09 Sep 2017

The truth about colour. This is stuff every film-maker should know about.

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A Series of Approximations

A useful way to think about colour is to distinguish between 'something coloured', such as an even area of colour defined by a graphic designer, and something 'in colour', where the system is more or less reproducing a scene before the camera. Games and CGI occupy a space somewhere between the two.

Pixel based screens make dramatic reinterpretations compared to the camera data, even when a colour profile has been set in post. A camera pixel very rarely maps to the same pixel on a screen, though it would seem logical to imagine that a 1920x1080 (or 2k, 4k, 5k 8k etc) image is coherent between the two. The system is based on a series of approximations. Depending on the pixel structure on the screen, the amended camera data will be sampled, mapped and re-presented in widely varying arrangements, including Bayered micropixels, the dots of traditional CRT's, or the overlapping bands of LED's. If that data originated as 4k with 4.4.4 sampling after finishing, the transcoding to HD 4.2.0 at even the highest quality levels involves a radical transformation. Bring in the issue of compression, intra, short or long GOP, the signal minimalisation through multiplexing for cable and satellite or even the fairly mundane task of cramming data onto a Blu-Ray disc or DVD and the issues compound rapidly.

Pointillism & Colour Mixing

There are at least three stages to the perception of screen pixels with respect to colour, part of which begins with the individual micro-pixel structure of the screen. The Pen-tiled amoled RGBW (red, green, blue, white) colour space brings screen images a similar increase in contrast from a darkish screen made possible with four colour printing, with three primaries and black on white paper. For screen presentations, like text, which are 'something coloured' rather than 'in colour', there are huge rewards. Graphic designers seek clearly defined boundaries and edges, between one area of colour and another. Movies seek to control detailed variations of shading and form to enhance veracity and evade the 'uncanny valley', when unsettling grain, compression artifacts and other perceptually undesirable elements disturb the illusion of scenes being 'in colour'.

For something 'in colour', even uncompressed data brings in a second level of colour mixing between individual pixels to create small patches of colour, which is then open to the kind of colour mixing we associate with 'Pointillism', a post-impressionist approach to painting associated with George Seurat in the nineteenth century. Impressionism was an approach to painting based on identifying the colour of a particular element in the field of vision and reproducing it as accurately as possible on the canvas, rather than merely creating an impression of the scene. Seurat's innovation was to recognise the possibility of creating small mosaics like spots of colour, rather than the elegant brushwork of his predecessors.
How 'pointillism' behaves for the viewer depends a great deal on their distance from the screen. For this final stage of 'pointillist' interaction, the viewer is responding to the interaction of perceived pixels, which are in turn based on the averaged character of groups of pixels, which are themselves the product of sub-sampled micro-pixels based on data transformed for the monitor electronics. Apple have a point when they categorise 'Retina' screens according to the typical viewing distance and pixels per degree of the field of vision, rather than a simple pixel count across the screen.


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