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