ColorPerfect, ColorNeg et al. and RGB / grayscale working spaces

User feedback tells us that there is considerable confusion about how ColorPerfect, ColorNeg et al. relate to RGB and Grayscale working space profiles in Photoshop. Many users believe this aspect of processing images with color integrity to be much more complicated than it actually is, involving conversions between such color spaces. In this article we are going to try to clarify how color profiles really relate to ColorPerfect. Essentially these considerations apply to all images captured by scanners or digital cameras that are processed with our plug-ins, including the previous versions of ColorNeg and ColorPos.

What color space does ColorPerfect use

What color space is being used for displaying the preview and final image in both RGB and monochrome mode actually depends on your settings in Photoshop alone. ColorPerfect does not convert between color spaces in the sense you might be familiar with. That this is so is not a shortcoming. Any conversion of this sort can hurt your image's color, especially in the early stages of image correction where large adjustments and calibration may be necessary.

The preview image shown by ColorPerfect is rendered by Photoshop using the currently active color profile for that image in Photoshop. The only color space-related operation in ColorPerfect is decoding the input image - normally from Linear form for negatives and from either Linear or Gamma C encoded form for positives - and encoding the output image - according to the Gamma C value specified. Gamma C refers to the tone reproduction curve used in the working space selected for the image and thus needs to be set accordingly.

Which Gamma C setting relates to what color space

ColorPerfect's Gamma C needs to be set to a value of 1.8 for Apple RGB, ColorMatch RGB or ECI RGB v1 and to a value of 2.2 for Adobe RGB 1998. sRGB uses a specific tone reproduction curve listed under the same name and ECI RGB v2 uses an L* curve. If you use another working space you will likely already know its intrinsic tone reproduction curve.

Do assign the color space of your choice prior to using ColorPerfect

The most common scenarios for Gamma C are either that your image is already encoded in your target color space which means that input Gamma C and output Gamma C are the same or that your input is linear while your output is not. Using an advanced configuration found among ColorPerfect's options it is also possible to set the two to differing settings of your choosing. In any case you will have to make sure that the color space desired for the image resulting from ColorPerfect is the one that's assigned to the source image file in Photoshop. The profile in place for the image when ColorPerfect is called controls how Photoshop renders the preview image ColorPerfect shows you, so the working profile should be assigned before starting ColorPerfect and inside ColorPerfect the Gamma C needs to be set to the value that corresponds to that color space.

There is no assign profile command in Photoshop Elements and color options are very limited so following the remainder of this article might be difficult for PSE users. If you use Photoshop Elements follow this link for specific instructions before reading on.

It is recommended to always actually assign a color space to your image because Photoshop's setting "don't color manage this document" still color manages that document - Photoshop interprets the image data to be encoded in its standard working space as defined under edit > color settings. The problem with that is that this setting will not be saved with individual image files and might well be different on another or future installation of Photoshop so that an image might change visually depending on where it's opened or printed.

Monochrome Images

All tone reproduction curves supported for color images also are supported for monochrome ones. Technically, the working space in use for grayscale images thus should not be a dot gain but must relate to a supported Gamma C setting. Common grayscale spaces directly supported by ColorPerfect are Gray Gamma 1.8, Gray Gamma 2.2 and sGray which uses an sRGB encoding. Any of these are suitable for 16 bit images. If you prefer a dot gain profile as the end result, the image can be converted (not assigned) to your preferred profile after using ColorPerfect or ColorNeg, but be sure to remain in 16 bit mode through the conversion. In practice you may find the difference between your preferred dot gain and one of the supported Gamma C settings to be small enough to ignore. The following combinations are the good practical matches: 15% Dot Gain => Gamma C 1.5, 20% Dot Gain => Gamma C 1.8, 25% Dot Gain => Gamma C 2.0 and 30% Dot Gain => Gamma C 2.2.

