Majority of the image compression techniques are usually associated with type of image. Enterprises or individuals who are interested to share high resolution documents in public domain require an effective lossy compression technique that can provide high rate of compression. Normally such documents are viewed in a web browser. To support display of such high resolution document in a web browser, the compression technique used should have fast decoding rate. DjVu is an option for such compression requirement.
In order to read DjVu files in a web browser, a decoder is implemented in the form of plug-in that is compact-able with all available standard web browsers. It is pretty much easy to zoom and pan a file with image with the help of this decoder. A user can see the initial version of a file very quickly and the quality of the file gradually improves when more bits arrives, Because of this, the DjVu compression technique is considered to be progressive in nature.
The DjVu techniques make use of the fact that various elements of a scanned file, such as text, images, and drawings, possess different perceptual characteristics. Digitized text and line drawings can be coded at high spatial resolution and little color resolution whereas high color resolution and at lower spatial resolution is required for images and backgrounds. Text usually requires one color but images and drawings requires more than one colour.
DjVu Compression Rate :
DjVu offers a compression factor up to 1000. This level of compression is 5 to 10 times superior compared to similar compression techniques available. For example, full color pages scanned at 300 dpi can be compressed from 25 Mb to 30-60Kb without compromising quality. The color documents having image and text compressed using DjVu method are mostly 5 to 10 times smaller when compared to JPEG files of similar quality and black-and-white documents compressed using DjVu method are mostly 10–20 times smaller when compared to JPEG files and five times smaller when compared to GIF files. Similarly files compressed using DjVu technique is about 5 times smaller compared to PDF files.
DjVu Components :
In DjVu compression technique, the document is decomposed into three components and they are mask, foreground, and background. The pixels that represent the image and the paper background are categorized as background component. The text and the lines in bi-level form are considered as mask component. The three components are decoded by the decoder. After that, the resolution of the background and foreground components is increased back to 300 dpi, and this will generate final decompressed image pixel based on the mask. A mask pixel value 0 implies the respective image pixel is decoded from the background. Similarly a mask pixel value of 1 implies the respective image pixel is created in the color of the foreground pixel.
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