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Focus Cues

Focus Cues. Kurt Akeley CS248 Lecture 20 6 December 2007 http://graphics.stanford.edu/courses/cs248-07/. Depth cues. Binocular geometric cues: Stereopsis (retinal-image disparity) Convergence of the lines of sight. Retinal-image disparity. Vergence.

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Focus Cues

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  1. Focus Cues Kurt Akeley CS248 Lecture 20 6 December 2007 http://graphics.stanford.edu/courses/cs248-07/

  2. Depth cues • Binocular geometric cues: • Stereopsis (retinal-image disparity) • Convergence of the lines of sight Retinal-image disparity Vergence Correct retinal-image disparity allows the viewer to “fuse” the scene

  3. Depth cues • Binocular geometric cues: • Stereopsis (retinal-image disparity) • Convergence of the lines of sight • Monocular geometric cues: • Motion parallax • Object size (relative and changing) • Perspective convergence

  4. Depth cues • Binocular geometric cues: • Stereopsis (retinal-image disparity) • Convergence of the lines of sight • Monocular geometric cues: • Motion parallax • Object size (relative and changing) • Perspective convergence • Color cues: • Occlusion • Lighting and shading • Atmospheric effects (attenuation, blue shift) • Texture gradient

  5. Focus matters too! This photo is of a real scene, not of a model

  6. Outline • Focus • Focus cues • Fixed-viewpoint volumetric display • Experimental results • Practical implications

  7. Focus

  8. Object points Aperture Image points Focus • Focus defines a 1-to-1 correspondence between • Object points, which (may) radiate light, and • Image points, where the radiated light converges • In practice image points capture only the object-point radiation that passes through an aperture.

  9. A lens provides the magic Index of refraction is greater than one

  10. so si Thin lens equation The correspondence Object point f Straight line through the center of the lens Image point d

  11. Diopters (D)

  12. 0.67 m 2 D Diopter distances Diopter number-line assignments are relative to a reference point 1.0 m 0.5 m 0.33 m 1 D 2 D 3 D Diopter distances are differences between number-line assignments:2 D = 3 D – 1 D Not computed as the reciprocal of the 0.67 m “distance” !

  13. Thin-lens equation (using Diopters) Object point f Reference point is the center of the lens Image point so si

  14. rb ra Image plane to ti E Out-of-focus blur Object point f so si

  15. Depth of field (DOF) • (sometimes also called depth of focus) • DOF is the amount of focus error that is inconsequential • Recall that • Therefore: • DOF measured in Diopters is (almost) invariant with respect to focus distance (So) • For a given aperture radius (ra) and an acceptable blur radius (rb) • “Almost” because there is a slight dependence we are ignoring • DOF is inversely proportional to aperture radius • Once the acceptable blur radius is determined

  16. Focus Cues

  17. Focus cues • There are two focus cues: • Accommodation (the focus response of the eye) • Retinal-image blur • Neither is a quality of the light field • Instead they are conditions in the human body that are stimulated by the light field

  18. Accommodation • Accommodation is the focus response of the eye: Resting focus is at infinity (0 D), or is corrected with fixed lenses to infinity. The ciliary muscles contract, allowing the lens to become more spherical. This increases the power of the lens, reducing the focal distance.

  19. All but one Diopter of focal range is within arm’s reach! Human accommodation range 0 1 Diopters 2 3 4 8 12 … ∞ 1 meters Me  Young adults Children

  20. Human depth of field • Human depth of field is approximately +/- 0.3 D • The optics of the eye are not perfect • This corresponds to • A DOF from 2 m to infinity, or • A DOF from 10” to 12” • So near-field scenes (with differing depths) are blurry, while far-field scenes are not 0 1 Diopters 2 3 4 8 12 … ∞ 1 meters

  21. Tilt-shift miniaturization

  22. Why “tilt-shift”? • Recall that focus is a correspondence • Before Photoshop the effect was created by tilting the image plane of the camera off the main axis:

  23. Make a model appear real

  24. Make a real scene appear miniaturized

  25. A stereo display gets all these right … • Binocular geometric cues: • Stereopsis (retinal-image disparity) • Convergence of the lines of sight • Monocular geometric cues: • Motion parallax • Object size (relative and changing) • Perspective convergence • Color cues: • Occlusion • Lighting and shading • Atmospheric effects (attenuation, blue shift) • Texture gradient

  26. But the focus cues are all wrong Vergence and accommodation are decoupled Incorrect accommodation cue No retinal-image blur cues

  27. Volumetric displays fix the focus cues … • And they are autostereoscopic: • Require no tracking of the viewer’s position or orientation • Support multiple simultaneous viewers • Stereopsis is “free” • References: • Downing et al. 1996 • Favalora et al. 2002 • Lightspace Tech. 2003

  28. But they fail in other critical ways • Binocular geometric cues: • Stereopsis (retinal-image disparity) • Convergence of the lines of sight • Monocular geometric cues: • Motion parallax • Object size (relative and changing) • Perspective convergence • Color cues: • Occlusion • Lighting and shading • Atmospheric effects (attenuation, blue shift) • Texture gradient No view-dependent shading is possible, because viewer position is not known

  29. Fixed-viewpoint Volumetric Display

  30. Fixed-viewpoint volumetric display • Fixed-viewpoint: • All geometric and color depth cues are correct • Volumetric: • All focus cues are near-correct • No need for gaze tracking • What’s the catch? • Display is head-mounted • Must track viewer position and orientation • Latency is a challenge • Must overcome ergonomic issues

  31. Required volumetric resolution • Autostereoscopic volumetic displays have huge pixel-count requirements in all three dimensions • Fixing the viewpoint allows spatial and depth resolutions to be optimized independently: • Spatial pixel density requirements are unchanged • Foveal limit requires 2 pixels/arc min • But depth pixel density is determined by depth of field • +/- 0.3 D is more than satisfied by two pixels per diopter • A display with 4 D range has a depth pixel-count of 7 ! 1000s 7 1000s

  32. Prototype display design

  33. Prototype display Bite bar

  34. Demo

  35. Depth blending

  36. Retinal image of a sine wave grating Lower contrast Eye image from www.wikipedia.com

  37. Modulation transfer function

  38. Retinal-image contrast with summed images

  39. Experimental Results

  40. Work done at UC Berkeley • Marty Banks • Simon Watt • Ahna R. Girshick • David M. Hoffman • … http://bankslab.berkeley.edu/

  41. Stimuli

  42. Forced-choice This ? Or this ?

  43. Experimental design

  44. Experimental results

  45. Results summary • Correct focus distance results in • Shorter time to “fuse” a depth-corrugation stereogram • Ability to fuse finer depth corrugations • Better estimations of depth • People consistently underestimate depth in VR environments • Less fatigue • No forced decoupling of vergence and accommodation • There is no performance penalty for depth blending • For details refer to the publications: http://bankslab.berkeley.edu/publications.html

  46. Practical implications

  47. Without a fixed-viewpoint volumetric display • Use long viewing distances when possible • Flight simulators use either • Large done display • Collimated (infinite focus distance) optics • Minimize accommodation/vergence conflict • High-quality stereo headsets have adjustable focal distance • Set it to the best average distance • Is there hope of a practical fixed-viewpoint volumetric display?

  48. Fixed optics f

  49. Adaptive optics Images from www.wikipedia.com

  50. Summary • Focus cues are • Accommodation (focus response) • Retinal blur • Correct focus cues matter • Tilt-shift miniaturization • Experimental results

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