Conversion to Gray Scale May Not Be Trivial how to make a grayscale image?

[Eben Eliason]

The context of the conversation is simply to display a temporary
grayscale still frame from a video file during a compression step,
which won't be saved and needn't have high contrast.  This thread is
certainly interesting, though, and may prove useful in other areas
later.

- Eben

On 11/2/07, Seth Woodworth <seth at isforinsects.com> wrote:
> There are around four completely different methods for converting to
> grayscale in photoshop or gimp.  Averaging luminosoty values, aka, Convert
> to grayscale, is by most accounts the least interesting method in most
> circumstances.  I've worked in pre-press in both color and grayscale
> separations, so I've played with each of the versions at one time or
> another.  What exactly are we talking about here?  Record activity or
> backlight-off mode?
>
>
> On 11/2/07, Ed Trager <ed.trager at gmail.com> wrote:
> >
> > Hi,
> >
> > On 11/2/07, Bert Freudenberg <bert at freudenbergs.de> wrote:
> > > On Nov 2, 2007, at 3:16 , Albert Cahalan wrote:
> > >
> > > > Eben Eliason writes:
> > > >
> > > >> Roughly speaking, you can calculate a colored pixel's effective
> > > >> luminance by:
> > > >>
> > > >> Y = 0.3*R+0.59*G+0.11*B
> >
> > I assume that this equation is taking into account the relative
> > sensitivity of the human eye to the 3 primary colors, correct?
> >
> > But even if this does take into consideration the relative sensitivity
> > of cones in the human retina, does this equation preserve contrast
> > sufficiently well or not?
> >
> > When we take a colored photograph of something like American Holly
> > (Ilex aquifolium) and convert it to grayscale, the contrast between
> > the red berries and the green leaves often disappears.  For example,
> > you can try this in Gimp and you'll see that the result is often not
> > very satisfying.  As far as I know, Gimp only provides one option for
> > converting from color to grayscale.
> >
> > I believe a lot of research has been done for many years in the area
> > of how to convert colored images to black and white.  Software like
> > the Gimp ought actually to provide several methods of doing the
> > conversion, but to the best of my knowledge only provides one.
> >
> > Having seen this topic arise on OLPC's mailing list, I am now
> > wondering out of curiousity to what extent OLPC has researched and
> > considered the possible solutions to this problem beyond the
> > "standard" equations?
> >
> > Another thing very worth considering is the fact that nearly 8% of the
> > male human population world wide is red-green color blind.  Color
> > schemes dominated by blues and yellows work best for such individuals.
> > The reason for this should be apparent from examining the attached
> > .png file comparing the appearance of 9 colors in (a) with how they
> > appear approximately to someone with red-green color deficiency in
> > (b).
> >
> > (FYI - I work at an Eye Center where I am often reminded by the
> > scientists and doctors around here about such things)
> >
> > Best - Ed
> >
> > > >
> > > > To be clear on why this is rough: it performs an operation
> > > > on non-linear data which is only valid on linear data.
> > > > That is, it ignores gamma.
> > > >
> > > > From best to worst:
> > > >
> > > > a. convert to linear, Y = 0.3*R+0.59*G+0.11*B, convert back
> > > > b. square, Y = 0.3*R+0.59*G+0.11*B, square root
> > > > c. Y = 0.3*R+0.59*G+0.11*B
> > > > d. Y = (R+G+G+B)>>2
> > > > e. Y = G
> > > >
> > > > FYI, most interesting image operations are only valid on
> > > > linear data. This includes scaling and alpha blending.
> > > > Lots of programmers degrade images by screwing this up.
> > > > We should all try to do better, especially when the image
> > > > is something that might be important to the user.
> > >
> > > Right. For example, am I the only one who is bothered by the huge
> > > change in perceived brightness of different colors when you switch
> > > the DCON to grayscale mode? This switch from swizzling to the per-
> > > pixel brightness calculation gives a huge difference.
> > >
> > > - Bert -
> > >
> > >
> > > _______________________________________________
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> > >
> >
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