What color space to choose for RGB images

For 16 bit/Channel color images tone reproduction curves (generic term for Gamma C) are used for encoding and later removed during decoding and so are normally not a problem. What makes a difference between the standard color spaces is their RGB primaries which traditionally have been based on physical data; the actual phosphors of CRT displays, the integral average spectral response of filters or sensors, etc. Technical history comes to our aid in this. Although red, green and blue primary colors do differ, most of the technology we use has had very similar goals and targets in this regard and so the physically-based RGB primaries tend to be very similar. Couple this with the fact that primaries based on integral average response are just that - averages rather than absolute physical values - and it turns out to be that there is no pat best answer to which profile is best. In Photoshop assign whatever physically-based profile works best for you to produce good color as you continue. Any normal or slightly extended gamut color space, including all of the aforementioned, can be used directly for images acquired by RGB imaging devices such as scanners or digital cameras.

Wide gamut color spaces like ProPhoto RGB or Wide Gamut RGB are not suitable to be assigned directly to such images. The RGB primaries used to expand the gamut are not physically realistic for RGB based capture devices in general because they are not close matches to the actual RGB primaries of devices like digital cameras and scanners. If you require a final image to be in such a color space, assign one of the physically-based working profiles for working through ColorPerfect or ColorNeg. After finishing all ColorPerfect and general image editing convert your image to the wide gamut profile in Photoshop. Work in 16 bit mode if at all possible. Converting to another physically-based color profile is unlikely to degrade color integrity once it has been produced as this was the intent of the ICC when they developed profiling. Converting profiles can do considerable damage to images for which color integrity still has to be produced by means of calibration however. This always is the case for color negatives and also applies to certain positive images. For this reason any conversions are best made after ColorPerfect has produced good color, not before.

Input Profiles

Do not use input type color profiles for scans or digital camera images. These usually are derived from methods referred to as profiling and are very often used in ways that violate the intentions of the ICC - this even applies to profiles produced by major manufacturers. To find out more about the problems of misapplied ICC profiling and to learn why the choice of RGB primaries really is a matter of preference rather than accuracy you might want to visit our Comments on Calibrating Digital Images.

Please understand that the color management and profiling systems as defined by the ICC (www.color.org) are very well designed. The problems come from imperfect implementations of these systems and, particularly in the case of RGB device input profiling, actual misapplication of the systems, rather like using a well-designed screwdriver to drive in a nail.

If the choice of RGB primaries is a matter of preference can they be used artistically?

The choice of RGB primaries is a matter of preference. To better come to terms with this think of the situation as if you had to choose between different brands or types of color slide film. Each film would have slightly different spectral sensitivities yet all film types could be used to produce acceptable results. There would be no best film type for all purposes however. Which film to use when would be a matter of preference. For those of you interested in experimenting with different RGB primaries we have prepared this zip archive with 10 test working spaces. We named them @RGB Primaries A through @RGB Primaries M. They are roughly organized by their color gamut from narrow to wide. All of them use a Gamma C of 2.2 and D50 as their white point. The RGB primaries used relate to the following specifications / standard color working spaces:

  • A-D: primaries used by a certain RAW converter
  • E: SMPTE-C (successor of NTSC)
  • F: Trinitron (Apple RGB)
  • G: EBU/ITU (sRGB, PAL)
  • H: P22-EBU (ColorMatch RGB)
  • L: Adobe RGB 1998
  • M: ProPhoto RGB

Wide Gamut Working Profiles

There are basically two ways in which wide gamut working profiles are commonly used. One use is to actually extend the gamut and gain access to a visually small range of highly saturated colors that printers can print but which cannot be rendered on RGB devices. Note that since these colors cannot be rendered, sensed or produced by normal RGB cameras and scanners there will be no such colors in the original image. For these colors to occur in the final result they must be created by artistic editing. Furthermore, since these colors cannot be produced by normal RGB devices including the display used for editing, they cannot be seen during editing, appearing only in the final print. We note that some recent RGB displays have somewhat expanded gamuts as a step toward addressing this, but the general situation remains.

The other common use of wide-gamut working spaces is as a color editing tool, most commonly to alter the color saturation in an image. Assigning a wide gamut RGB working profile to an image produced with a normal gamut RGB device will produce an apparent increase in color saturation. Moreover, if done correctly the increase in color saturation is done in accord with ICC rules and often will be more realistic than most of the other color saturation adjustments that have been available. That is, until the color saturation adjustment in ColorPerfect was developed. Unlike all previous color saturation adjustments, the ColorPerfect method is based on physical and physiological principles and produces a more realistic saturation effect.