to ISO standard viewing conditions.

The default measurement parameters for the profile connection space and all other color spaces defined in this specification are based on the ISO 13655 standard, "Graphic technology - Spectral measurement and colorimetric computation for graphic arts images." Essentially this defines a standard illuminant of D50, the 1931 CIE standard colorimetric observer, and 0° /45° or 45° /0° measurement geometry measured with a black backing behind the print for the reflectance measurements. The reference viewing condition is that defined in ISO 3664 as viewing condition P2 using the recommended 20% surround reflectance. This is a graphics arts and photography print viewing environment with a D50 illumination level of 500 lux.

One of the first steps in profile building involves measuring a set of colors from some imaging media or display. If the imaging media or viewing environment differ from the reference, it will be necessary to adapt the colorimetric data to that appropriate for the profile connection space. These adaptations account for such differences as white point chromaticity and luminance relative to an ideal reflector, maximum density, viewing surround, viewing illuminant, and flare. Currently, it is the responsibility of the profile builder to do this adaptation. However, the possibility of allowing a variable illuminant in the PCS is under active consideration by the International Color Consortium. (International Color Consortium undtd, viewed 22 March 2012,<http://www.color.org/profile.xalter>)

"... a colour managed system will require three basic components, namely: -

A device-independent colour space - this is usually referred to as the Working Space or Reference Colour Space.
ICC/ColorSync device profiles for each device (printer, scanner, monitor, digital camera, etc.) that describe the colour characteristics of the specific device.
A Colour Matching Module (CMM) that will interpret the information contained within a device profile and carry out the instructions on the way the colour gamut of each device should be treated."

(Lyons I 2002, Photoshop 7 Colour Settings, viewed 22 March 2012 <http://www.computer-darkroom.com/ps7_colour/ps7_1.htm>)

6. What is the difference between ‘calibrating’ and ‘profiling’ ? (http://www.adorama.com/alc/article/8525) viewed 22 March 2012.

In photography and digital printing, calibrating means to bring a device, such as a flat screen monitor, to a repeatable operating state, usually with the expectation that it will maintain that state over days or months before having to be recalibrated. ... LCD computer displays are shipped to users in a similar fashion, with the brightness, contrast, or color level adjustment set at specific default settings. A monitor can then be calibrated more accurately using various software or hardware tools.

Also keep in mind that the environment in which your monitor is being operated also plays a major role in the way your images will look. Light from windows, overhead lighting, and even the color of the walls and ceiling in the space where you work will all influence the overall look of your display. Some hardware devices allow you to take ambient light measurements, which can be helpful.

After you calibrate your monitor, you can create and install a profile for it so that your images will look more accurate on screen. To help you understand what a monitor profile does, think of it as being like a Photoshop adjustment layer or curve that is applied to and “fixes” the images you see on your screen.

The issue of calibration also applies to inkjet printers. Most inkjet printers have printheads and use multiple inks, and as they age or when ink is changed, the tiny nozzles that eject ink from the print head can clog or become misaligned over time, leading to subtle changes in performance, such as lower ink output or less overall color saturation.These changes are referred to as drift.

Indoor temperature and humidity can also affect consistency over time, or even right out of the box. For example, two printers that are calibrated at the factory may perform differently if they are operatedin locations with very different climates or elevations.

Printer profiles are small files that describe the gamut (number of printable colors) of a specific printer's paper and ink combination (they have a .icc or .icm extension). Profiles are often downloaded when you install a printer driver, or you can download them online from the websites of many paper manufacturers, companies, or individual photographers.

Profiles are generally categorized in the following ways: input profiles (for scanners and cameras); display profiles (for monitors and projectors); editing space (or working space) profiles, which are often embedded in digital files, such as JPEG or TIFF files; and output profiles (also known as printer profiles).

7. What is a Rendering Intent ? (http://www.cambridgeincolour.com/tutorials/color-space-conversion.htm) viewed 22 March 2012

The translation stage attempts to create a best match between devices — even when seemingly incompatible. If the original device has a larger color gamut than the final device, some of the those colors will be outside the final device's color space. These "out-of-gamut colors" occur with nearly every conversion and are called a gamut mismatch. Each time a gamut mismatch occurs, the CMM uses the rendering intent to decide what qualities of the image it should prioritize. Common rendering intents include: absolute and relative colorimetric, perceptual, and saturation. Each of these types maintains one property of color at the expense of others (described below). [See question 8 response.]

8. Which Rendering Intents are most useful to photographers, and when would you use each of them ? (http://en.wikipedia.org/wiki/Color_management) viewed 22 March 2012

Relative colorimetric

The goal in relative colorimetry is to be truthful to the specified color, with only a correction for the media. Relative colorimetry is useful in proofing applications, since you are using it to get an idea of how a print on one device will appear on a different device. Media differences are the only thing you really would like to adjust for. Obviously there has to be some gamut mapping going on also. Usually this is done in a way where hue and lightness are maintained at the cost of reduced saturation. Relative colorimetric is the default rendering intent on most systems.

Perceptual and Saturation

The perceptual and saturation intents are where the results really depend upon the profile maker. This is even how some of the competitors in this market differentiate themselves. These intents should be created by the profile maker so that pleasing images occur with the perceptual intent while eye-catching business graphics occur with the saturation intent. This is achieved through the use of different perceptual remaps of the data as well as different gamut mapping methods. Perceptual rendering is recommended for color separation.

