Raster Image Compression (Lossy and Lossless) and Compression Ratio (Especially for GATE-Geospatial 2022)

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Image compression is an application of data compression that encodes the original image with few bits. The objective of image compression is to reduce the redundancy of the image and to store or transmit data in an efficient form. It affects the tiles of raster before storing them in geodatabase. It reduces the size of a file or database. Compression improves data handling, storage, and database performance. Examples of compression methods include Quadtrees, run-length encoding and wavelets.

Types of Image Compression

A variety of techniques are available for image compression. Compression techniques can be lossless or lossy.

This Figure Shows Data Compression Techniques

1. Lossy Compression

JPEG files use lossy compression which can achieve high compression ratios but cannot reconstruct the original image fully. MrSid (Multi-resolution Seamless Image Database) has capability of recalling image data at different resolution or scales and can compress a large image.

This Image Shows Raster Data Compression

The wavelet transforms the latest technology for image compression, treats an image as a wave and progressively decomposes the wave into simpler wavelets. JPEG 2000 uses wavelength compression. Following are important points:

  • Higher compression ratios (JPEG – 5: 1,10: 1,20: 1)
  • Cannot fully reconstruct the original image
  • Useful for raster data used as background (faster loading and retrieval)
  • Amount of reduction in data size depends on type of pixel data
    • Ex. Homogenous images will have higher compression ration
    • Higher have already been lossy compressed
This Image Shows Lossy Compression

2. Lossless Compression

  • Preserves the cell or pixel values
  • Allows the original raster to be reconstructed
  • MrSID and JPEG 2000 (1: 1,1: 3)
Image Shows Lossless Compression Examples
  • Should be used if:
    • Deriving new data for visual analysis
    • Required compression is no higher than 3: 1
    • Need to preserve information content (cell values)
    • Inputs have already been lossy compressed

Compression Ratio

Data compression ratio is defined as the ratio between the uncompressed size and compressed size: Thus, a representation that compresses a file՚s storage size from 10 MB to 2 MB has a compression ratio of 10/2 = 5, often notated as an explicit ratio, 5: 1 (read “five” to “one” ) , or as an implicit ratio, 5/1. Following are important points:

  • The compression ratio of lossy video codes is nearly always far superior to that of the audio and still-image equivalents.
  • Wavelet compression, used by raster formats such as MrSID, JPEG 2000, and ER Mapper՚s ECW, takes time to decompress before drawing.
  • Compression A series of techniques used for the reduction of space, bandwidth, cost, transmission, generating time, and the storage of data.
  • It is a computer process using algorithms that reduces the size of electronic documents so they occupy less digital storage space.

Advantages of Image Compression

  • First, the transfer of data from disk to memory is considerably slower than the speed with which the same information can be processed once it is held in memory – therefore smaller files means quicker execution times.
  • Second, the smaller the file size, the more images can be held in memory at one time.
  • Data compression allows storing and sending a smaller number of bits resulting in cost savings.

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