Free Scale Gamut, Free Scale Log (FS-Gamut, FS-Log)

SMPTE has adopted Standard ST 2048-1, entitled 2048x1080 and 4096x2160 Digital Cinematography Production Image Formats FS/709.

The standard endorses wide-gamut imagery in production. “FS” stands for “Free Scale;” imagery having arbitrary chromaticity can be conveyed. The standard uses the notation R’FSG’FSB’FS for such wide-gamut colour components. The “709” component in the name reflects the option to convey imagery having BT.709 colourimetry.

The colourspace is defined by chromaticity coordinates of the primaries and white. Apart from the potentially nonlinear toe and shoulder regions, no accommodation is made for footroom or headroom. The standard does not specify how image data values are to be carried, but presumably more than 10 bits per component will be used (despite the quasilog).

The quasilog OECF is described by a set of four numerical parameters and a (fifth) “exposure” value kEXT, 0 ≤ kEXT < 1 indicating underexposure, kEXT = 1 indicating correct exposure (default!), and 1 < kEXT indicating overexposure.

The standard defines an ancillary data (ANC) packet carrying colour metadata – the “Color VANC” (pronounced “color-VEE-ants,” ideally located in the early portion of the active interval of line 18 of 1125). It carries the chromaticities of the primaries and reference white, the four parameters of the quasilog OECF function, kEXT, and 12 numerical parameters concerned with the toe and knee (or shoulder) of the OECF. Presumably, DI ingest should construct a transform through the parameters carries by the Color VANC.

The default values for FS primaries apparently reflect Sony “wide gamut” delivered by the F23, F35, and F65 cameras. Apparently there are no default values for the quasilog OECF.

Please consult the standard for details.

p.s. There's a company called FreeScale; there’s no relation, and we hope they’re not ticked.

p.p.s. Colour, color, colourspace, colorspace, colorimetry, color primary, color primaries – whatever it takes to keep Google happy.

Copyright © 2011-10-22
Charles Poynton