When black and white is not black and white

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When black and white is not black and white

rich2005
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 (#FF0000) next to blue, the grayscaled-blue
may match the grayscaled-red depending on the tone *and* the method used.
Method 1: Absolute red (#FF0000) will grayscale-match absolute blue (#0000FF)
Method 2: Absolute red (#FF0000) 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: When black and white is not black and white

Joao S. O. Bueno Calligaris
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 <[hidden email]> 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 (#FF0000) next to blue, the grayscaled-blue
> may match the grayscaled-red depending on the tone *and* the method used.
> Method 1: Absolute red (#FF0000) will grayscale-match absolute blue (#0000FF)
> Method 2: Absolute red (#FF0000) 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:    [hidden email]
> List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
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Re: When black and white is not black and white

Casey Connor-2
In reply to this post by rich2005
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&W 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 (#FF0000) next to blue, the grayscaled-blue
> may match the grayscaled-red depending on the tone *and* the method used.
> Method 1: Absolute red (#FF0000) will grayscale-match absolute blue (#0000FF)
> Method 2: Absolute red (#FF0000) 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: When black and white is not black and white

Rick Strong-2
In reply to this post by rich2005
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&W (greyscale) if what they **want** to end up with is a
greyscale "B&W" image.

Rick S.

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Re: When black and white is not black and white

Liam R E Quin
In reply to this post by rich2005
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

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ankh on irc
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Re: When black and white is not black and white

Alex Vergara Gil-3
In reply to this post by rich2005
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 <[hidden email]>
To: [hidden email]
Cc: [hidden email]
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 (#FF0000) next to blue, the
grayscaled-blue
may match the grayscaled-red depending on the tone *and* the method used.
Method 1: Absolute red (#FF0000) will grayscale-match absolute blue
(#0000FF)
Method 2: Absolute red (#FF0000) 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)
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When black and white is not black and white

rich2005
In reply to this post by rich2005
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: When black and white is not black and white

Rick Strong-2
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: [hidden email]
Cc: [hidden email]
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

--
Lancer (via www.gimpusers.com/forums)
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Re: When black and white is not black and white

Akkana Peck
In reply to this post by Liam R E Quin
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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