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

[Casey Connor <gimp-user-list caseyconnor org>]

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?