08-1 - Image compression

08-1 - Image compression - Image compression compression...

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4/28/2008 1 Image compression Image compression fundamentals Spring 2008 ELEN 4304/5365 DIP 1 by Gleb V. Tcheslavski: [email protected] http://ee.lamar.edu/gleb/dip/index.htm Preliminaries Image compression – reducing or eliminating of redundant or irrelevant information – is one of the most used techniques in the field of image processing. Considering a two-hour standard definition (SD) TV movie that we need to store digitally… A digital movie (video) is a sequence of video frames, which are full-color digital still images. The standard rate for an NTSC video is close to 30 fps (frames per second), therefore, the bit rate required for SD video is 30 (720 480) 33 1 ,104,000 / sec bytes ⋅ = Spring 2008 ELEN 4304/5365 DIP 2 Image resolution Frame rate Pixel depth And a storage requirement for a two-hour movie would be 11 31,104,000 60 60 2 2.24 10 bytes ⋅ ≈
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4/28/2008 2 Preliminaries Approximately 224 GB is needed to store an uncompressed two-hour SD movie. Therefore, to fit such movie on a standard DVD-9, data must be compressed by a factor of approximately 26.3. The compression must be higher for high definition (HD) TV, where image resolution is up to 1920x1080 pixels. Similarly, an 8-MP digital camera could store about 41 uncompressed full-color images (roughly 24 MS each) on a 1 GB memory card. Spring 2008 ELEN 4304/5365 DIP 3 Compression can also significantly reduce transmission time needed to transmit an image over the web… Fundamentals Data compression is a process of reducing the amount of data required to represent a given quantity of information. Note that data and information are not the same; data are the means by which information is conveyed. Since various amounts of data can be used to represent the same amount of information, representations that irrelevant or repeated information are said to contain redundant data . Denoting the number of bits in two representations of the same information as b and b’ , the relative data redundancy of the representation with b bits is 1 1 R Spring 2008 ELEN 4304/5365 DIP 4 C = Where C is the compression ratio : ' b C b =
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4/28/2008 3 Fundamentals If C = 10, for instance, the larger representation has 10 bits of data for every 1 bit of data in the smaller representation. The corresponding relative data redundancy of the larger representation is R = 0.9 indicating that 90% of its data is redundant. b is the number of bits needed to represent an image as a 2D array of intensity values. We notice that such arrays being preferred formats for human viewing, are not optimal from a compact data representation viewpoint. Such arrays have three types of data redundancy: Spring 2008 ELEN 4304/5365 DIP 5 1. Coding redundancy : intensities represented by 8 bits contain more data than needed; 2. Spatial and temporal redundancy : each pixel is similar to or dependent on (correlated) neighboring pixels. In video, pixels are correlated temporally. Therefore, information is replicated.
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This note was uploaded on 12/08/2010 for the course ELEN 4304 taught by Professor Staff during the Spring '08 term at Lamar University.

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08-1 - Image compression - Image compression compression...

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