Re: [Gimp-user] When black and white is not black and white

[Akkana Peck]

Liam R E Quin writes:
> I don't know of any accessibility checkers for GIMP; there are
> PhotoShop plugins. It'd be a good Google Summer of Code project I
> suppose, if that's still going. I might even be able to drum up some
> funding for work in the area, and/or technical resources.

There's View->Display Filters, "Color Deficient Vision".

That's just one type, and there are lots of different variants
of color vision. I think I've seen other GIMP color filters but
don't have a specific reference, but a web search of 
GIMP color-blind
gets some hits that might lead to more filters.

> Because of the differences in people's vision, I don't think it matters
> which method is used to convert. 

+1. It's amazing how much color vision varies among people. We think
"green" is an obvious concept that means the same thing to everyone,
but even among people with "normal" vision, color perception varies
tremendously.

And think of Ansel Adams and all his work in the darkroom. There is
no one "correct" black and white version of a scene; the art is in
creating the one that shows what you want to show.

...Akkana
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Re: [Gimp-user] When black and white is not black and white

[Rick Strong]

An interesting test. I believe your test image has to be flattened to work?

Cheers,
Rick S.

-Original Message- 
From: Lancer

Sent: Wednesday, June 07, 2017 8:13 PM
To: gimp-user-list@gnome.org
Cc: notificati...@gimpusers.com
Subject: [Gimp-user] When black and white is not black and white

Some very interesting responses here, thank you :-)

This would be interesting material for students wanting an extra study at a 
more
advanced level beyond level 1 NCEA; simply creating a poster or brochure 
with

"good" design principles (contrast, alignment, repetition, proximity etc).

It would make a good topic for Level 3, having a student analyse the 
different
hex levels of the grayscale conversion methods and to try and reverse 
engineer

the algorithms which may have been used.

I'm attaching a gimp image in color which "detects" which method is used 
when

turning it to grayscale. (Could be used to demonstrate to students that
grayscale conversion is not always the same). I was entertaining spending 
more
time with the image, perhaps making a version where there is blue snow that 
make
the writing invisible until grayscale is applied. (Just a bit busy with 
marking

and lesson planning at the moment)

But thank you for your replies - great information.

Attachments:
* 
http://www.gimpusers.com/system/attachments/603/original/TEST_you_have_used_01.xcf


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Re: [Gimp-user] When black and white is not black and white

[Alex Vergara Gil]

You may try Colors -> Component -> Decompose, then select LAB space, BW 
image is the L, thats a lot better than anything else I have tried

Regards 

-Original Message-
From: Lancer 
To: gimp-user-list@gnome.org
Cc: notificati...@gimpusers.com
Date: Tue, 06 Jun 2017 06:02:29 +0200
Subject: [Gimp-user] When black and white is not black and white

I am a school teacher. One of the checks I ask students to do in order to 
test
the contrast of their graphics work, is to convert the images to grayscale 
and
see whether images are still clear.

There are two methods students are using to convert their images to 
grayscale
for this test...


Method 1: flatten image, then Colors > Hue-Saturation => slide the 
saturation
slider down to zero.
Method 2: image => mode => grayscale

Either of these methods results in a grayscale image, but the grays are not
exactly the same.

For example, if I have absolute red (#FF) next to blue, the 
grayscaled-blue
may match the grayscaled-red depending on the tone *and* the method used.
Method 1: Absolute red (#FF) will grayscale-match absolute blue 
(#FF)
Method 2: Absolute red (#FF) will grayscale-match a slightly lighter 
shade
of blue  (#2626FF)

Why are the two methods of grayscale having a different result? I would have
thought that conversion to grayscale would be the same process as dragging 
down
the saturation of an image.

...and given that they are different, which is the better method to use in 
terms
of testing for contrast in media assignments?

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Re: [Gimp-user] When black and white is not black and white

[Liam R E Quin]

On Tue, 2017-06-06 at 06:02 +0200, Lancer wrote:
> I am a school teacher. One of the checks I ask students to do in
> order to test the contrast of their graphics work, is to convert the
> images to grayscale and see whether images are still clear.

As you discovered, there are different ways to convert to greys.
However, people's colour perception varies, with more than one in 10
having some form of "colour blindness" (depending on how you measure)
or eye difficulty - and more than that percentage unable to read small
text, of course.

I don't know of any accessibility checkers for GIMP; there are
PhotoShop plugins. It'd be a good Google Summer of Code project I
suppose, if that's still going. I might even be able to drum up some
funding for work in the area, and/or technical resources.

Because of the differences in people's vision, I don't think it matters
which method is used to convert. The people who have poor colour vision
will be exactly the ones who see the brightnesses differently, e.g. if
their eye doesn't respond well to reds (the most common problem with
human males) then reds will likely appear darker to them. My father
couldn't tell the difference between a traffic light that was all dark
and one with just red showing. So all the methods will be "wrong".

If the goal is just to make sure the image reproduces OK on a black-
and-white printer, have them send the RGB image to the laser printer,
then convert to greyscale in several different ways and compare, and
they can learn a lot, it's a good exercise. The default dot screen on
PostScript printers is actually fairly mediocre and most professional
graphic design software replaces it, or used to.

