perception - spatial vision

Question 1

learning objectives

Answer 1

• What is meant by spatial frequency?
• Spatial frequency unit (cycles/degree)
• Spatial grating patterns, frequency, contrast,orientation; Fourier Analysis
• Contrast sensitivity function (CSF) and its relationship with Optical and Neuronal Modulation Transfer Functions.
• Comparison of different visual systems
• Spatial frequency channels

Question 2

examples of squarewave gratings at different spatial frequencies

Answer 2

slides
many lines and close together - high frequency
less lines and further apart - low frequency

Question 3

examples of sinusoidal gratings at different spatial frequencies

Answer 3

the lines merge into each other, patter that goes gradually up and down

Question 4

the difference between the brightest and lowest part of gratings (black and white)

Answer 4

contrast C =(Lmax-Lmin)(Lmax+Lmin)

Question 5

spatial luminance gratings
-the greater the number of waves…..

Answer 5

the greater the number of waves ,the greater the number of stripes (higher spatial frequency)

Question 5

the greater the amplitude (depth) of the wave, the greater the ____ of the stripes

Answer 5

contrast (blackness to whiteness) of the stripes

Question 5

Gratings can also ____ in orientation

Answer 5

vary

Question 6

contrast C

Answer 6

contrats C=(Lmax-Lmin) / (Lmax+Lmin)

Question 7

-gratings have 3 characteristics:…..
-these characteristics can be manipulated to form any ___ ____

Answer 7

-spatial frequency, contrast and orientation
-visual pattern

Question 8

degrees concept

Answer 8

the cone like diagram
lecture around 15-20 mins
10 degrees for example the further you go away the larger it is

Question 9

why sinosoidal gratings
fourier analysis

Answer 9

-it can be shown mathematically (gourier analysis) that any wave pattern can be broken down into a series of sine waves with different frequencies and amplitudes

Question 10

high and low frequency image
2 identities

Answer 10

image has 2 identities
-if you look it the high frequencies you see the outline (white lines sketches)
-if you look at the low frequency you see the blurry image identity

Question 11

how is camera quality measured pt 2
-how is a lens evaluated

Answer 11

-a lens is evaluated by measuring the luminance intensity of the transmitted stripes in the image in comaprison with the original intensities of the stripes in the object

-you can get an image of an object
-the object are those sinusoidal gratings, and the you take a photo of the object
-can measure differnce between black and white
-can measure the contrast for frequencies
-given the contrast what can the device produce?
-can do it between all frequnecies and look at the ratio between the 2
-if its a good camera you should get a 1.1 ratio

Question 12

image contrast
object contrast

Answer 12

contrast is the difference divided by the sum of an image
image contrast - Lmax-Lmin / Lmax+Lmin
object contrast - Lmax-Lmin/Lmax+Lmin
you get the ratio of these and then plot it

Question 13

the measuremnets are done for a series of different ______ _________

Answer 13

spatial frequencies

Question 14

image to object contrast ratio on graph

Answer 14

high line (high up) camera doing well
low down (camera not doing well)

low spatial frequency - very good
high spatial frequency - not good

Question 15

modulation transfer function

Answer 15

a measurement of its ability to transfer contrast at a particular resolution from the object to the image.
-relationship of ratios of image to object intensities agaisnt spatial frequency is called the MTF of the lens

of this device it is the object (whats out there) compared to the image (what it produces)
-determines how much contrast in the original object is maintained by the detector (in this case the device)
characterizes how well the spatial frequency content of the object gets transferred to the image.

Question 16

demonstration

Answer 16

around 25-30 mins

Question 17

eye and brain role in vision

Answer 17

-eyes send a signal for processing to the brain
-and the brain does further processing for you to see things
-so we have the optics of the eye, like camera lens
-and we have the brain
-when you see a picture ,theres a perception in your head which is mixture of your eye and how well it saw it and your brain and how well it interpreted it

Question 18

perceptual transfer function or contrast sensitivity function

Answer 18

-a combination of your eye and brain-optical transfer function and neural transfer function
-the perceptual transfer depends on the optical transfer function and the neural transfer function
-how well the eye and brain work together to see the wolrd

Question 19

how do we measure the quality of the image produced by the human visual system (as opposed to that of a camera lens)

Answer 19

look at the contrasting lines in the slides
-the curve you see is a measure of how well your perceptual system (combination of your eyes and brain) can see things around you.
-will be curved shape
-

Question 20

peak sensitivity changes depending where you are /where your sitting
-how can we manipulate peak sensitivity

Answer 20

-simplest way of manipulating peak sensitivity is by changing the brightness level

Question 21

contrast sensitivity function at three different light levels

Answer 21

slides
-dark scotopic
twilight mesopic
bright photopic

the peaks of each vary, peak of bright one is to the right
peak means what frequency you see best
-photopic peaks at a higher spatial frequency
-shifted the sensitivity from low to very high spatial frequencies
-at high spatial frequency you see more detail

Question 22

contrast sensitivity function and age

Answer 22

-infants have a different window of visibility than adults
-babies peaks at very low spatial frequnecy, meaning everything is blurry, they don’t see anything in detail
-the CSF of the infant gradually improves during the first year of life
-the CFS then changes later in life

Question 23

CSF of other species

Answer 23

graph in slides
-can obtain contrast sensitivy function for all sorts of frequenvcies
-on left hand side there is goldfish (goldfish have many photoreceptor levels-have 5-6 diff ones they see many colours) but detail is very low spatial frequency-so they just see colourful blur (high contrast but low spatial frequency)
- cats and dogs see more detail, cat and dog have lower peak sensitivy compared to humans, cats cant see the amount of detail we see
-cats are good at motion detection
-falcon birds of prey etc , they have high spatial resolution , to see their prey (higher spatial frequency low contrast sensitivity)
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