Hi,

I am brand new to GIMP and am trying to statistically match 2 picture sets on brightness, contrast, and luminosity for a psychology research study. I've done a ton of online searching and figured out how to find values for brightness and contrast. Using the histogram, I used the mean of the "Value" channel to determine each picture's brightness, and the standard deviation of the "RGB" channel to identify contrast.

However, I am at a standstill for extracting values for each image's luminosity. Please let me know if you're aware of how I may go about this. I know spatial frequency can be used, but am unable to locate the value for each image in GIMP.

Thanks! Will

You might get an answer here, or you might not.

My advice, ask the same question on https://discuss.pixls.us/ lots of clever people there.

On 03/06/18 15:46, WMusc wrote:

Hi,

I am brand new to GIMP and am trying to statistically match 2 picture sets on brightness, contrast, and luminosity for a psychology research study. I've done a ton of online searching and figured out how to find values for brightness and contrast. Using the histogram, I used the mean of the "Value" channel to determine each picture's brightness, and the standard deviation of the "RGB" channel to identify contrast.

However, I am at a standstill for extracting values for each image's luminosity. Please let me know if you're aware of how I may go about this. I know spatial frequency can be used, but am unable to locate the value for each image in GIMP.

Thanks! Will

Something like:

Color>Desaturate (luminosity) and check the average or median in the Histogram dialog.

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 the 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.

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 the
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 pixls.us thread on this topic: https://discuss.pixls.us/t/image-luminosity/6904

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 pixls.us 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.

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 chroma).

-- Stratadrake strata_ranger@hotmail.com
--------------------
Numbers may not lie, but neither do they tell the whole truth.

From: gimp-user-list  on behalf of Elle Stone 
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
>> the
>> 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 pixls.us thread on this topic:
https://discuss.pixls.us/t/image-luminosity/6904

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 pixls.us 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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