Kostya's Boring Codec World

Dedicated to all young werehedgehogs.

So, let’s talk about colour in multimedia. To summarise it so you can skip the rest: proper colour representation hardly matters at all.

What is colour from physical point of view? It’s a property of light in visible range (i.e. between infrared and ultraviolet though some people are born without proper UV filters). Even better, you can clearly define it via spectroscopy because it’s a mix of certain wavelengths with certain energies. Another approach is to have reference colours printed on some surface (aka Pantone sets)—and that is the very thing you use to make sure you get what you want when taking a photo (especially on other celestial body) or ensuring consistency of production at typography.

The problem is that either approach is too bulky for use outside certain specific areas, for example it’s too expensive to store the whole spectrum for each pixel even in palette form (also image or video compression would be extremely inconvenient). Good thing is that our eye has its own variant of psychoacoustic masking and you can use several basic colours to achieve the mix. And from this most colour models (or spaces) were born where the range of real (aka present in spectrum) and perceivable colours (like purple or white, which are a mix of several colours) are represented as a composition of some primary colours like red+green+blue or cyan-magenta-yellow. And of course there is famous CIE 1931 model with basis being theoretical components corresponding to sensitivity of cone cells in human eye.

And there came the other problem: most colourspaces (XYZ, HSV and such) are as good as π-based computing system—it’s incredibly convenient for certain kinds of calculations but it’s next to impossible to convert results from and into decimal with good precision. Even RGB with its primary colours widely available has a problem: for instance, the colour of sky outside Britain (in case you didn’t know the etymology of word ‘sky’, it comes from Scandinavian word for cloud) can be represented only with IIRC red component being negative.

So how to deal with it? By mostly not caring as humans usually do. In places where higher colour reproduction fidelity is required (mostly typography) they simply use more primary colours. But overall humans don’t care much if the colours are slightly wrong. On one side, human brain has an internal auto-correction scheme for colour tint and white auto-balance (you might remember that optical illusion with seemingly red strawberries covered by green or blue tint with no pixels being actually red); on the other side each pair of human eyes is unique and perceives colours and shades differently. So if most people won’t agree about actual shade and would recognize picture anyway why bother at all (again, some specific areas excluded)?

So all those TV-related standards that define fine details of colour models are good only for mastering stuff (i.e. to keep consistency for the final product because you might not care about colour being slightly wrong but you’ll spot slight shade mismatch for sure). And speaking about TV-related standards, so-called TV-range (i.e. having component values fit into 16-240 range instead of 0-255 as you’d expect) is an archaism that should’ve been buried long time ago along with analogue TV broadcasting. But it still exists in digital world standards along with interlacing and KROPPING! not fully purged yet.

And speaking about shade differences, some of you might remember the era of VGA where each component actually had only 64 possible values and yet it was enough to create very convincing moving pictures. You may argue that the underlying issue was masked by palette mode I should point out that for rather long time after that people had to live with laptops and displays that had cheap LCDs with actual 18-bit colour depth (i.e. the same 6 bits per component as on VGA) as well (and let’s not talk about black colour representation there). So people didn’t care much about that and all this high-bitdepth stuff seems to be more of marketing creation than actual technical necessity (again, I understand that it’s needed somewhere like medical imaging, but common people don’t care about quality).

In the conclusion I want to say that the main reasons for introducing higher bitdepth wherever possible are: because we can (I understand and respect that), because it keeps many engineers and marketers employed (I understand that but don’t agree much) and because it helps fixing some other problems introduced elsewhere (like TV-range helped to deal with filtering artefacts—that I understand as well and try to respect but fail). Now be a good hedgehog and set proper colour profile in IMF metadata.

This entry was posted on Thursday, November 9th, 2017 at 2:38 pm and is filed under Useless Rants. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.