Photographic colors that exceed the very small sRGB color gamut
Even today the occasional photographer or software developer will claim that all photographically important colors fit within the sRGB color gamut. Contrary to this quaint and thoroughly outdated point of view, commonly encountered photographic colors do exceed the very small sRGB color gamut.
Written April 2014. Updated June 2014.
Introduction: sRGB vs raw file camera input profiles
This article examines target chart shots and also shots of colorful natural objects to see whether and to what extent photographic colors exceed the very small sRGB color gamut.
The target charts and colorful natural objects shown below were shot raw, interpolated with all image enhancing algorithms disabled, and then converted to 32-bit floating point unbounded sRGB. In a normal "bounded" ICC profile conversion, out of gamut colors are clipped to 0.0 or 1.0 in one or more RGB channels. Unbounded ICC profile conversions allow otherwise out of gamut RGB channel values to go less than 0.0 or greater than 1.0.
High bit depth GIMP 2.9 from git was used for all unbounded ICC profile conversions and also to produce the screenshots shown below.
Which colors are driven out of gamut after converting to sRGB?
The older model, entry-level DSLR Canon 400D
My own digital camera is the older model, entry-level Canon 400D. I applied my ArgyllCMS-made (simple linear gamma matrix) camera input profile to an interpolated Wolf IT8 Target shot and then converted the target shot to 32-bit floating point unbounded sRGB. In Figure 1 below, the color patches with dots in the middle exceeded the sRGB color gamut, being negative in one channel:
The newer model, high-end Nikon D800
The objection can be made that my old Canon 400D camera and/or my custom camera input profile are somehow atypical. The Nikon D800 is an impressive camera. Its DXOMark sensor rating is 95, a score that as of April 2014 is exceeded only by its cousin the D800E, which has a sensor rating of 96.
The excellent camera review site Imaging Resource (Dave's Camera Reviews) provides Multi-Target raw files for the Nikon D800. I interpolated the Imaging Resource Nikon D800 100 ISO Multi-Target raw file, using Darktable and the "standard" (not enhanced) camera input profile. All image enhancements were disabled.
I did not ask permission to post screenshots, so of course I won't. However, the GIMP color picker shows that as interpolated by the standard D800 input profile, the following MacBeth ColorChecker SG Target color patches are out of gamut with respect to the very small sRGB color gamut (the color patches in the Table are listed in order by the color locations on a standard color wheel):
|Chart location||Color Wheel location||Descriptive color name||Negative channel|
|6-C||cyan-blue||dark turquoise blue||red|
|8-B||cyan-green||light turquoise green||red|
|9-D||cyan-green||medium sea green||red|
Again, real world colors that appear in front of a photographer's camera can easily exceed the most saturated colors on the MacBeth ColorChecker SG Target.
Summary: colors that are out of gamut with respect to the very small sRGB color gamut
In the sRGB color space:
- Saturated yellow greens, yellows, and oranges are negative in the blue channel.
- Saturated cyans, cyan greens, and cyan blues are negative in the red channel.
- Saturated reds are negative in the green channel.
- In my limited photographic experience, I rarely encounter any true blues and violet blues that exceed the sRGB color space (I photographed a deep blue glass that had some dark violet blues that exceeded the sRGB and also the ProPhotoRGB color gamut). However, saturated blues that heads toward cyan rather than violet very quickly exceed the sRGB color gamut.
- Magenta is not an especially common color out there in the real world. Printable saturated magenta seems to be well within the sRGB color gamut.
Colors that have been captured by digital cameras and then interpreted by camera input profiles, can easily be far more saturated than the colors that are printed on an IT8 Target or a MacBeth ColorChecker SG Target.
Target charts are one thing. What about actual images?
Target charts are one thing. Photographically captured real world colors are another. None of the target chart colors had RGB channel values that exceeded 1.0. However, photographs of subjects out there in the world can very easily have channel data that is less than 1.0 in the source (camera input) color space, but greater than 1.0 in the sRGB color space.
The images below show channel data before and after a selection of naturally saturated (no post-interpolation image enhancements were performed) interpolated camera raw files, before and after conversion from the camera input profile to sRGB:
Sample images with saturated colors
Camera input profile vs sRGB Red, Green, and Blue channel data
Color information is composed of channel data. In the digital darkroom, channel data can also be used as blending layers, for making channel-based selections, and for black and white conversions.
The screenshots below compare channel data for the camera input profile color space and sRGB. For the sample array of images, converting to sRGB drove 1.5% of the red channel, 13% of the green channel, and 23% of the blue channel to values that are either less than 0.0 or greater than 1.0. In other words, converting the interpolated camera raw files to sRGB means much potentially useful channel data is no longer available.
In the screenshots below, the out-of-sRGB-gamut areas in the red, green, and blue channels are circled in, respectively, red, green, and blue.
To summarize, converting the image to sRGB drove 1.5% of the red channel, 13% of the green channel, and 23% of the blue channel to values that are either less than 0.0 or greater than 1.0. These out of gamut channel values represent channel data that you, the photographer, no longer have available to you for use while editing if you convert such colorful images from your camera input profile or other larger RGB color space to the very small sRGB color space.
Conclusion: Commonly encountered photographic colors do exceed the sRGB color gamut
Contrary to any quaintly outdated opinions to the contrary, when shooting raw it's very easy to capture photographic colors that exceed the very small sRGB color gamut.
Stepping back to gain a wider perspective, RGB color spaces by their very nature are device-oriented color spaces. The sRGB color space is based on the light-emitting phosphors used in consumer grade monitors from the 1990s. Digital cameras capture color data when lightwaves stimulate charge-collecting sensels. The two technologies have nothing in common, and neither do the respective device-based profiles: