Digital Images

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Digital Images

• CS 1308 Computer Literacy and the Internet

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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

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A 15x12 picture
One Pixel
15x12 170 total pixels
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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

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Aliasing
Zoomed in 5 times
Notice the Pixelation
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Colors

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

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Image Colors
256 Colors
45,000 Colors
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Image Colors
16 Colors
45,000 Colors
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Image Colors
45,000 Colors
2 Colors
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Image Colors
45,000 Colors
250 Grey scale tones
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Pixel Densities

• Resolution Example

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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)

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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)

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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
  wont 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

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Digital Cameras

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

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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

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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?

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Displaying Images

• Display types
• CRT
• LCD
• Gas Plasma
• Typical resolutions
• 800x600
• 1024x768
• 1152x864
• 1280x1024
• 4 to 3 ratio
• Movies are 169
• Refresh rate
• Interlacing
• Graphics Card

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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

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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

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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