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Understanding Optical Illusions

Understanding Optical Illusions. Mohit Gupta. What are optical illusions?. Perception: I see. Light (Sensing). Truth: But this is an !. Oracle. Optical Illusion in Nature. Image Courtesy: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter19/graphics/infer_mirage_road.jpg.

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Understanding Optical Illusions

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  1. Understanding Optical Illusions Mohit Gupta

  2. What are optical illusions? Perception: I see Light (Sensing) Truth: But this is an ! Oracle

  3. Optical Illusion in Nature Image Courtesy: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter19/graphics/infer_mirage_road.jpg

  4. A Brightness Illusion

  5. Different kinds of illusions • Brightness and Contrast Illusions • Twisted Cord Illusions • Color Illusions • Perspective Illusions • Relative Motion Illusions • Illusions of Expressions

  6. Lightness Constancy Our Vision System tries to compensate for differences in illumination

  7. Why study optical illusions? • Studying how brain is fooled teaches us how it works “Illusions of the senses tell us the truth about perception” [Purkinje] • It makes us happy  : Al Seckel

  8. Simultaneous Contrast Illusions

  9. Low-level Vision Explanation Negative Photo-receptors Positive Photo-receptors Receptive Fields in the Retina - Inhibitory Light Excitatory Light - + - -

  10. Low-level Vision Explanation Positive Gradient Negative Gradient - - - - + + - - - -

  11. High-level Vision Explanation: Context Less Incident Illumination More Incident Illumination Higher Perceived Reflectance Lower Perceived Reflectance Brightness = Reflectance * Incident Illumination

  12. The Hermann grid illusion

  13. The Hermann grid: Low level Explanation - - + - - Lateral Inhibition

  14. The Hermann grid illusion Focus on one intersection

  15. Why does the illusion disappear? Receptive fields are smaller near the fovea (center) of the eye

  16. The Waved Grid: No illusion!

  17. Scintillating Grids: Straight and Curved

  18. Adelson’s checkerboard illusion

  19. Adelson’s checkerboard illusion

  20. Where is the illusion?

  21. Where is the illusion?

  22. Rubik’s Cube Color Illusion Same Color?

  23. Rubik’s Cube Color Illusion

  24. Todorovic’s Gradient Chessboard Illusion Are all the dots the same color?

  25. Craik-O’Brien-Cornsweet Illusion Actual Profile Perceived Profile

  26. White’s Illusion

  27. Footstep Illusion Contrast affecting Perception of speed Click

  28. Twisted Cord Illusions

  29. Are the lines parallel? Straight?

  30. Café Wall Illusion

  31. Café Wall Illusion

  32. A variant of the Café Wall Illusion

  33. The Fraser Spiral Illusion

  34. Pinna’s Intertwining Illusion

  35. Illusions involving perspective

  36. Shepard’s table-top illusion Which is bigger?

  37. Shepard’s table-top illusion Which is bigger?

  38. Shadow Illusions

  39. Shape-from-Shading Shadows influence the perception of shape

  40. Kersten’s Ball and Shadow Illusion Click Shadows influence the perception of motion and position

  41. Relative Motion Illusions

  42. Out of Focus Illusion

  43. Spinning Wheel Illusion

  44. Motion Induced Blindness Click

  45. Anamorphoses Leonardo’s Eye [Leonardo Da Vinci, 1485]

  46. Good things come to those who wait Image Courtsey: http://www.apprendre-en-ligne.net/blog/images/guinnessbase.jpg

  47. Why study optical illusions? • Studying how brain is fooled teaches us how it works “Illusions of the senses tell us the truth about perception” [Purkinje] • It makes us happy  : Al Seckel Click

  48. Have fun with Optical Illusions Escher’s Waterfall Then think about them!

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