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.

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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 Retinal Stabilization Temporal Response –Persistence of vision –Zoetropes –Stroboscopes

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

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

Lateral Inhibition & Edge Enhancement 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 X

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

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

Saccades and Reading Text

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

Bridget Riley, “Waves”

Reginald Neal, “Square of Two”

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

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. What is seen with retinal stabilization Actual image

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 flashvisual system response

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

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

Appearance of Motion time actual flashvisual system response time actual flashvisual system response delay persistence time time actual flashvisual system response time between frames overlap between persistent images

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

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

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

Flicker in Motion actual flashvisual system response time actual flashvisual system response delay persistence time time actual flashvisual system response time between persistent images

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

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

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.

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

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.

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.