Re: purpose of Granger rainbow chart?
Re: purpose of Granger rainbow chart?
- Subject: Re: purpose of Granger rainbow chart?
- From: Marco Ugolini <email@hidden>
- Date: Mon, 08 Mar 2010 11:23:33 -0800
- Thread-topic: purpose of Granger rainbow chart?
In a message dated 3/8/10 10:58 AM, Jacob Rus wrote:
> Marco Ugolini wrote:
>> "Tonal discontinuities" such as which, exactly?
>
> Sorry, I suppose there are no "discontinuities" in the luminous
> landscape max-for-each-hue-and-luma, but neither will there be in the
> same thing piped through a CMM and printed. What I meant to say is
> that the function is not smooth (continuously differentiable), and in
> particularly noticeably has a nasty diagonally slanted edge in
> lightness (L*), where the derivative changes abruptly.
"Derivative"? I can't follow you. Is that calculus?
Plain English, please, if this is to be a conversation, rather than a
monologue. Speaking for myself, I'm not a Bachelor of Science.
> This visual artifact is clearly visible
> <http://www.luminous-landscape.com/images31/Granger_Chart.jpg>. I
> don't understand why it wouldn't make more sense to create test images
> with lightness rather than luma on the vertical axis, especially
> considering that most CMMs use CIELAB as a PCS. I suppose it would
> take crunching some numbers to make such a test image, and wouldn't be
> quite as easy to manage in Photoshop.
Incidentally, as far as I know, isn't "luma" a term specifically meant for
video signals?
My understanding is that, in the case of digital images, we use LUMINOSITY
(or LIGHTNESS), not luma, independently of whether they are shown on a
self-luminous display or output as a reflective print.
Marco Ugolini
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