Pro colour management on Mac
Adam Banks explains how to ensure your digital colours always look the same...
When you take a photo, the image is generated by detecting photons striking a silicon CCD, using red, green and blue filters to capture colour. When you view the photo on your iMac, it’s displayed by shining a backlight through IPS LCD RGB subpixels. After tweaking the RGB values in a photo editor, you can print it using dots of cyan, magenta, yellow and black (CMYK) inks, which each absorb different wavelengths.
These are all different processes, and each piece of hardware will differ from others, even if based on the same technology. Yet the photo looks the same throughout. There’s a lot of colour science making that happen.
SQUARING THE CIRCLE
For thousands of years, philosophers worked on models of colour that assumed it had the perfection of geometry. Spoiler: it didn’t. After key discoveries in the 19th century about human vision by Thomas Young and Hermann von Helmholtz, in the early 1900s, Prof Albert H Munsell came up with the first proper model of the colours we actually see. He painted patches of each hue (colour of the rainbow) with steps in value (brightness) and chroma (saturation), then, by asking people which ones looked equal, arranged them according to ‘perceptual uniformity’. The result was a lumpy blob. CIE colour gamut diagrams (see below) are a 2D version of this. A colour space is the blob, or gamut, of colours that a system can reproduce, or a device-independent space, such as sRGB.
Formulae specified by the International Colour Consortium (ICC) convert between colour spaces. Numbers that represent colour are only meaningful in terms of a colour model, so ideally, an ICC profile should be stored with each image — otherwise whatever device reads it has to guess. For images on the web, though, we don’t want any unnecessary bytes. So web designers save images in sRGB but without a profile, and web browsers assume they’re in that space.
ROOM WITH A VIEW
Saving an image in a colour space imposes limits on its fidelity, and the colour space in which your editing software works affects the accuracy of your edits; pushing colours outside the gamut will clip information. So you should aim to work in a suitably large colour space. The most popular
working space for print designers is Adobe RGB (1998), which extends sRGB at the blue-green end, while video editors have used Rec709 but are moving to wider spaces like DCI P3 and Rec2020.
Premium smartphones, tablets and displays today cover all of the sRGB gamut, while cheaper displays usually manage at least 70%. Within a space, colours may be mapped more or less accurately, expressed as a Delta E value: below 1 is excellent, below 2 is decent. As colour technology improves, wider gamuts are getting more common. Apple has adopted P3 for recent iMacs, iMac Pros and iPad Pros.
A wider-gamut screen means you can edit photos and videos more confidently and see the benefit of HDR (high dynamic range) content, but it can also bring problems. Software that assumes images are sRGB will make colours too vivid in P3. And although these displays cover more of the Adobe RGB gamut than before, quite a lot of it falls outside P3. Here, though, we’re into distinctions that even many pros find esoteric.
In editing software, your working RGB space should be a generic space such as sRGB, not a display or printer profile. Many designers will prefer Adobe RGB. Save your work in the same space, with a profile; copies can be saved as sRGB if necessary. The display profile set in ‘System Preferences > Displays > Colour’ should take care of on-screen colour.
RESPECTING PROFILES
iOS also manages colour, and app developers are encouraged to use profiles and cater properly for screens that use the P3 colour space, so colour should ‘just work’. On both macOS and iOS, Safari now respects display profiles and will make images that are assumed to be sRGB appear correctly on wider-gamut displays.
If you work in CMYK for print, a CMYK ‘press characterisation’ such as Coated FOGRA39 can serve as both working space and press profile. With photos, though, you’ll normally work in RGB and convert to a CMYK profile on output.
Larger colour spaces need more bits per pixel to avoid posterisation (banding). The huge ProPhoto space requires 16 bits. While many apps can work in 16 or even 32 bits, most displays are 8-bit. 10-bit colour (also known as 30-bit, since there are three channels) is supported by OS X 10.11 and later, but only Mac Pro graphics cards support 10-bit displays.
“Each piece of hardware will differ from others, even if based on the same technology.”