the contrast sensitivity function 資工三 91321003 黃明進 資工三 91321016 李啟毓...
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The contrast sensitivity function
資工三 91321003 黃明進資工三 91321016 李啟毓資工三 91321032 邱彥霖
http://vision.bhs.mq.edu.au/~peterw/csf.html
Introduction
Some objects may not be visible at all because their contrast is too low. The invisible objects are said to be below
some threshold. The visible objects are said to be above
some threshold. Ex. Driving a car in a very thick fog.
Can you see the black writing on a white billboard ?
Black-White edge Black has low intensity. White has high intensity.
Black-white image
Represent edge graphically
Grating (repeating bars) High contrast grating: light bars have a very high intensity
and dark bars have a very low intensity. Low contrast grating: light bars are not very light and the
dark bars are not very dark.
Sameaverage
Grating (repeating bars) Square wave gratings: the luminance lines go straight up
and down and are flat on top. Sine wave gratings: the luminance lines change in sine
wave fashion.
Sine wave grating Grating of different contrast but the same
average luminance.
Definition of contrast Definition of contrast:
(LMAX - LMIN ) / (LMAX + LMIN)
LMAX: maximum luminance LMIN: minimum luminance
High contract: approximate 1.0. Low contract: approximate 0.
Sine wave grating with one frequency
Contrast decreases as the picture goes up.
Less
sensitive
Moresensitive
Sine wave grating with varying frequency
thickHarder to see
ThinHarder to see
Moresensitiv
e
Spatial Frequency High spatial frequency-
Lots of bars across particular distance
Low spatial frequency- few bars across particular distance
Thin bars
Thick bars
Spatial Frequency
Too obscure… Depend on the distance you look, because
the more closely you look, the lower spatial frequency it will be
Visual angle Definition-
an object 1 cm tall at a distance of 57 cm subtends a visual angle of 1°
Easy way of visual angle Because one arm is about
57 cm long and a fingernail is about 1 cm in diameter, your fingernail at arm's length subtends about 1°
So this spatial frequency is one cycle per degree of visual angle.
One black bar + one white bar = one cycle
Contrast, Sensitivity, Frequency
Human vision
The most sensitive spatial frequency is about 2-5 cycles per degree for humans
the more sensitive you are the less contrast you need to see the object, as the following figure shows.
Extra information Channel:
A group of cells or neurons of human visual system
There are many “channels“ in human visual system
Every channel responds to small ranges of spatial frequency
CSF is the "envelope" which just covers the peaks of all these individual channels.
Build your own contrast sensitivity function (CSF)
Gabori Attack It’s like a shooting game to measure the co
ntrast sensitivity function Demo