Confused by ColorRamp input values
When I connect the overlay to colorramp as shown, it seems like all the input values to the colorramp are really close to 0. With default ramp settings, I see the following:
If I open this image in an editor and use the eye dropper on the chest, I find that the brightest parts of the wood have a brightness of around 120 (out of 256) and the darkest around 40. So I'd expect the range of input values to the colorramp to range from 40/256 = .16 to 120/256 = .47. But in fact it seems like all the input values are around 0.1 or lower. Here I've added another stop to the ramp, and observe how even with this extra stop the chest doesn't show any blue:
Only when I move the stop's position to .1 does blue start appearing, suggesting that none of the input values are much greater than .1:
Why is this? If a color value of 120/256 corresponds to a position of only about 0.1, then do all the positions above 0.25 correspond to colors "brighter than pure white"?