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Understanding Optical Illusions Mohit Gupta

Understanding Optical Illusions Mohit Gupta. Truth: But this is an ! What are optical illusions? Light (Sensing) Oracle Perception: I see

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Understanding Optical Illusions Mohit Gupta Slide 2 Truth: But this is an ! What are optical illusions? Light (Sensing) Oracle Perception: I see Slide 3 Optical Illusion in Nature Image Courtesy: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter19/graphics/infer_mirage_road.jpg Slide 4 A Brightness Illusion Slide 5 Different kinds of illusions Brightness and Contrast Illusions Twisted Cord Illusions Color Illusions Perspective Illusions Relative Motion Illusions Illusions of Expressions Slide 6 Lightness Constancy Our Vision System tries to compensate for differences in illumination Slide 7 Why study optical illusions? Studying how brain is fooled teaches us how it works It makes us happy : Al Seckel Illusions of the senses tell us the truth about perception [Purkinje] Slide 8 Simultaneous Contrast Illusions Slide 9 Low-level Vision Explanation Receptive Fields in the Retina Negative Photo-receptors Positive Photo-receptors + - - -- Inhibitory Light Excitatory Light Slide 10 Low-level Vision Explanation + - - --+ - - -- Positive Gradient Negative Gradient Slide 11 High-level Vision Explanation: Context Less Incident Illumination More Incident Illumination Brightness = Reflectance * Incident Illumination Higher Perceived Reflectance Lower Perceived Reflectance Slide 12 The Hermann grid illusion Slide 13 The Hermann grid: Low level Explanation + - - -- Lateral Inhibition Slide 14 The Hermann grid illusion Focus on one intersection Slide 15 Why does the illusion disappear? Receptive fields are smaller near the fovea (center) of the eye Slide 16 The Waved Grid: No illusion! Slide 17 Scintillating Grids: Straight and Curved Slide 18 Adelsons checkerboard illusion Slide 19 Slide 20 Where is the illusion? Slide 21 Slide 22 Rubiks Cube Color Illusion Same Color? Slide 23 Rubiks Cube Color Illusion Slide 24 Todorovics Gradient Chessboard Illusion Are all the dots the same color? Slide 25 Craik-OBrien-Cornsweet Illusion Actual Profile Perceived Profile Slide 26 Whites Illusion Slide 27 Footstep Illusion Click Contrast affecting Perception of speed Slide 28 Twisted Cord Illusions Slide 29 Are the lines parallel? Straight? Slide 30 Caf Wall Illusion Slide 31 Slide 32 A variant of the Caf Wall Illusion Slide 33 The Fraser Spiral Illusion Slide 34 Pinnas Intertwining Illusion Slide 35 Illusions involving perspective Slide 36 Shepards table-top illusion Which is bigger? Slide 37 Shepards table-top illusion Which is bigger? Slide 38 Shadow Illusions Slide 39 Shape-from-Shading Shadows influence the perception of shape Slide 40 Click Kerstens Ball and Shadow Illusion Shadows influence the perception of motion and position Slide 41 Relative Motion Illusions Slide 42 Out of Focus Illusion Slide 43 Spinning Wheel Illusion Slide 44 Motion Induced Blindness Click Slide 45 Anamorphoses Leonardos Eye [Leonardo Da Vinci, 1485] Slide 46 Slide 47 Image Courtsey: http://www.apprendre-en-ligne.net/blog/images/guinnessbase.jpg Good things come to those who wait Slide 48 Why study optical illusions? Studying how brain is fooled teaches us how it works It makes us happy : Al Seckel Click Illusions of the senses tell us the truth about perception [Purkinje] Slide 49 Have fun with Optical Illusions Then think about them! Eschers Waterfall Slide 50 Slide 51 Mach Band Illusions Slide 52 Intrinsic Image Separation Slide 53 Footstep Illusion: Contrast affecting Perception of speed http://journalofvision.org/6/12/5/images/Figure_7b_Demo.html Kick-back Illusion http://www.michaelbach.de/ot/mot_feet_lin/index.html Stepping feet Illusion http://journalofvision.org/6/12/5/images/Figure_8a_Demo.html http://journalofvision.org/6/12/5/images/Figure_8b_Demo.html http://journalofvision.org/6/12/5/images/Figure_8c_Demo.html http://journalofvision.org/6/12/5/images/Figure_8d_Demo.html http://journalofvision.org/6/12/5/images/Figure_2a_Demo.html Reverse-phi phenomenon[Howe et al, Journal of Vision] Slide 54 Levels of Processing Low-level Vision Mid-level Vision High-level Vision Light receptive cells at the retina level Surfaces, contours, grouping Cognition knowledge about materials, objects, scenes, illumination Slide 55 Slide 56 Slide 57 http://www.michaelbach.de/ot/ Slide 58 Slide 59