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Perception and VR

MONT 104S, Fall 2008Lecture 4

Lightness, Brightness and Edges

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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.

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Neuron 2:Positive response

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Neuron 3:Zero response

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Neuron 4:Negative response

Light Dark

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Neuron 1:Zero response

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Neuron 5:Zero response

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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.

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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.

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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.

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ΔII

=C

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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?

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Other contrast effectsThe Koffka Ring

1. The Koffka Ring(demo)

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Other contrast effectsSurface Orientation

Brightness depends on surface orientation

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Attention can affect brightness

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Seeing Things?The Hermann Grid:

Can we explain this with neural receptive fields?

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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.

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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.

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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.

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Smoothing

Intensity

Smoothing

More

Smoothing

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Intensity Derivative

SmoothedIntensity

First Derivative

Second Derivative

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Zero Crossings and Edges

ImageImage after smoothing and second derivative

Black = Negative

White = Positive

Zero Crossings

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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.

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Detecting a zero-crossing with center-surround RF's

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Neuron 1:Positive response

Neuron 2:Zero response

Neuron 3:Negative response

Detecting a zero crossing with center surround receptive fields.