[Tig] What's being done to bring out-of-gamut colors intended for P3 color space into P3 range?

Richard Kirk richard at filmlight.ltd.uk
Thu Nov 4 16:22:10 GMT 2010

Truelight interpolates in L*a*b* space. If a colour is out of gamut, it 
will project along a line in L*a*b* joining the colour to a neutral 
point (a*=b*=0) until it meets the gamut surface. The particular grey is 
somewhere between the colour's luminance and 50%. This should mean the 
line goes up at a shallow angle for deep colours, and the intercept will 
be more luminous as well as less saturated; and down for bright colours 
so biased towards the colour luminance, so the intercept will be more 
luminous as well as less saturated.

Suppose you have the setting sun in your image, with huge RGB values, 
and R > G > B > 1. This will map into a pinkish colour.

It is possible to turn this gamut clipping off. The RGB image values 
would then get clipped, and the setting sun would then be white.

There is no single best way of projecting colours back into gamut. 
Anything that does not preserve the hue is unlikely to be popular. 
Reducing the saturation works well over the mid-tones, but at the 
luminance limits, the saturation will go almost to zero. This is why our 
grey point is always biased towards the middle of the colour space.

Richard Kirk

FilmLight Ltd.  	Tel: +44 (0)20 7292 0400 or 0409 224 (direct)
Artists House,		Fax: +44 (0)20 7292 0401
14-15 Manette Street
London W1D 4AP

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