[argyllcms] Re: help with camera profile
- From: Stephen T <stwebvanuatu@xxxxxxxxxxxx>
- To: "argyllcms@xxxxxxxxxxxxx" <argyllcms@xxxxxxxxxxxxx>
- Date: Thu, 2 Aug 2012 07:07:22 -0700 (PDT)
"Cannot the problems with shadows/highlights be related with the dynamic range
of the camera?"
No single exposure of a test chart can fully exercise the dynamic range and
colour gamut of a modern digital camera. What we need is a profile model (and
workflow) that can extrapolate beyond the bounds of the test chart and
consistently deliver pleasing results.
Linear transforms are well-behaved and predictable. I read somewhere
that Adobe DNG profiles are essentially matrix profiles (with a LUT for
tweaking if needed) - someone please educate us if I am wrong.
I have not yet experienced shadow problems with matrix profiles. My profiles
render shadows too light and I simply apply an RGB tone curve (in the RGB
working space) to darken the shadows, increase contrast in the mid-tones,
smoothly roll-off the highlights and even adjust exposure.
Highlight clipping (L* > 100) during conversion from camera RGB to the working
space RGB is more difficult. One workaround is to underexpose the raw data (in
software) and then correct the exposure and compress the highlights in the RGB
working space. Adobe's DNG profiles seem to apply this strategy. To check this:
1) Take a correctly exposed photo of a test chart.
2) Apply a DNG profile without any exposure correction or tonal adjustments.
3) Check if L* = 50 for any middle grey patch. I find L* << 50 (in Raw
Therapee).
With linear exposure scaling and linear matrix transforms, the above process is
no less accurate for test chart patches. In real world photos, highlights and
deep shadow colours are based on extrapolation.
For general photography, matrix profiles seem to perform well, meaning results
are not far from the CIE standard observer (with appropriate exposure
adjustment if required, see above). For well-controlled studio work, where the
illuminant is fixed and highlights are controlled, then LUT-based profiles
should be more accurate but perhaps more limited?
ST.
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