Numerically, it's about colour spaces and gamma and precision and the
purpose ("intent") of the conversion. The gegl c-to-g filter sometimes
gets much better results than either the mono mixer or desaturating.

Liam

-- 
Liam R E Quin 
ankh on irc
Web slave for fromoldbooks.org
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Re: [Gimp-user] When black and white is not black and white

[Rick Strong]

For a quick & dirty contrast test I would get them in the habit of 
converting to greyscale. This converts the colour space from RGB (or CMYK) 
to Greyscale...but it may not give them the best perceptual rendering of a 
colour scene in B (greyscale) if what they **want** to end up with is a 
greyscale "B" image.


Rick S. 


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Re: [Gimp-user] When black and white is not black and white

[Casey Connor]

I'll take a stab at this, but other more knowledgeable people may weigh 
in: as I understand color theory, the notion that there is a canonical 
equivalent grayscale value for every color is somewhat of a fallacy; any 
such translation relies on pyscho-perceptual assumptions about how we 
perceive different colors. As a result, translating from a color image 
to a B image is somewhat of an art (heavily informed by lots of research).


I'm not intimately familiar with the various algorithmic options in GIMP 
to do this conversion, but different options use different methods, as 
you seem to have found. I'm guessing, but: sliding the saturation to 
zero likely applies a standard transformation to go from RGB to HSV, 
then sets "V" to zero, and then translates back to RGB. This may involve 
different assumptions and techniques than "mode -> grayscale" which 
might do a more (or less) subtle transformation.


Here's another detailed link on the subject: 
https://patdavid.net/2012/11/getting-around-in-gimp-black-and-white.html


I'm fond of the "C2G" GEGL filter (which in the latest GIMP betas is 
under Colors -> Desaturate -> Color to Gray, but in older versions is 
apparently under Tools -> GEGL Operation (see the tutorial)), but it 
does a bit more than just convert the colors.


Also, a minor pedantic note: Colors -> Hue-Saturation -> 
slide-saturation-to-zero doesn't technically change the image to a 
grayscale image, as it's still an RGB image internally. And other exotic 
methods of making an image "black and white" might actually preserve 
some imperceptible amount of color information in the pixels to achieve 
the effect. (I think I've read about that, but can't cite anything off 
hand.)


-c

On 06/05/2017 09:02 PM, Lancer wrote:

I am a school teacher. One of the checks I ask students to do in order to test
the contrast of their graphics work, is to convert the images to grayscale and
see whether images are still clear.

There are two methods students are using to convert their images to grayscale
for this test...


Method 1: flatten image, then Colors > Hue-Saturation => slide the saturation
slider down to zero.
Method 2: image => mode => grayscale

Either of these methods results in a grayscale image, but the grays are not
exactly the same.

For example, if I have absolute red (#FF) next to blue, the grayscaled-blue
may match the grayscaled-red depending on the tone *and* the method used.
Method 1: Absolute red (#FF) will grayscale-match absolute blue (#FF)
Method 2: Absolute red (#FF) will grayscale-match a slightly lighter shade
of blue  (#2626FF)

Why are the two methods of grayscale having a different result? I would have
thought that conversion to grayscale would be the same process as dragging down
the saturation of an image.

...and given that they are different, which is the better method to use in terms
of testing for contrast in media assignments?



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Re: [Gimp-user] When black and white is not black and white

[Joao S. O. Bueno]

Good luck documenting the diferences between the different ways to
convert an image to grayscale:

https://graphicdesign.stackexchange.com/questions/69308/how-to-convert-color-images-to-black-white-in-gimp/69372#69372

On 6 June 2017 at 06:02, Lancer  wrote:
> I am a school teacher. One of the checks I ask students to do in order to test
> the contrast of their graphics work, is to convert the images to grayscale and
> see whether images are still clear.
>
> There are two methods students are using to convert their images to grayscale
> for this test...
>
>
> Method 1: flatten image, then Colors > Hue-Saturation => slide the saturation
> slider down to zero.
> Method 2: image => mode => grayscale
>
> Either of these methods results in a grayscale image, but the grays are not
> exactly the same.
>
> For example, if I have absolute red (#FF) next to blue, the 
> grayscaled-blue
> may match the grayscaled-red depending on the tone *and* the method used.
> Method 1: Absolute red (#FF) will grayscale-match absolute blue (#FF)
> Method 2: Absolute red (#FF) will grayscale-match a slightly lighter shade
> of blue  (#2626FF)
>
> Why are the two methods of grayscale having a different result? I would have
> thought that conversion to grayscale would be the same process as dragging 
> down
> the saturation of an image.
>
> ...and given that they are different, which is the better method to use in 
> terms
> of testing for contrast in media assignments?
>
> --
> Lancer (via www.gimpusers.com/forums)
> ___
> gimp-user-list mailing list
> List address:gimp-user-list@gnome.org
> List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
> List archives:   https://mail.gnome.org/archives/gimp-user-list
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