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 <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> wrote: > > > > Hi, > > > > On 11/2/07, Bert Freudenberg <[EMAIL PROTECTED]> 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 - > > > > > > > > > _______________________________________________ > > > Devel mailing list > > > [email protected] > > > http://lists.laptop.org/listinfo/devel > > > > > > > _______________________________________________ > > Devel mailing list > > [email protected] > > http://lists.laptop.org/listinfo/devel > > > > > > > > > _______________________________________________ > Devel mailing list > [email protected] > http://lists.laptop.org/listinfo/devel > > _______________________________________________ Devel mailing list [email protected] http://lists.laptop.org/listinfo/devel
