If a pixel has luminance of 240 then adding more luminance is meaningless, so after adding the positive delta (even just 1) you have to clamp the value to 240. Opposite for negative deltas that would take luminance below 0.

That's what "clamping" means.

Maybe I misunderstood your point?


Also, we don't simply "see RGB" that's a gross oversimplification. Only one among many issues is the different spectral widths and positions each type of cone responds to, not to mention color sensitivity of each type. There is also the contribution of rod input, as well as person to person differences including a 4th type of cone in some people.

But the various colorspaces are just simplified abstractions facilitating different technologies. CYMK anyone?