Re: Media Testing for maclife.de
Re: Media Testing for maclife.de
- Subject: Re: Media Testing for maclife.de
- From: Eric Chan <email@hidden>
- Date: Mon, 15 Sep 2008 13:53:37 -0700 (PDT)
> I'm not denying that it is important how close the
> profiling process
> can approach the "raw device", but that's not
> different from profiling
> a printer, a scanner or a display.
It's different with raw converters, compared to printers & displays, because different raw converters treat color differently. For example, Camera Raw's color profiles are all based on scene-referred colorimetry and deliberately ignore tone curves (i.e., since everything is linear in the raw stage, any required exposure adjustment is just a simple scaling of the raw data by a constant). The idea being that the profile sets up the initial scene-referred color and it's then up to the user to guide the rendering process, mostly through the use of a non-linear tone mapping curve (looks like a big gamma adjustment with an S-curve, by default). So if you're evaluating the final output, which it seems like you're doing, it's not at all surprising that you're seeing large mismatches, esp. if you're including L* differences in your error metric.
> In your terminology, is this still scene-referred (still
> original
> camera color space) or already output-referred (white
> balance and tone
> curve set)?
In my opinion if the data has been tone mapped it's no longer scene-referred.
> As far as *profiling* is concerned, clearly the best
> approach would be
> the one that gets closest to the device. However, note that
> even
> ProfileMaker (hardly an application for "home
> users") requires a
> target TIFF with white balance and tone curve already set,
> or else it
> won't work.
And that's why Adobe didn't go down that path.
> > Where did you get the reference patch values for the
> SG chart?
>
> From the accompanying reference file. But I additionally
> validated
> these values with my reference i1Pro (mean deltaE(1976)
> ~0.3).
Ok, but the reference XYZ/Lab values provided in the PM text files are all relative to D50. As I recall you are testing under multiple different lighting conditions. So, I assume you are starting from the reference __spectral__ values of the chart and then, for each lighting condition that you're shooting under, you derive the final XYZ/Lab values based on the actual spectrum of your light (measured with your Eye-One).
True?
(In other words, your reference Lab values should ideally be computed relative to your actual light.)
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