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Digital Images

Learn about the process of digitizing images, the importance of resolution and pixel density, different file formats for graphics, digital cameras, displaying images, graphics cards, and the basics of digital animation.

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Digital Images

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  1. Digital Images CS 1308 Computer Literacy and the Internet

  2. Creating Digital Pictures • A traditional photograph is an analog representation of an image. • Digitizing a picture is the act of creating picture elements (pixels) to represent the image. • The process is similar to the sampling of analog audio • The more pixels, the better the quality • The more bits for each pixel, the closer the approximation • The human eye can be tricked into filling in the missing pieces

  3. A 15x12 picture One Pixel 15x12 = 170 total pixels

  4. Resolution • The number of pixels • The higher the resolution, the clearer the picture • Increased resolution allows you to zoom in and see more detail • Higher resolution increases the size of the file in memory

  5. Aliasing Zoomed in 5 times Notice the Pixelation

  6. Colors • RGB (Red, Green, Blue) • How many bits for one color in this cube?

  7. Image Colors 45,000 Colors 256 Colors

  8. Image Colors 45,000 Colors 16 Colors

  9. Image Colors 45,000 Colors 2 Colors

  10. Image Colors 45,000 Colors 250 Grey scale tones

  11. Pixel Densities • Resolution Example

  12. How much space? • For each pixel you have to store a color or grey scale. • Resolution 300x200 = 60,000 pixels • 8 bit greyscale (256 tones) – 60KB • 24 bit true color – 180KB (3 bytes per pixel)

  13. Graphics File Formats • Raster Graphics • Stores information on a pixel-by-pixel basis • BMP, GIF, JPEG, PNG • Vector Graphics • Stores a description of shapes in the picture • Scalable Vector Graphics (SVG)

  14. JPEG vs GIF • JPEG • Joint Photographic Experts Group • Forgent Networks claims patent on key algorithm • 24 bit color • Lossy compression • Cleverly removes portions of picture that humans won’t see • Very good for pictures, “realistic” scenes • Allows use to select amount of compression • GIF (Graphics Interchange Format) • Patented by Compuserve • 256 colors • Lossless compression • Good for line drawings, clip art

  15. Digital Cameras • Much like a normal camera in terms of optics • Has advantage of knowing what a picture looks like immediately

  16. Digital Pictures • Image is acquired by absorbing reflected light and recording the wavelengths (very similar to sampling audio) • The sensor takes the place of the film • Stored in proprietary RAW format before download to computer • Sensor sends digital signals to be processed and stored in non-volatile memory

  17. Megapixels? • What does it mean? • Amount of sensor information • Allows for more pixels at maximum resolution • Why is 5 megapixels better than 3 or 4? • Can they really zoom in on pictures like on CSI?

  18. Displaying Images 15” • Display types • CRT • LCD • Gas Plasma • Typical resolutions • 800x600 • 1024x768 • 1152x864 • 1280x1024 • 4 to 3 ratio • Movies are 16:9 • Refresh rate • Interlacing • Graphics Card 12” 19”

  19. Graphics Cards • Hardware that connects monitors to your PC • Have processors, memory, clocks • Almost sub-computers • Controls resolution and refresh rate • Drivers • Software that allows graphics cards to interface with monitors • Specific to Operating Systems • Linux often has worse support because fewer people use it • Some card include “tuners” for television • NTSC

  20. MPEG Compression • MPEG-1 • video resolution of 352-by-240 at 30 frames per second (fps) • quality slightly below the quality of conventional VCR videos • MPEG-2 • offers resolutions of 720x480 and 1280x720 at 60 fps • full CD-quality audio • sufficient for all the major TV standards, including NTSC, and even HDTV • MPEG-2 is used by DVD-ROMs. MPEG-2 can compress a 2 hour video into a few gigabytes • decompressing an MPEG-2 data stream requires only modest computing power, encoding video in MPEG-2 format requires significantly more processing power • MPEG-4 • graphics and video compression algorithm standard that is based on MPEG-1 and MPEG-2 and Apple QuickTime technology • Wavelet-based MPEG-4 files are smaller than JPEG or QuickTime files • designed to transmit video and images over a narrower bandwidth and can mix video with text, graphics and 2-D and 3-D animation layers

  21. Digital Animation • Programs used to describe desired output • Maya • Images are “rendered” one screen at a time • 24 fps • More for slow motion • Takes a lot of computer time • Graphics machines need A LOT of memory, processor speed, and secondary storage

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