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Office Hours

Office Hours. Office hours are posted on the website. Molly: Tuesdays 2-4pm Dr. Keister: Wednesdays 10am-12 Prof. Goldman: Wednesdays 2-3:30pm All office hours are in the help room downstairs. Map to Help Room (G2B90). Lecture room. Help room. Chapter 7: Image Processing. Eye Movement

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Office Hours

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  1. Office Hours • Office hours are posted on the website. • Molly: Tuesdays 2-4pm • Dr. Keister: Wednesdays 10am-12 • Prof. Goldman: Wednesdays 2-3:30pm • All office hours are in the help room downstairs.

  2. Map to Help Room (G2B90) Lecture room Help room

  3. Chapter 7: Image Processing • Eye Movement • Retinal Stabilization • Temporal Response • Persistence of vision • Zoetropes • Stroboscopes

  4. Eye Movements • Scanning serves two purposes: • It moves different parts of the scene over the high resolution fovea • It produces a temporal change at a fixed point on the retina

  5. Concept Question • Why would you want to scan your eyes to move the scene across the fovea? • The photoreceptors there are more sensitive to low light levels • Cones (high resolution color vision) are concentrated in the fovea • The image will be less distorted because the fovea is in the middle of the retina • The nerve cells at the fovea work faster than in other areas of the retina

  6. Remember this illusion? It only works if you don’t move your eyes. If you move your eyes, you are causing a change in illumination on the same part of the retina Lateral Inhibition & Edge Enhancement X

  7. Eye Movement • Drifts: Slow smooth movements • Tremors: Rapid jittery movements • Saccades: Sharp, abrupt movements

  8. Saccades and Reading Text • When reading text, your eyes skip between words and lines, both forwards and backwards, by means of saccades

  9. Saccades and Reading Text

  10. Involuntary Eye Movements • Even when you think you are staring fixedly at something, your eyes are still moving. • These involuntary eye movements are responsible for the shimmering effect of certain op art pieces • The negative afterimages of edges are overlapped with what you are actually looking at on a very fast timescale

  11. Bridget Riley, “Waves”

  12. Reginald Neal, “Square of Two”

  13. Retinal Stabilization • While it is not possible to prevent your eye from moving, it is possible to cancel out the effects of the motion of the eye. • This is called retinal stabilization • A moving mirror is used to reflect an image such that the image is moved to exactly counteract the motion of the eye

  14. Retinal Stabilization Actual image What is seen with retinal stabilization A ganglion cell desensitizes within a few seconds, even if there is an edge in its receptive field. Eye movement is necessary for our vision to work.

  15. Temporal Response • The visual system response to a flash of light is twofold: • The response is delayed slightly from the actual flash, called “latency” • The response lasts longer than the actual flash, called “persistence” time actual flash visual system response

  16. Persistence of Vision • The image of a flash or image lasts for between 1/20s (in low light) and 1/50s (in bright light) • If two images are presented in rapid succession, faster than the persistence time, they will merge into one image • For a series of images, this creates the illusion of motion

  17. Motion • A series of images, each only slightly different from the previous one, produces continuous motion. • Each image is shown too quickly to process separately, and your visual system fills in between the images

  18. Appearance of Motion persistence time delay time actual flash visual system response overlap between persistent images time between frames time actual flash visual system response time actual flash visual system response

  19. Concept Question • The visual system fills in a lot of things. Which of these are examples of this? • Uniform color areas between edges • In between successive images on a tv screen • Edges that aren’t really there • A and B • B and C

  20. Zoetropes • Invented in the 1830s, zoetrope means, roughly, “wheel of life”

  21. Flicker • If the time between images is not fast enough, the motion will not be smooth, but will be jerky or flickery

  22. Flicker in Motion persistence time delay time actual flash visual system response time between persistent images time actual flash visual system response actual flash visual system response

  23. Which second flash will appear to flicker? persistence time delay time actual flash visual system response A time B time C time

  24. Stroboscope • A strobe light flashes for a short time at some frequency

  25. Stroboscopic Photography Using a strobe light to illuminate a scene for a very short period of time, you can “freeze” motion that otherwise would be a blur in the image.

  26. Stroboscopic Photography You can also take long exposures of a moving object

  27. Wagon Wheel Effect • When a regularly spoked wheel is filmed, or illuminated with a strobe light, it will sometimes appear to be stopped or even moving backwards.

  28. Wagon Wheel Effect • When a regularly spoked wheel is filmed, or illuminated with a strobe light, it will sometimes appear to be stopped or even moving backwards.

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