Compression is something that is used by everybody. Usually when you have a big file like a Word document that you need to email to someone or to backup you will compress the file first using algorithms like Zip. But compressing files does not always work.
If you take a large Word file or text file and compress it most likely you will end up with a compressed archive file that its size is half or even less of the original file. But sometimes when trying to compress files nothing happens and although you setup the compression software utility to maximum compression and you wait patiently for it to finish its work the compressed archive file is almost the same size as the original or sometimes even bigger.
To understand why this happens and to be able to figure out which files are good candidates for compression and which are not you need to understand the core concept behind compression. Compression is a process in which data is converted from one format to another where the new format is more efficient in terms of storage required to store it. In other words compression is a process that utilizes inefficiencies in the original file in order to create what is known as a compressed file format that efficiently represents the data in the original file.
For example if you have a large text file that only contains the letter A lets say a million times. The file would require million characters of storage also known as one mega byte. If you compress this file than a more efficient way to represent this file would be to create a file with a content that says This file includes million As. The result is a file that is just a few characters in size much more efficient.
Generally speaking text files like Word files or other files that include human created data like spreadsheets are inefficient and can be easily compressed. Now lets look at examples of files that can not be compressed. One example that comes to mind is a compressed file. If you create a compressed archive file and then try to compress that file again you will get almost the same file size. Why? Because the compressed archive file already represents the original data in an efficient way and there is not much that can be done to further optimize its storage.
Another examples of files that are already compressed are JPEG photos. JPEG is a special compression algorithms intended for compressing photos and images. It is also known as lossy compression since the compressed JPEG file does not exactly represent the same data as in the original photo file but almost the same data to the extent that the human eye looking at the original and the compressed photo can not tell the difference. This loose requirement not needing to exactly represent the original data allows much higher compression ratio.
If you take a JPEG photo and try to further compress it using for example the Zip algorithm the result will be an archive file with is almost the same size as the JPEG photo file or even a bit bigger. The reason again is that the JPEG photo file already represents its data in a very efficient way and there is not much that the Zip software can further do.
Generally speaking multimedia files like JPEG photos or MPEG movies can not be compressed using Zip or similar algorithms since they are already compressed.
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