Grayscale vs. Desaturation

[jbw]

Not exactly sure I can answer your question...

If you desaturate an image it still retains the 3 (rgb) or 4 (cmyk) channels and gives them appropriate grey values i.e. R-125, G-125, B-125. BUT if you convert an image to greyscale it flattens the 3/4 colour channels and creates a single grey channel.
Was that what you wanted to know??

ta,
jbw

[Stroker]

When you convert to Greyscale, the channels are weighted.
I can't remember the exact numbers, but it's something like:
Red 35%
Green 15%
Blue 50%

Those number are way off, but Greyscale does weight the channels. No idea why Adobe went this route. I suspect it may have something to do with one of the CIE models.

With de-saturating, of two formulas is used, but I don't remember which

V = maxcolor(R,G,B)

or

L = (maxcolor + mincolor)/2

I can't remember which because I get HSV/HSB and HSL mixed up sometimes. Although, I'm pretty sure it's the first formula.

[chach]

well, another facet of that in terms of working is that alot of your tools are disabled in PS once you move out of RGB.

[Nutter]

Sorry for being rather late with this, but being a programmer I don't come around here very often.

The actual weighting is R: 29.9%, G: 58.7%, B: 11.4%. They are approximations of the importance of each colour channel in human visual perception (meaning that what we consider to be the luminance of an image is actually based 58.7% on the green colour, 29.9% on the red, and 11.4% on the blue).
Think of it this way - create 3 solid red, green, and blue images and look at how much more intense the green looks compared to the blue - that's what these weights are approximating.

[conio]

well, another facet of that in terms of working is that alot of your tools are disabled in PS once you move out of RGB.

just apply an channel mixer on top of your channels, check monochrome and set the channels to the percentages mentioned by Nutter...

[MasonDoran]

converting to greyscale will also reduce the file size tremendously...

ie: 1 pixel in rgb has 3 seperate values(0->255) x 3...1 for each color. Greyscale has only one color, hence 1 value.

If the pixel has transparency...then u have an additional value.

cmyk would have 4...creating even bigger file sizes.

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