Lab Coordinates in a Processed Image

The process of conversion from RGB to the Lab color space is determined by the profile in use. However, when using different color engines, there is some discrepancy in the way the data is translated.


When measuring colors (e.g., from patches in color chart) using readouts, the intention is to match the colors in the processed output file to the chart’s reference data. The camera profile describes how the colors of the captured image are to be transformed into a reference color space known as a Profile Connection Space (PCS), while the output profile describes how the colors from that are to be transformed in the output image file.

In Capture One, it is only necessary to process and output an image when using the file in third-party analysis software, otherwise it is enough to measure the colors using the output profile, as Capture One converts the results in the Viewer. The output profile determines the destination color space, which is set by the selected Process Recipe. (The Proof Profile option may be used instead, however, note that, if an output file is required for analysis, the selected Process Recipe must use the same profile.) Most modern applications, not to mention the chart maker’s reference data, specify colors as Lab color space coordinates or values, therefore Capture One Pro can now display Lab values as an option.

While Lab is a reference color space adopted by the ICC in the PCS, there is variation in the way that third-party color engines convert the data in RGB profiles to the Lab color space. That is assuming an output file is required in an RGB color space and is to be interpreted using a ICC output profile. Note that, some third-party software analyzes colors directly without interpreting a ICC profile. For more information, see the section on Matching Third-Party Applications. The following describes some of the challenges involved during the conversions.

White point

The ICC specification defines that the white point for the profile connection space is D50, and this is specified in the ICC profile. However the same specification states that D50 should have certain XYZ coordinates when in fact ICC profiles round the coordinates and most color engines adopt that or close to it. In addition, RGB coordinates at 255, 255, 255 should be neutral white, but in some profiles that’s simply not the case, however, it can be used to infer that. Many profiles in use do not strictly adhere to the definitions, and although it’s possible to construct them so that they do, color engines vary in they method used to interpret the results.

Chromatic Adaptation

Although the ICC state clearly that the white-point for the profile connection space is D50, the native white-point of the actual color space, sometimes referred to as media white, is often different. For example, standard working spaces sRGB and Adobe RGB are both specified to a media white of D65. Some validation applications adopt the standards for the color space directly, without using an ICC profile for the definition, based on the media white point. When an ICC profile is used, color engines must convert between white-points, which is known a chromatic adaption. Two commonly used methods, known as the von Kries and the Bradford transforms, can be employed to convert between these white-points. However, some more generalized applications describe Lab coordinates in the PCS, which assumes D50 as the light source, and uses simple XYZ-scaling between that at D65, which can result in errors exceeding 8 delta E ab. Capture One supports several methods when converting between white-points.

Black Point Compensation (BPC)

Black point compensation (BPC) helps maintain shadow detail when converting between source and destination color spaces. The black point of the source space is mapped to that of the destination space, and if not enabled, any colors that are darker than the destination space are clipped and details in those darkest black areas are lost, which is sometimes referred to as "crushed-" or "blocked-up-" shadows.

Although BPC algorithms differ between color engines, a simple linear transform on individual XYZ values is common. As a consequence, this method results in a shift of hue, saturation, or lightness, and leads to inaccurate color values. Therefore Capture One does not support color black point compensated Lab readouts, and it may be necessary to configure third-party applications to match (i.e., disable BPC) when validating values. The issue can be avoided by using output profiles that can represent black, where known, and is highly recommended, unless the image is being processed to a device such as a printer.

Note that, Perceptual rendering intent, when selected in Adobe Photoshop, always maps the darkest black of the source space to the destination space, therefore when validating readouts with values in Photoshop it is recommended that Relative Colorimetric intent is adopted, with BPC disabled and Use Dither. This improves the gradations in shadow tones in 8-bit images during conversion. As BPC is a highly destructive process, it should be limited to the last operation in a color-managed workflow, where possible.

Black point compensation (often abbreviated as BPC) can be applied by a color engine to convert between color spaces where at least one of the spaces is unable to represent black.

Slope Limiting Profiles

Several standard profiles including Adobe RGB and ProPhotoRGB have a shallow gamma tone curve at its base (close to x=0). Many color engines adopt their own interpretion of the specification, thereby handling the shadows and deeply saturated colors differently. Some engines such as the Adobe Color Engine (ACE) apply what’s called “slope-limiting” to help in practice, but even slope-limits vary between various color engines. Although the effects are subtle, it can account for differences of 0.4 delta E ab at certain RGB coordinates. While Capture One supports the interpretation of profiles with, and without, slope-limiting, the issue can be avoided by adopting profiles that are based on tables, or by using v4 profiles that include slope-limiting.

The Adobe Color Engine, and others who follow the same convention, limits the slope for pure gamma tone curves.

Rendering Intent

An ICC profile may include several interpretations for different rendering intents. Although not typically adopted for the type of output profiles considered here, it may be an issue for certain profiles. Therefore, Capture One will choose the colorimetric intent when a choice is required. Note that, the distinction between relative and absolute colorimetric intents is dictated by the choice of chromatic adaptation.

Profiles based on 3D LUTs

Capture One does not support Lab readouts from profiles based on 3-D look-up tables (LUTs). Profiles based on 3-D LUTs include the majority of CMYK profiles, printer profiles and a small number of others that describe a complex relationship between color and the coordinates. The ICC specification does not detail an interpolation method for 3-D LUTs, and is therefore open to interpretation by various CMMs.