Lightness Dependencies and the Effect of Texture on Suprathreshold Lightness Tolerances
1 Perception and VR MONT 104S, Fall 2008 Lecture 4 Lightness, Brightness and Edges.
-
Upload
madlyn-cummings -
Category
Documents
-
view
212 -
download
0
Transcript of 1 Perception and VR MONT 104S, Fall 2008 Lecture 4 Lightness, Brightness and Edges.
1
Perception and VR
MONT 104S, Fall 2008Lecture 4
Lightness, Brightness and Edges
2
Center-Surround Receptive fields can find edges
A single retinal ganglion cell cannot unambiguously signal an edge.
To find edges, one must evaluate the difference in responses of neighboring cells.
The response must go from positive to negative across a region of space, crossing through zero.
+-
Neuron 2:Positive response
+-
Neuron 3:Zero response
+-
Neuron 4:Negative response
Light Dark
+-
Neuron 1:Zero response
+-
Neuron 5:Zero response
3
Brightness and Luminance
Luminance is the physical intensity of light from a surface.
Brightness is the perceived intensity of light.
Brightness is related to luminance, but is not completely determined by it.
Brightness depends on the level of adaptation of the eyes to light and the contrast of neighboring surfaces.
4
Dark Adaptation
•When one turns off the light at night, at first one cannot see much.
•After a few minutes, you can start to make out objects.
•This increase in sensitivity is called Dark adaptation.
•Cones adapt in about 7 minutes; Rods in about an hour.
•Rods reach a higher level of sensitivity.
•Adaptation is the result of regeneration of pigment molecules (Rhodopsin) that are bleached in the light.
5
Contrast Effects
•The amount of brightness perceived depends on the surrounding scene.•Lighting a candle in a dim room has a larger perceived effect than lighting it in a lighted room.
Weber's Law:The smallest difference in intensity that can be detected is directly proportional to the background intensity.
€
ΔII
=C
6
Simultaneous Contrast
The perceived brightness of a surface depends on the luminance of the surrounding surfaces.
Two uniformly gray squares will appear different when placed on light or dark backgrounds.
Demo
Can we explain this with neural receptive fields?
7
Other contrast effectsThe Koffka Ring
1. The Koffka Ring(demo)
8
Other contrast effectsSurface Orientation
Brightness depends on surface orientation
9
Attention can affect brightness
10
Seeing Things?The Hermann Grid:
Can we explain this with neural receptive fields?
11
Why Find Edges?•Want to find meaningful objects or surfaces
•Usually the border of an object is defined by a change in light intensity.
•Changes in intensity can also signal a change in depth or orientation of the surface.
•Must first find the intensity changes.
•Then must find what led to the intensity change.
12
Human Psychophysics
• Humans can detect sharp intensity changes:•Craik-Obrien-Cornsweet illusion #1
Intensity
DistanceDemos: (Viperlib.org)
•Humans are not good at detecting gradual intensity changes.
13
Stages for Edge Detection
Detecting Edges:
• Smoothing--Eliminates noise. Determines spatial scale.
• Differentiation--Localizes the intensity change
Feature Extraction:
• Determine the feature that caused the intensity change.
14
Smoothing
Intensity
Smoothing
More
Smoothing
15
Intensity Derivative
SmoothedIntensity
First Derivative
Second Derivative
16
Zero Crossings and Edges
ImageImage after smoothing and second derivative
Black = Negative
White = Positive
Zero Crossings
17
Localization of an edge•Changes in intensity are not instantaneous, particularly in a smoothed image.•Humans can localize edges to within a few seconds of arc (a few mm for a line a distance of 1 meter from the observer).•Accurate localization is necessary for stereo vision.•Differentiation allows us to find the location of the most rapidintensity change.•The first derivative gives a peak at the location of the most rapid change.•The second derivative gives a zero at this location.• Marr and Hildreth suggested using these zero crossings to indicate edges.
18
Detecting a zero-crossing with center-surround RF's
+-
+-
+-
Neuron 1:Positive response
Neuron 2:Zero response
Neuron 3:Negative response
Detecting a zero crossing with center surround receptive fields.