1. 1 Perception and VR MONT 104S, Fall 2008 Lecture 4 Lightness, Brightness and Edges

2. 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. 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. 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. 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.

6. 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. 7 Other contrast effects The Koffka Ring 1.The Koffka Ring (demo)

8. 8 Other contrast effects Surface Orientation Brightness depends on surface orientation

9. 9 Attention can affect brightness

10. 10 Seeing Things? The Hermann Grid: Can we explain this with neural receptive fields?

11. 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. 12 Human Psychophysics Humans can detect sharp intensity changes: Craik-Obrien-Cornsweet illusion #1 Intensity Distance Demos: (Viperlib.org) Humans are not good at detecting gradual intensity changes.

13. 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. 14 Smoothing Intensity Smoothing More Smoothing

15. 15 Intensity Derivative Smoothed Intensity First Derivative Second Derivative

16. 16 Zero Crossings and Edges Image Image after smoothing and second derivative Black = Negative White = Positive Zero Crossings

17. 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 rapid intensity 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. 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.