In practice, photographers almost always use relative or perceptual intent, as for natural images, absolute causes color cast, while saturation produces unnatural colors.^[4] Relative intent handles out-of-gamut by clipping (burning) these colors to the edge of the gamut, leaving in-gamut colors unchanged, while perceptual intent smoothly moves out-of-gamut colors into gamut, preserving gradations, but distorts in-gamut colors in the process. If an entire image is in-gamut, relative is perfect, but when there are out of gamut colors, which is more preferable depends on a case-by-case basis.

The follows from question 7 using the same reference (http://www.cambridgeincolour.com/tutorials/color-space-conversion.htm) viewed 22 March 2012

Perceptual and relative colorimetric rendering are probably the most useful conversion types for digital photography. Each places a different priority on how they render colors within the gamut mismatch region. Relative colorimetric maintains a near exact relationship between in gamut colors, even if this clips out of gamut colors. In contrast, perceptual rendering tries to also preserve some relationship between out of gamut colors, even if this results in inaccuracies for in gamut colors. The following example demonstrates an extreme case for an image within a 1-D black-magenta color space:

Original Image:
A = Wide Gamut Space B = Narrow Gamut Space (Destination Space)

	Relative Colorimetric	Perceptual
A
↓
B
	Converted Image:	Converted Image:

Note how perceptual maintains smooth color gradations throughout by compressing the entire tonal range, whereas relative colorimetric clips out of gamut colors (at center of magenta globules and in the darkness between them). For 2D and 3D color spaces, relative colorimetric maps these to the closest reproducible hue in the destination space.

Even though perceptual rendering compresses the entire gamut, note how it remaps the central tones more precisely than those at the edges of the gamut. The exact conversion depends on what CMM is used for the conversion; Adobe ACE, Microsoft ICM and Apple ColorSynch are some of the most common.

Another distinction is that perceptual does not destroy any color information — it just redistributes it. Relative colorimetric, on the other hand, does destroy color information. This means that conversion using relative colorimetric intent is irreversible, while perceptual can be reversed. This is not to say that converting from space A to B and then back to A again using perceptual will reproduce the original; this would require careful use of tone curves to reverse the color compression caused by the conversion.

To answer which one would I use? It depends. If the image were going on a website or projected I wouldn't worry about it and go with the default. If I am printing I would make the decision when I view the soft proof and view it with both renderings and the out of gamut warnings and determine which to use by the one that is most pleasing to ME in the soft proof.

Dynamic Range is the ratio between the bright and dark areas in a scene. HDR is a technique for creating image files which can encompass the entire range of real world tonal values.

To produce a HDE image aof a scene with an excessively high luminance range you udst capture a number of 8 or 16 bpc (bits per channel) images using different exposures, and merge them into a single DHR 32 bpc image. The number of images required will depend on the lumanance range of the scene.

A 32 bpc HDR image file contains far too many tones to be viewed on a monitor or sent to a printer. It is only possile to view a small range of tones at a time. Tone Mapping is a process of controlling how the extended range of tones in a 32 bpc HDR image file are compressed and manipulated to produce a conventional 8 or 16 bpc image, which can be viewed or printed.
(Above was taken from the Technology 3 handout High Dynamic Range Imagery)

My Canon EOS 5D Mark II as do most Canon SLR camera have Auto Exposure Bracketing (AEB) settings that allows the camera to automatically bracket exposure up to ± 2 stops in 1/3 stop increments for three successive shots. In addition to the exposure bracketing, the user can set the exposure compensation so the bracketting can be shifted to the ± 4 stop exposure but still only bracket within 2 stops.

(Canon Inc. 2009, Canon EOS 5D Mark II Instruction Manual, pg 97)

I have seen discussions on various web forums about the number of shots that are needed to producing HDR and there seems to be differing opinions if 3 are sufficient or more should be used due to the dynamic range of the scene. Those wanting to expose more than 3 images complain about the Canon's ability to shoot only a 3 shot bracket. There does appear to be a relatively easy work around as follows.

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Ridiculous. AEB can also be used very easily for bracketing for HDR. Manually adjusting each exposure can certainly be done, but increases the chances of movement between frames.

Even with Canon's 3-exposure AEB limit, I can bracket 9 to 12 shots extremely rapidly -- much, MUCH faster than I could manually shot-by-shot. For shots that include the sky (in which the clouds are ALWAYS moving), this can mean the difference between virtually unnoticeable ghosting in the clouds and an unusable final image.

Getting more than 3 shots isn't really all that complicated. Your exposure meter in your viewfinder shows you where all three brackets are placed. I use 1-stop brackets most of the time (because I found that with the software I use, the smaller gap between exposures produces better results).

Meter your shot as you would for a single exposure. If you normally "expose to the right," ignore that and meter for "0" in your viewfinder for HDR work. Set your camera to bracket 3 shots. Set your dial to jump in 1-stop increments. Dial your exposure until the longest exposure of your bracket (the notch on the left) is two stops above "0" (to the right) of your meter. Take three shots. Dial back to put your center notch at "0." Take three shots. Dial back to put the shortest exposure (the notch on the right) two stops below (to the left) of "0."

9 exposures, from -4 to +4, in 1-stop increments. That will give you a pretty good range for most scenes and just adjust how many brackets you need accordingly.

With a little practice, you can take a 9-bracket set like this without losing much more time than someone shooting a camera capable of more bracketed shots.

If you choose to bracket at something different than 1-stop increments, you may have to adjust your math a bit to determine how far to dial for your starting bracket, but the basic concept remains the same.

(Flickr 2012. flickr canon photography/ discuss group AEB setting. (Using the Auto Exposure Bracket), viewed 18 March 2012 <http://www.flickr.com/groups/canon_photography/discuss/72157627494534787/ viewed 19 March 2012>