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The Brain
from retina to extrastriate cortex
Neural processing responsible for vision
• photoreceptors• retina
– bipolar and horizontal cells– ganglion cells (optic nerve)
• optic nerves• optic chiasma (X)• lateral geniculate body• striate cortex• extrastriate cortex
Photoreceptors
Ganglion cells
Light
Lateral inhibition
• Edge detection and contrast enhancement
• Bipolar, Horizontal and Ganglion cells
1000 0
100
Lateral inhibition
• If no activity in neighboring photoreceptors,
output = output of photoreceptor
100100 100
0
Lateral inhibition
• if activity in neighboring photoreceptors,– output is decreased, possibly absent
100100 100
0
(-.5) (-.5)
-50 -50
+
(1.0)
100
200200 200
0
(-.5) (-.5)
-100 -100
+
(1.0)
200
Lateral inhibition via addition and negative weights
cornea
crystallinelens
retina: photoreceptors = rods + cones
opticnerve
Optic nerve
• axons of the ganglion cells– 1 million optic nerves– 120 million photoreceptors
From light to vision
Lateral Geniculate Nucleus (LGN)
StriateCortexGeniculo-Striate Pathway
(LGN)
StriateCortex
Striate cortex(primary visual centre)
• Neurons are edge detectorsfires when an edge of a particular orientation is present
(LGN)
StriateCortex
Striate cortex(primary visual centre)
• Neurons are edge detectorsfires when an edge of a particular orientation is present
frequent output
vertical bar
(LGN)
StriateCortex
Striate cortex(primary visual centre)
• Neurons are edge detectorsfires when an edge of a particular orientation is present
infrequent output
diagonal bar
Edge detection
• each cell “tuned” to particular orientation– vertical– horizontal– diagonal
• cats: only horizontal and vertical• humans: 10 degree steps• edges at particular orientations and positions
Extrastriate cortex(beyond the striate cortex)
V1
Extrastriate cortex
• Each area handles separate aspect of visual analysis– “V1-V2 complex”: Map for edges– V3: Map for form and local movement– V4: Map for colour– V5: Map for global motion
• Each is a visual module– connects to other areas– operates largely independently
Douglas A. Lyon, Ph.D.Chair, Computer Engineering Dept. Fairfield
University, CT, USA
[email protected], http://www.DocJava.com
Copyright 2002 © DocJava, Inc.
Background
• It is easy to find a bad edge!• We know a good edge when we see it!
The Problem
• Given an expert + an image.
• The expert places markers on a good edge.
• Find a way to connect the markers.
Motivation
• Experts find/define good edges
• Knowledge is hard to encode.
Approach
• Mark an important edge
• Pixels=graph nodes
• Search in pixel space using a heuristic
• A* is faster than DP
Assumptions
• User is a domain expert
• Knowledge rep=heuristics+markers
Applications
• Photo interpretation
• Path planning (source+destination)
• Medical imaging
Photo Interpretation
Echocardiogram
•3D-multi-scale analysis
Path Plans, the idea
Path Planning-pre proc.•hist+thresh
•Dil+Skel
Path Planning - Search
The Idea
• The mind selects from filter banks
• The mind tunes the filters
Gabor filter w/threshold
• The Strong edge is the wrong edge!
Canny Edge Detector
Mehrotra and Zhang
Sub bands for 3x3 Robinson
Sub Bands 7x7 Gabor
Gabor-biologically motivated
Log filters=prefilter+laplacian
2 1
2 2 ex 2 y 2
2 2
1
4 1 x2 y2
2 2
ex 2 y 2
2 2
2 f (x, y) 2 f
x2 2 f
y2
Mexican Hat (LoG Kernel)
The Log kernel
Oriented Filters are steerable
Changing Scale at 0 Degrees
Changing Phase at 0 degrees
Summary
• Heuristics+markers= knowledge• Low-level image processing still needed• Global optimization is not appropriate for
all applications• Sometimes we only want a single, good
edge