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

[Lancer <forums gimpusers com>]

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)