3ContrastEnhancementLectureNotes

3ContrastEnhancementLectureNotes - GIS 4037c Dr Charles...

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GIS 4037c Dr. Charles Roberts The Contrast Enhancement Introduction to Image Enhancement There are thresholds beyond which humans cannot detect differences. Humans can see between 8 and 16 grey tones at a time. VGA graphics systems display only 16 grey tones. If the data contains 256 grey tones and we can only see 16, how do we see the others? They are combined as textures and patterns in our brains with the 16 that we see. An important part of image analysis is image enhancement. There are several image enhancement techniques. We have already had an exercise in which we color the image to highlight the information we want to emphasize. This coloring process is called LEVEL SLICING or DENSITY SLICING. Using this technique, you combined grey tones to create and interprete a map of the Berry Islands in the Bahamas. You generalized the information to make meaningful inferences about the environment. The overwhelming experience for remote sensing professionals is that humans can see more elements of image interpretation than the computer can see. The most useful elements in image interpretation are: -Shape -Size -Pattern - T e x t u r e -Tone -Color -Shadow -Site and Situation -Context When we do image enhancement, in essence what we are doing is enhancing these elements of the image. For instance, we can use contrast enhancement to brighten or darken particular tones so that they stand out and reveal a Pattern or Texture (forest canopy) or else reduce the tone contrast to produce good shape (fields). In essence, we are making it easier to visually interpret the image. Skill in image enhancement comes with practice: Like art or writing, you must do it over and over again to get good at it. It is not scientific, in that we are distorting the spectral information to get an image that is easy to interpret visually. It is really a graphic art. But it is increasing in importance for two reasons: 1) GIS users are coming to rely on digital imagery as a background for other data layers. 2) Digital imagery and image products are becoming widely available from individual state GIS data clearing houses on the World Wide Web. Currently, Florida, Texas, New Mexico, New York, Kentucky, Alabama, North Carolina
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3ContrastEnhancementLectureNotes - GIS 4037c Dr Charles...

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