>algorithm is making a guess at what level of black to convert a pixel to
Can you not just average the RGB values? Perhaps adjusting a bit for relative intensities of those values (I'm just guessing but red is probably less bright than blue or green). It's not really a "guess" is it unless it's a sophisticated algorithm. It's more akin to rotating or skewing an image.
Or you mean it's a guess compared to how non-colour film would actually record the light?
Can you not just average the RGB values? Perhaps adjusting a bit for relative intensities of those values (I'm just guessing but red is probably less bright than blue or green). It's not really a "guess" is it unless it's a sophisticated algorithm. It's more akin to rotating or skewing an image.
Or you mean it's a guess compared to how non-colour film would actually record the light?