Why Wide Colour? A guide to its gains

Among the announced improvements in macOS 10.12 Sierra is more extensive support for Wide Colour. It sounds impressive, but is it of any real value, and who stands to benefit?

Early computer colour systems worked entirely in 8-bit colour, in which each of the red, green and blue channels was represented by a single byte. The number of colours, and their range, was visibly inferior to that of the human visual system, and good quality images readily appeared posterised, with step changes in colour instead of smooth gradation.

Modern colour systems work not with such limited ranges of integers, but floating-point values between 0 and 1.0, and have added an alpha (transparency) channel too. All good Mac and iOS apps now use at least four 16-bit floating point values to represent each pixel. This approximates human visual capabilities much better.

Problems come when processing these colours, and particularly when rendering them into a colour space. macOS offers developers a wide range of facilities for processing colours and working with images, in frameworks such as Core Graphics, Core Image, and most importantly in its new high-performance Metal, which makes best use of the GPU in modern graphics cards. Until macOS 10.12 arrives, those frameworks generally support the use of wide colour spaces (or gamuts), but there are still some gaps.

macOS 10.12 Sierra changes that by extending support for wide colour spaces throughout the graphics frameworks.

Provided that software developers stick to using the recommended 16-bit floating-point colour format, Sierra’s improved graphics frameworks should ensure that colours are rendered properly into wide colour spaces.

Early digital cameras had CCDs which did not perform as well as those in modern cameras. Not only was their resolution much lower, but the colour space (gamut, the range of colours which they could capture) was much smaller. The same goes for colour displays. So there was little point then in trying to use wide colour spaces, as the additional space could not be used by the camera (as an input device) or display (as an output device).


The CCDs in modern cameras can capture a much wider range of colours than can be expressed using the sRGB colour space (above), and may surpass Adobe RGB. By using a more restrictive colour space than that of the camera, you are effectively wasting some of your camera’s ability to capture natural colour. The same is true for high-quality modern displays, which also support a wider range of colours than traditional colour spaces like sRGB or even Adobe RGB.


The answer is to use a wide colour space, such as Adobe Wide RGB (above). This uses pure spectral primary colours to provide a gamut which is going to be much larger than that of your camera’s CCD, your display, or other output devices such as a printer.

Considering the range of visible colours within the standard Lab colour space as shown in the diagrams above, Adobe Wide RGB encompasses nearly 80% of the space (as area). sRGB covers just over 35%, and Adobe RGB just over 50%. So by using Wide Colour, whenever your image is rendered into a colour space, more of its original colour will be preserved.

And there’s more. When rendering into a colour space, the most popular (and almost invariably most appropriate) methods of converting colours (‘rendering intent’) not only have to adjust the colours which fall outside the destination colour space, but to preserve colour relationships, adjust other colours which are within that space. So using Wide Colour doesn’t just ensure that more ‘natural colour’ is preserved, but it improves colour throughout the image.

By making Sierra fully capable of supporting Wide Colour, Apple is offering you the ability to work with more faithful colour from your camera, to see it more faithfully rendered on your display, and to maintain that higher fidelity through your workflow. This should apply to all apps which use Apple’s frameworks for images and graphics. Every Mac user should benefit, but those to whom faithful colour is most critical will of course see the greatest benefits.

The chart of sRGB colour space is by Spigget, via Wikimedia Commons; that of the Adobe Wide RGB by Entirety, via Wikimedia Commons.