Re: [Gimp-user] Image luminosity

Another option that might work is luma.  This can be easily determined by converting the image to grayscale, 
or decomposing the color channels using a YCbCr color model (yielding one layer for luma, and two layers for 

-- Stratadrake
strata_ranger hotmail com
Numbers may not lie, but neither do they tell the whole truth.
From: gimp-user-list <gimp-user-list-bounces gnome org> on behalf of Elle Stone <ellestone ninedegreesbelow 
Sent: Saturday, March 10, 2018 6:47:53 AM
To: gimp-user-list gnome org
Subject: Re: [Gimp-user] Image luminosity

On 03/10/2018 03:55 AM, Ofnuts wrote:
On 03/07/18 15:10, WMusc wrote:
Something like:

Color>Desaturate (luminosity) and check the average or median in the
Histogram dialog.
Thanks for the tip. Do you know whether that approach provides the
mean (median,
etc.) luminosity value before or after desaturation? It looks to apply
effect so it's unclear to me. If the program applies it uniformly
across all
images I may be able to use it as a proxy for luminosity but would
like to avoid
that if possible.

I expect Color>Desaturate>Luminosity to not change the luminosity of the
pixel. The grey value after is normally the luminosity you would have
computed from the RGB components you have in the initial image.

This said, in 2018 I'm surprised you are engaging in such manual labor.
Find someone (intern?) to write a small program that will extract all
these data directly from the image files and create a spreadsheet with
the results.

Before finding an intern to write a program, it might be a good idea to
figure out whether the metrics you've decided to extract are actually
useful to the task at hand. Garbage in/garbage out, as I've been trying
to say over in the thread on this topic:

As far as extracting information from an image, ImageMagick might be a
better choice than GIMP - more available metrics, though again it's an
open question as to whether any of the metrics actually help you decide
whether an image is "too this" or "not enough that". Remember the red
square in the middle of an otherwise solid gray image that I posted to
the thread - the relative luminance of the red square is
exactly the same as the relative luminance of the background, but I'm
fairly sure just about everyone will say the red square is "brighter",
as indeed it according to the definition of "brightness" used in current
color appearance models.

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