11Lec. 26, Thursday, Nov. 18Digital imaging (not in the book)• Matrices and bit maps• How many pixels• How many shades? •CCD• Digital light projector• Image compression: JPEG and MPEG1We are here1Chapter 8: Binocular vision What is a digital image?First: What is a matrix?The matrix M is rows and columns of numbers. M1,1M1,2M1,3M2,1M2,2M2,3M3,1M3,2M3,3 22Dot matrix printer3The numbers in the matrix are 0 for OFF and 1 for ON. The 0 or 1 is a bit. The matrix for an image is a bit map. One dot is a pixel. Each pixel is one “bit” (a zero or a one) in the memory. Example: highway sign with weather warningWhy do I want more pixels?4Answers:• To make an enlargement• To read the fine print3How many pixels does my display have?640 x 480 VGA used in older 14 in. displays800 x 600 SVGA (super video graphics array)1024 x 768 XGA (extended graphics array)1280 x 1024 SXGA1600 x 1200 UXGA used in 20 inch displaysPixels in digital cameras (examples): 2240 x 1680 (4 megapixels) 4064 x 2704 (11 megapixels) In color images, there are 3 subpixels, one for each color.5Two designs for color pixels. WikipediaHow many shades of gray?Instead of 0 for black and 1 for white: 0 to 255 for shades of gray (256 values) 64How many possible colors on your screen?256 shades of red x 256 shades of blue x256 shades of green = 16.7 million colors 7Demo: in MS Word, the draw program, fill colorsDigital CamerasFilm is replaced by a CCD chip (and a memory chip.)Wikipedia5The digital camera dial• Auto: camera chooses shutter speed and exposure time• S or Tv: shutter priority, you choose the shutter speed to freeze moving subjets, camera selects f/number.• A: aperture priority, you choose aperture to control depth of field• M: manual, you choose f/number (aperture) and shutter speed • P: program mode, partially automaticMore camera modes• Macro mode (flowers)– Camera focuses close to capture a bee in a flower, for example– 4 x 6 inch prints are life size• Landscape mode– Chooses small aperture (big f/number) for depth of field• Portrait mode– Chooses large aperture for small depth of field• Sports mode– Chooses fastest shutter speed• Night mode– Chooses long exposure for background and flash for subject6CCD is a charge coupled device A CCD is rows and columns of sensors which convert light to electric signals. It produces a bitmap image directly. One sensor elementColor CCD chips have filter arrays7CCD chip seen with a microscope.How is the image stored?• The easiest file type to understand in the bitmap (.bmp) which is a matrix of numbers. • Bitmaps files are often too large (2 MB or more). You could not store many images on a small “memory stick.”• JPEG (joint photographic experts group) files are smaller (compressed about 10:1) and some detail is lost. They are converted back to bitmaps to be displayed.8Examples of JPEG compression8.4 KB23 KB4 KBMPEG filesMPEG (moving picture experts group) files are for movies and DVDsThe basic idea:1. Send a complete picture every now and then2. Between complete pictures, send only the difference between the images (consider a car moving against a fixed background)3. Send fewer colors than the whole range9What is a digital light projector (DLP)?It is an array of tiny mirrors which can tilt. Each mirror is 16 micrometers square. Maximum resolution now is 1280 x 720. Mirror array with insect leg. Light shines on the mirrors and is projected (or not) onto the screenFor color images, color filters are needed. Often a rotating wheel with color filters is used.10Internal components of an old 1998 InFocus LP425z single-chip DLP projector, with a 4-segment color wheel. Interior view of a single-chip DLP projector, showing the light path. Light from the lamp enters a reverse-fisheye, passes through the spinning color wheel, crosses underneath the main lens, reflects off a front-surfaced mirror, and is spread onto the DMD (red arrows). From there, light either enters the lens (yellow) or is reflected off the top cover down into a light-sink (blue arrows) to absorb unneeded light. Wikipedia.com202020Chapter 8: Binocular vision• Why two eyes• Binocular disparity• Conveying depth in art• Stereo images20We are here201121Why two eyes?1. Wider field of view: one eye sees 130 degrees, two eyes see 208 degrees2. Depth perception: the two images are not the same. The difference, binocular disparity, tells us the distance. Animal eyes:Predators: eyes in front, like catsPrey: eyes on sides of head, like cows, ducks, rabbitsExceptions: prey living in trees; monkeys, squirrels2122How do we perceive distance?Accommodation:Eye muscles must exert a force to focus more closely. You can feel the strain associated with accommodation.Convergence:Your eyes must both orient toward the center of the field of view in order to focus on something close. Parallax:A different view is seen from different positions. Hold up a finger in front of your face. Look at a distant object through one eye and then the other. 22Demo: hold up a finger near your face. Close one eye then the other.1223Convergence (again)2324What is parallax?What is binocular disparity?24Left eye view Right eye viewThe items are a basketball and a grapefruit. The brain interprets the difference in the images as due to distance.The image difference is binocular disparity. Parallax is an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight.1325Stereo Photographytwo cameras make two images to reproduces binocular disparity25The three-d view looks “just right” when the cameras have the same spacing apart as your eyes. Is this answer a surprise? The spacing of eyes is about 6.5 cm or about 2 ½ inches. 26Viewing a stereo image26This 19thcentury device is a stereopticon or stereoscope.1427Lenticular screenLenticular screens were used for 3-d “baseball cards.”These have little lenses embossed in plastic (see diagram below) which send to the left and right eyes different images which are printed in small vertical strips under the lenses. 27PlasticlenticulesThere is an LCD version of the lenticular screen that shows 3-d. However, you must be standing in front at the right distance. 282828Chapter 8: Binocular vision• Why two eyes• Binocular disparity• Conveying depth in art• Stereo images28We are here281529Conveying 3-d in 2-d artSix ambiguous depth cues (having more than one