Notes1011 - Honors 208W Fall 2011 Lenna Soderberg -...

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Honors 208W Fall 2011 Image Compression (Information Compression) Lenna Soderberg - Playboy, November 1972
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Notes © 2005-2011 : Evan Golub (egolub@acm.org) Bitmaps • The simplest way to represent a photographic image digitally is to store a color code for each individual pixel. • None of the available color data is lost. • The more pixels you have, and the more colors available, the better the quality of the image reproduction (recall the letter “e” from the aliasing slides). • The size of a BMP is prohibitive. As the width and height are doubled, the size is squared! – A 1024x768 bitmap (sub-megapixel) using 24-bit colors would be a file of size over 2MB. – The size of a 2048x1536 bitmap file would be over 9MB.
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Bits per pixel Even with a bitmap approach, each pixel needs to have enough bits reserved for it to store the information available. Consider a camera that produces 12-bit values for each color channel from a photosite but a bitmap file format that can only store 8-bit values for each color channel in a pixel. Something must be lost. Notes © 2005-2011 : Evan Golub (egolub@acm.org)
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How a 12-bit RED value looks. Base-10 Value Base-2 Equivalent 0 000000000000 255 000011111111 1492 010111010100 2730 101010101010 3381 110100110101 4032 111111000000 4095 111111111111 Notes © 2005-2011 : Evan Golub (egolub@acm.org)
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How to go from 12-bit to 8-bit Channel? 12-bit value high 8-bit middle 8-bit low 8-bit 0 0 0 0 255 15 63 255 1492 93 117 84 2730 170 170 170 3381 211 77 53 4032 252 240 192 4095 255 255 255 Notes © 2005-2011 : Evan Golub (egolub@acm.org)
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Other ways to go from 12-bit to 8-bit? Image processing tools can use different formulas and algorithms to take the 12-bit data and go to an 8-bit value. These can alter the brightness levels or tonal qualities with less visual aberration than performing those adjustments with an image than already only has 8-bit values being used… Notes © 2005-2011 : Evan Golub (egolub@acm.org)
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Notes © 2005-2011 : Evan Golub (egolub@acm.org) Raster -vs- Vector Graphics Image representation systems that are pixel-based are referred to as raster graphics systems. Image representations that use geometric shapes such as points, lines, and curves to define an image are referred to as vector graphics systems. – Elements such as lines can essentially be scaled without “loss” of quality, though anti-aliasing might need to be used to avoid jaggedness when drawing. – Very useful for things like drawing lines to be made by a laser cutter. – Not very good for photographs, so we will probably not be discussing them again…
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Notes © 2005-2011 : Evan Golub (egolub@acm.org) Sensor Recap We have discussed the digital camera’s sensor as typically being a grid of filter-covered photosites where each is able to report the amount of light of a certain color it detects. Most sensors use RGB color channels.
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This note was uploaded on 01/13/2012 for the course HONR 208W taught by Professor Golub during the Fall '11 term at Maryland.

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Notes1011 - Honors 208W Fall 2011 Lenna Soderberg -...

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