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

["Joao S. O. Bueno" <gwidion mpc com br>]

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 <forums gimpusers com> 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)
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