> Values for RGB are about responses to light in three frequency ranges.

Yes that's true, but the frequency ranges used for RGB are different than 
the frequency ranges in your eyes (which are significantly overlapping). 
You cannot describe all visible colours using only positive RGB values.  To 
describe the ones outside of positive RGB you can either use negative values 
or introduce different colours.

> You can't have negative responses.

Nope, in the same way that you can't possess a negative number of any 
physical objects, or you can't have a complex-valued voltage, but they are 
still useful concepts with many uses.

Some visible colours that you can see with your eyes are simply not possible 
to represent as positive RGB, allowing negative RGB allows these colours to 
be correctly represented within the RGB system.  They might not be 
displayable on normal monitors, but the data is there and complete, should 
anyone wish to use it for some other purpose (eg calculating the dominant 
wavelength, converting it to another colour space, displaying it on 
specialised hardware, etc).

Note that allowing negative values in RGB is not the same as allowing a 
negative brightness.  RGB of (-1,0,-1) doesn't make much physical sense, but 
(-5,100,-5) might correctly describe the colour of a green LED.

Post a reply to this message