Re: Colorimetric Accuracy in the Field
Re: Colorimetric Accuracy in the Field
- Subject: Re: Colorimetric Accuracy in the Field
- From: Robin Myers <email@hidden>
- Date: Wed, 05 Jun 2013 10:59:46 -0700
A matrix profile in this case is a best fit of the transformation from the color chart image RGB values to the reference XYZ values. A relative colorimetric transform is the adjustment of the source neutral axis to that of the destination neutral axis without adjusting the primaries. In the case of the pigments used in the ColorChecker family, the neutral axis is skewed in XYZ space slightly from that of the illuminant, D50 (the Prophoto RGB white point). Mathematically if the relative colorimetric shift in XYZ space is transformed into the source RGB space the result is equal RGB values.
Robin Myers
On Jun 5, 2013, at 10:30 AM, Ben Goren wrote:
> On Jun 5, 2013, at 10:20 AM, Robin Myers <email@hidden> wrote:
>
>> Adjusting the RGB values for the neutral patches to be equal in value is roughly equivalent to making the values "relative colorimetric".
>
> Eh, I don't think it works like that.
>
> First, the patches aren't perfectly spectrally flat. They're very close, but they do actually have a very small bit of color to them, especially the N9.5 patch.
>
> Second, if they *were* perfectly spectrally flat, any failure to render them on the neutral axis would represent an improper white balance, something that should be fixed before the data makes it to the profiling engine. Otherwise, you're ``baking in'' the white balance, which will result in actually-neutral objects being rendered with a bit of color in shots where you do actually get the white balance correct. And my personal experience suggests that all sort of other things get screwed up as well when you make a profile from a raw development with less-than-perfect white balance and an exposure that hasn't been perfectly normalized.
>
> Cheers,
>
> b&
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