What Compression Means
Compression is the shrinking of the size of a file for saving disk space and transmission time. Data can be compressed by shrinking only the data content or the whole transmission unit depending on a number of factors. Compression of content involves the removal of extra space characters,insertion of a single repeat character indicating a string of repeatedcharacters, and replacing frequently occurring characters with smaller bit strings. Such compression can shrink a text document to half of its initial size. For compression, a program is used that applies a formula or algorithm to determine how to compress or decompress. It is thisalgorithm on which depends the quality of the result of compression.
PDF Files and Compression
The PDF format is designed to compress data as much as possible becausePDFs tend to have a large file size. So, by compression of PDF files weunderstand image compressing. Image compression is of two basic types:lossy and lossless. In lossy compression some information is permanentlylost, whereas in lossless compression all the information can berestored. A lossless algorithm produces no change in the content of thefile being compressed. When you decompress the file you get the exact original file that you compressed. A lossy algorithm, on the other hand,achieves a higher compression ratio by selectively discarding some ofthe data in the PDF document. PDF is a page description language; youmay call it a simplified and advanced version of a postscript. It differs from a postscript in that it has restricted functionality and islightweight. The reason for its being so is that it has a better data structure and it uses very efficient compression algorithms that canshrink the file size to about half of that of an equivalent PostScript file.
Two Most Common Algorithms
ZIP works suitable for simple images that are created with paintprograms. It is a lossless method. JPEG is a lossy compression algorithm that is designed to shrink the file size of natural, photographic-like true-color images to a great extent without producing any change in thequality of the image noticeable to the human eye.