Camera gamut outside horseshoe?

Capturing
49

I’ve done some work on this. For a particular photo of a tree from a Nikon D800, using the raw .NEF file, the xy diagram showing white where any pixel is that chromaticity, is:

The circles show the sRGB primaries and white point. Some chromaticities are outside the triangle of primaries, and a few are outside the horseshoe.

Telling dcraw to make an sRGB image, then converting that to xyY gives the following diagram:

dcraw has heavily reduced chroma (roughly speaking, “saturation”), and has changed some hues.

Instead of using dcraw’s sRGB conversion, we do some tweaking on chromaticities, and we get this…

… which is closer to the original xyY chart, and contained within the sRGB primaries, but has some clipping.

The tweaking is done by these Windows BAT commands:

set INPRIM=0.56,0.44,0.35,0.65,0.08,0.06
set OUTPRIM=0.56,0.44,0.35,0.65,0.12,0.07

%IMDEV%convert ^
  cbc_xyy.miff ^
  -set colorspace xyY ^
  -process 'barymap channels xyY inPrim %INPRIM% gain 0.85 outPrim %OUTPRIM% v' ^
  -process 'barymap channels xyY ign inPrim sRGB clampBarycentric outPrim sRGB v' ^
  -depth 32 ^
  -define quantum:format=floating-point ^
  cbc_tx9.miff

I chose those numbers manually. Further automation is possible.

More detail is at Colours as barycentric coordinates. I haven’t yet published the source code for “barymap”.

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