Nice illustration.

To be a bit picky, there’s no unprocessed photo. They start with a minimally processed photo and take it from there.

The reason I clicked is that when I saw the title, I’m tempted to think they might be referring to analog photo (ie film). In that case I think there’s a well defined concept of “unprocessed” as it is a physical object.

For digital photo, you require at least a rescaling to turn it to grayscale as the author did. But even that, the values your monitor shows already is not linear. And I’m not sure pedagogically it should be started with that, as the authors mention later about the Bayer pattern. Shouldn’t “unprocessed” come with the color information? Because if you start from gray scale, the color information seems to be added from the processing itself (ie you’re not gradually adding only processing to your “unprocessed” photo).

To be fair, representing “unprocessed” Bayer pattern is much harder as the color filter does not nicely maps to RGB. If I were to do it I might just map the sensor RGB to just RGB (with default color space sRGB) and make a footnote there.