Re: Luminosity channel...
Re: Luminosity channel...
- Subject: Re: Luminosity channel...
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Mon, 24 Sep 2001 17:30:20 -0500
Carl Stawicki asked:
>
Does anyone know how PhotoShop calculates the luminosity channel? I know
>
this channel represents the lightness of an image, and it doesn't actually
>
exist, but it is calculated somehow, with a 256 scale. Is there an actual
>
mathematical relationship between the luminosity channel and the color
>
channels (RGB or CMYK), if so what is it?
Bruce Fraser replied:
>
It's the L* (Lightness in Photoshop terminology) channel that you'd
>
get if you converted the image from whatever space it's in to Lab.
Hmm... It doesn't seem to work that way for me. The "Luminosity" histogram
of an RGB image is similar to, but different from the L* histogram of the
same image after converting to Lab.
Photoshop used to do it like this:
Luminosity = 0.299 * R + 0.587 * G + 0.114 * B
but I don't know if this method has been replaced by one that uses profiles.
I could find out, if you really want to know.
--
Bruce J. Lindbloom, Pictographics Intl. Corp.