Re: Erroneous color transformations with Apple CMM
Re: Erroneous color transformations with Apple CMM
- Subject: Re: Erroneous color transformations with Apple CMM
- From: Marco Ugolini <email@hidden>
- Date: Tue, 26 Jan 2010 18:56:44 -0800
- Thread-topic: Erroneous color transformations with Apple CMM
In a message dated 1/26/10 2:53 PM, Klaus Karcher wrote:
> Of course you can get any result you want if you assing a different
> profile, but as I already said: this is not the point.
If a problem appears only with one specific profile but not with a number of
other tried-and-tested ones, I tend to think that the problem lies in the
profile itself, not the CMM or other elements in the chain.
Unless there are other profiles in widespread usage using which one can
observe similar misbehavior.
> ... but OK -- if you demand for the whole storry:
I wasn't demanding, but I'm interested.
> It's a small detail of a photo with very high dynamic range. There are
> large areas in the uncropped image that are rather under- than overexposed.
Yes, that seems self-evident. Yet, the embedded Canon profile assigns
near-white appearance to values in a range that should normally carry
visible detail (say, between 200 and 230 in a 256-step tonal scale). In
other words, it markedly WORSENS the loss of detail in the highlights,
specially in an overexposed shot. That feels like bad tonal modeling to me.
> The image was processed in a linear workflow -- assigning anything else
> but a profile with gamma 1 is simply inappropriate. The main part of the
> image gets almost black if you assign a profile with gamma 2 or higher.
Not in my own test: assigning AdobeRGB (gamma 2.2) makes the very darkest
measurable point in the image appear like L* 38, and near-neutral -- L* 38
is not what I would consider to be "near black", though it is indeed a
dark-looking cloud, as I suspect it would be in the actual scene.
> The profile was made from an underexposed shot while the photo is
> overexposed.
>
> I have chosen this image as it is a good visual demonstration for the
> bugs I seemingly found.
"Seemingly" is the operative word. Right as you may be, we should rule out a
malfunctioning profile before we look at a possibly malfunctioning CMM.
Marco Ugolini
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