> >>>Does anyone know how photoshop converts RGB values into a single
> >>>grayscale value when we convert a color image into grayscale mode?
> >> it's 30% Red, 59% Green, 11% Blue for a straight greyscale
> >> conversion.
> >It depends on the display. The numbers you gave are for the 1953 NTSC
> >phosphors, which nobody uses any more. Chances are excellent that the
> >weights for your monitor will be different.
> I have to disagree with you ... why would the program convert RGB to
> grayscale differently for different monitors?
Because the individual red, green and blue channels are *different
colors* on different monitors, so they have different relative
Quote:> I checked again in a few Photoshop books I rely on and the 30/59/11
> weighting is what John Paul Caponigro, Bruce Fraser, Dan Margulis
> and Barry Haynes all agree is taking place with the basic Image >
> Mode > Grayscale command. This is a wide range of experts and I
> seriously doubt that you are right and all of them are wrong.
Perhaps Photoshop does it that way. If so, it is at best an
approximation. I just tried it in Photoshop 6; here's what I got for
an sRGB profile.
255,0,0 -> 130
0,255,0 -> 220
0,0,255 -> 69
Assuming a gamma of 2.2 (an approximation to real sRGB), I get weights
of .227, .723, .056.
Quote:> Maybe you're referring to how the image gets displayed on the screen
> when it's run thru the monitor ICM profile, but that's totally
> different than how the pixel values get converted when creating a
> new grayscale file.
To convert a color image to a grayscale image while retaining relative
luminance of each color, you need to perform the Y part of the
RGB->XYZ transformation. And that transformation depends on the
particular RGB primaries involved.
See <http://www.poynton.com/ColorFAQ.html>, especially
"What weighting of red, green and blue corresponds to brightness?"
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.