CS/ECE 545 Class 2

More on Human Vision

We demonstrated in class that the human eye is not generally responsive to the absolute level of illumination, but to the relative changes of brightness at the edges between objects. This is because the visual response is approximately logarithmic so the diference between two signals is invariant with overall illumination changes.

So white is not necessarily MAX_BRIGHTNESS. See, for example, Debby Krim, The Colors of White

If two diffusely-reflecting objects have reflectivities and then the apparent brightnesses when a light of intensity I reflects off of them, are: . The difference in the brightnesses is:

which is independent of the illumination intensity I. As a corollary, to have objects appear to be equally spaced in brightness, the ratios of their reflectivities should be approximately constant.

When designing images, it is useful to accommodate this visual non-linearity.

Image Operations

racepoint image
racepoint image -- threshold = 64
racepoint image -- threshold = 192

Contrast stretching
comet -- max brightness = 15
comet -- sqrt brightness
comet -- log brightness
comet -- max brightness = 255
comet -- stretch, sqrt
comet -- stretch, log

comet stretched
comet log

Bit planes
fire -- bit plane 7 = msb
fire -- bit plane 6
fire -- bit plane 5
fire -- bit plane 4
fire -- bit plane 3
fire -- bit plane 2
fire -- bit plane 1
fire -- bit plane 0 = lsb

Color bit planes
EAP1 -- bit plane 7 = msb
EAP1 -- bit plane 6
EAP1 -- bit plane 1
EAP1 -- bit plane 0 = lsb

CS/ECE 545 Staff
Contents ©1997 - 2011 Norman Wittels and Michael A. Gennert