"Standard" RGB to Grayscale Conversion

To harold's point about the "Y plane": standard color JPEGs are encoded using the YCbCr colorspace, where Y is the luminance component (i.e. the brightness) and Cb and Cr are the blue-difference and red-difference chroma components. So one way of turning a color JPEG into a grayscale one is to simply drop the Cb and Cr components.

There is a utility called jpegtran than can do this losslessly, using the -grayscale option. (The lossless part would really only matter if you wanted to end up with a JPEG and not PGM, to avoid generation loss.) In any case, this would probably be the fastest way to do this transformation, because it doesn't even decode the image into pixels, much less do math on each one.

The problem is that I can't understand how the "official" JPG->PGM convertitors work in terms of what value to assign to the final pixel (i guess, 0->255) starting from the classic RGB format.

There is likely a gamma adjustment in the conversion those "official" tools are using.
That is, it is not just a linear transform.

See this Wikipedia section for the details: Converting color to grayscale

I believe you want to use the formula for Csrgb.
Try it out and see if it matches the results you're expecting.

Basically, you'll do this:

1. Take R, G, B color (each in [0,1] range)
   • If they're in the range 0..255 instead, simply divide by 255.0
2. Compute Clinear = 0.2126 R + 0.7152 G + 0.0722 B
   • This is likely the linear transform you were applying before
3. Compute Csrgb according to it's formula, based on Clinear
   • This is the nonlinear gamma correction piece you were missing
   • Check out this WolframAlpha plot
   • Csrgb = 12.92 Clinear when Clinear <= 0.0031308
   • Csrgb = 1.055 Clinear1/2.4 - 0.055 when Clinear > 0.0031308