RGBcam to ACR
RGBcam to ACR
- Subject: RGBcam to ACR
- From: "Olivier Desmaison" <email@hidden>
- Date: Thu, 12 Jul 2007 10:04:34 +0200
Hoping I'm not off-topic, I'd like some enlightening on the camera raw to
output process regarding possible colors.
This paper give the 1DsMkII spetral sensitivities:
http://digitalkamera.image-engineering.de/downloads/spectral/Canon_EOS_1Ds_M
arkII_spectral_sensitivities.pdf
The below is an extract from dcraw :
"/*
Thanks to Adobe for providing these excellent CAM -> XYZ matrices!
*/
void CLASS adobe_coeff (char *make, char *model)"
The 1DsMkII given matrix is :
6517 -602 -867
-8180 15926 2378
-1618 1771 7633
As per the DNG specs, this matrix needs to be inverted to deliver RGBcam to
XYZ. So I did and I computed from the camera spectral sensitivies what I
think are the primaries :
Rprim = 255 / 36 / 1
Gprim = 39 / 162.75 / 18
Bprim = 2 / 61 / 188
converted to RGBcam output 12b and applied the inverted matrix to have
R(x,y), G(x,y),B(x,y). The results together with AdobeRGB and SRGB (x,y) is
:
1ds MkII aRGB sRGB
R (0.61;0.33) (0.64;0.33) (0.64;0.33)
G (0.33;0.63) (0.21;0.71) (0.30;0.60)
B (0.12;0.07) (0.15;0.06) (0.15;0.06)
This would means a camera like the 1DsMkII outputs a range of colours close
to AdobeRGB-sRGB.
This contradicts ACR behaviour in that changing the working space output
from say AdobeRGB to Prophoto prevents channel clippings, so some colors are
out of AdobeRGB and in Prophoto.
Any comment ? Did I do wrong ?
Olivier
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