View
216
Download
0
Tags:
Embed Size (px)
Citation preview
Processing in the human visual system(maybe, sort of)
eyeearlyvision
(striate cortex)
contourdetection
salience attention
objectrecognition
(IT cortex)
spatial memory(PP cortex)
“what” pathway
“where” pathway
grouping
The eye
Lens Depth of field Chromatic aberration
Photoreceptors Rods Cones Fovea, macula, and
periphery Retinal processing
Gain control Edge enhancement? Simple motion detection
lens/irisrods
conesretinal
ganglion
Photoreceptors
Rods Found mostly in the macula and periphery Very sensitive to light But don’t detect color
Cones Found in the fovea Less sensitive Color sensitive
Colors seem to fade in low light
Trichromacy
Having different cones for every possible wavelength would be bad
We just have three kinds of cones “Blue” cones: short wavelengths “Green” cones: intermediate
wavelengths “Red” cones: long wavelengths However, their responses overlap
The eye reduces all the wavelengths at a given pixel to just the total “amount” of “red”, “green”, and “blue”
Intensity and chromaticity
Color image Intensity (R+G+B) Chromaticity (B-Y)white = coolblack = warmgrey = neurtral
Intensity and chromaticity
Color image
Intensity
Chromaticity (B-Y)white = coolblack = warngrey = neutral
Chromaticity (R-G)white = redblack = greengrey = balanced
Code
[define gray [i → [color i i i]]]
[define signed-gray [i → [gray [+ 128 [∕ i 2]]]]]
[define b-y [c → [− [blue c] [∕ [+ [red c] [green c]]
2]]]]
[map-bitmap [c → [signed-gray [b-y c]]] cones]
Contrast versus absolute intensity
The visual system responds less to the amount of light or color
Than to variations in the amount
This is also known as contrast
Contrast detection
Many of the neurons in the visual system are designed to respond to contrast
Center/surround receptive fields Stimulated by the presence
of light in a specific area Inhibited by its presence in
the area immediately around it
− −+
inhibitory region(light prevents firing)
excitatory region(light stimulates firing)
Edge detection
When applied to image intensity (R+G+B),
Center/surround processing responds to lightness contrast
Which often occurs at object edges
Oriented edge detection
Other neurons Are stimulated by light
in an elongated region And inhibited by it in
the region next to it These can detect the
orientation of edges
− +
−+
−+
Constancy
Illumination varies over a surface in intensity in chroma
But you (usually don’t perceive the color of the surface as changing
This is called perceptual constancy Color constancy Lightness constancy
Georges de la tour,The Repentant Magdalene, C. 1640
Depth cues
The world is 3D But the image is 2D Your vision system uses
depth cues to recover lost information Stereopsis Motion Linear perspective Aerial perspective Relative size
Occlusion
The interruption of edges of one object by the edges of another object
Shows the interrupting object is in front
C. Coles Phillips,Brittish Tank on Fifth Avenue, 1917
Stereo vision
Your eyes are in slightly different positions
So they receive slightly different images The disparity between
images decreases with distance (simple case)
So by matching features between the two eyes Your brain computes
disparity And hence, depth
Motion parallax
Objects farther away seem to move slower So the background seems to remain stable And the foreground moves Or, the opposite, if the camera tracks the foreground
This is frequently used in cell animation
Linear perspective
Parallel lines converge at a vanishing point
One-point perspective One set of lines converging Gives a sense of immersion in
the scene Background seems to pop out
Two-point perspective Two sets of converging lines The object seems to pop out
Three-point perspective Rarely used Very dramatic
Gestalt theory
Theory of how we organize components of a percept into wholes Max Wertheimer, Kurt Koffka,
Wolfgang Köhler
Law of prägnanz (simplicity) We choose the simplest possible
interpretation of a scene
Formulated laws of grouping Disks or pacman?
Figure and ground
We tend to separate the scene into One or more objects
(“figure”), in front of or on top of
A ground plane or background (“ground”)
We use a number of tricks to do it Depth, grouping
But it can be hard in degenerate cases
Attention
Remember our fovea is only about 2° So we have to view scenes by scanning
the fovea over the image
Attention is driven both top-down by cognition
Task Knowledge, expectations
And bottom-up by Size Intensity Local contrast Motion Odd-man-out
As well as recency(don’t look if we’ve just looked there)
Attention and intensity
All things being equal, our attention is driven to intense stimuli Bright lights Saturated colors Rapid motion
Attention and color
Warm, bright colors draw attention
Cooler, darker colors tend to recede into the background
Claude Monet, Sunrise, 1872
Attention and contrast
But one of the biggest drivers of attention is contrast We focus on unusual
elements of the scene Elements that are
different from their neighbors
Top-down attention
Attention is also driven by expectations If I see something
face-like I look for the facial
features Or vice-versa
And by task
Change blindness
We only really see what the fovea is fixating
If anything changes, we don’t notice unless we see motion
Oh, and we don’t see motion while our eye is moving …
Continuity in films Pretty woman
Neisser et al’s demo Simons and Chabris’ demo
Supply Pattern
Work area
Adapted from Ballard et al.