color management on darktable 5.6 user manual

overview

Mon, 01 Jan 0001 00:00:00 +0000

darktable employs a fully color managed workflow:

Input color specifications are taken from embedded or user-supplied ICC profiles or (in the case of raw files) from a library of camera-specific color matrices.
darktable automatically reads the display profile of your monitor (if properly configured) for accurate on-screen color rendition. Multiple-screen setups are fully supported as long as a system service like colord is in place and properly configured to inform darktable of the correct monitor profile.

display profile

Mon, 01 Jan 0001 00:00:00 +0000

For darktable to faithfully render colors on screen it needs to find the correct display profile for your monitor. In general this requires your monitor to be properly calibrated and profiled, and it needs the profile to be correctly installed on your system. darktable queries your X display server’s xatom as well as the system service colord (if available) for the right profile. If required you can enforce a specific method in preferences > miscellaneous.

rendering method

Mon, 01 Jan 0001 00:00:00 +0000

darktable can render colors either with its own internal algorithms or by using the external library LittleCMS2. darktable’s internal method is, by an order of magnitude, faster than the external one. The external option gives you a choice of the rendering intent and might offer a slightly higher accuracy in some cases.

You can change the default method in preferences > processing > always use LittleCMS 2 to apply output color profile

rendering intent

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If rendering with LittleCMS2 is activated (see rendering method) you can define how to handle out-of-gamut colors when converting between color spaces. A selection box in the export, output color profile, and soft proof modules gives you a choice of the following rendering intents:

perceptual: Best suited to photographs as it maintains the relative position of colors. This is usually the best choice.
relative colorimetric: Out-of-gamut colors are converted to colors having the same lightness, but different saturation. Other colors remain unmodified.
saturation: Saturation is retained but lightness is slightly changed.
absolute colorimetric: Keep the white point.

darktable's color spaces

Mon, 01 Jan 0001 00:00:00 +0000

Input images are either RGB files (like JPEGs or TIFFs) or camera RAWs. Both store visual information as a combination of primary colors (e.g. red, green and blue) which together describe a light emission to be recreated by a display.

The following image illustrates this concept.

The left-hand-side of the image depicts a colored light that we need to represent digitally. We can use three ideal color filters to decompose this light into three colored primary lights at different intensities. In order to recreate the original colored light from our ideal decomposition (as illustrated in the center of the image) we simply need to recombine those three primary lights by addition.

unbounded colors

Mon, 01 Jan 0001 00:00:00 +0000

Screens and most image file formats can only encode RGB intensities confined within a certain range. For example, images encoded on 8 bits can only contain values from 0 to 255, images on 10 bits from 0 to 1023, and so on… Graphic standards postulate that the maximum of that range, no matter its actual value, will always represent the maximum brightness that the display medium is able to render, usually between 100 and 160 Cd/m² (or nits) depending on the actual standard. We generally call this maximum “100 % display-referred”. The minimum of the range, encoded 0 no matter the bit-depth used, becomes then “0 % display-referred”. 100 % encodes pure white, 0 % encodes pure black.

possible color artifacts

Mon, 01 Jan 0001 00:00:00 +0000

There are some infrequent situations that still can lead to problematic results unless the user takes some action. Some modules in Lab color space, like levels and monochrome, rely on the fact that the L channels carries all lightness information, with the a and b channels purely representing chroma and hue. Unbounded colors with negative L values are especially problematic to these modules and can lead to black pixel artifacts.

It has been found that highly saturated blue light sources in the image frame are likely candidates for pixels with negative L values. If you are engaged in stage photography you should pay close attention to such lights appearing in images.

darktable's color dimensions

Mon, 01 Jan 0001 00:00:00 +0000

This section defines the perceptual properties of color, both conceptually and quantitatively, in order to characterize and quantify the creative and corrective adjustments made to color in darktable.

🔗definitions

Color properties like “saturation”, “brightness” or “lightness” have passed into common usage but are largely misused and often used to mean different things. In color science, each of these terms has a precise meaning.

There are two frameworks within which color properties may be analyzed and described: