“Standard” RGB to Grayscale Conversion

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

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