Compression

Definition

Data compression is decreasing the size of a file. There are many different compression techniques.

Uses of compression

Compression is used to reduce the storage space that is required to store a file. But since disk space is becoming less and less of a problem, compression's new main use is sending stuff through the internet. The reduced file size helps to reduce the amount of buffering that is required whilst retaining the quality.

Lossy Compression

Lossy compression is a compression technique that decreases file size by discarding bits of unnecessary data. This means that the original file cannot be recreated. Lossy compression will create a new image which is similar to the original, but has a reduced quality. Another example of lossy compression is used to reduce the file size of a sound file by reducing the bitrate used in the original.

All of the above use lossy methods of compression to save data and space. This isn't the best method to use as it gets rid of some of the data, so this would be a unsuitable method to use if the original file needs to be used. Lossy compression cannot be used on binary or text files because all data is need to convey the correct meaning. People who would need the original file would be: Photographers, Audio Producers and Printing Firms. These could produce lossy compressed images for sample purposes or as draft prints.

Lossy compression is also ideal for online pages, as it allows faster load times due to a lower amount of data being loaded.

Lossy Methods

Some methods used are to delete sounds which are not heard, either because of the frequency or if another sound will drown it out. Images could be compressed more in the background than the foreground, the focus is the foreground so compression will not be noticed in the background. Also images could merge together adjacent colours just like the human eye. A pixel of black next to a pixel of white will actually be seen as two grey pixels.

Lossy compression formats:

• JPEG
• MPEG-1
• MP3

JPEG

JPEG stands for "Joint Photographic Experts Group". The JPEG format uses an algorithm in order to remove details that will not be seen by the human eye. It also reduces the quality of the background of the image since the main focus will be on the foreground or the main focus of the image. This helps reduce the file size whilst not reducing the main focus of the image.

MP3

MP3 stands for MPEG-2 audio layer 3. It uses different techniques such as removing inaudible frequencies and removing sounds that will be drowned out by louder noises. Bitrate is the number of bits a second that are encoded by MP3 per second. A higher bitrate will lead to a better quality sound at the cost of a larger file whilst a low bitrate will reduce the file size at the expense of sound quality. A variable bitrate is available in which the bitrate adapts to the sound in different parts of the recording.

Lossless Compression

Lossless compression is a compression technique that decreases file size while keeping all of the data. This means there is no loss in quality, and the original file can be recreated exactly as it was prior to compression.

This is possible because of repeating patterns in the data.

Examples of lossless compression methods include Run Length Encoding and Dictionary Based Methods. Run Length Encoding replaces repeating pixels or codes. Dictionary Based Methods rely on patterns within a file and are more effective with larger files. Each pattern can has an ID number.

Lossless compression formats:

• FLAC (Audio)
• WAV (Audio)
• PNG (Image)
• BMP (Image)

(Note that there are not many types of lossless video compression, the closest being H.264, as the file size tends to be smaller to start with)

Run Length Encoding

This method of lossless compression counts the bits that are repeated consecutively. For example, if a picture contained 3 red pixels one after the other, rather than storing each pixel individually, the file would instead store the pixel colour and the amount of times it is repeated.

However, if a file does not contain many repetitions then this method of compression can actually increase file size, as a single pixel would be stored as its colour and then the information that it is repeated only once.

Dictionary Based Methods

This is used when there are lots of repeating patterns of data.

For example, when writing a document about Computer Science, key words like "Computer Science" would be repeated throughout the document. Instead of storing the bit pattern for the word over and over, it stores the phrase in a dictionary with a reference number and stores the number in place of the phrase. ?This means that whenever the phrase is needed, it calls up the dictionary and replaces that number with the phrase.

A disadvantage of this method is that additional data is needed to store the dictionary as well as the file.

Difference between lossy and lossless compression

The main difference between lossy and lossless compression is the fact that when compressed, lossy loses some of the original quality*, whilst lossless retains all of the initial quality, hence the names "lossy" and "lossless".

Although sometimes lossy compression only removes the information that is not needed, for example it may remove some of the frequencies that cannot be heard by humans, so in this sense the reduced quality may not be detected.

The only other main difference is that, due to compression methods, lossy tends to be of a lesser file size when compressed, as some duplicate or unnecessary information tends to be left out, although this can also depend, as the original file might not have any duplicate or unnecessary information, in which case, the file size would be the same.

Revision Questions

1. Which of the following uses lossy compression

	.JPEG
→	Correct
	.PNG
→	PNG uses lossless compression
	.TIFF
→	TIFF uses lossless compression
	.ZIP
→	ZIP Uses lossless compression

2. Which of the following is the correct definition for lossy compression

	Lossy file compression does not discard parts of a file, But it does reduce file size by merging colours and pixels.
→	False
	Lossy file compression simply stores instructions on how to reproduce the image. So instead of storing pixels and colours, Instructions are actually stored, Which reduces file size.
→	False
	Lossy file compression discards parts of a file that aren't that important to save space, It also merges colours and pixels which lowers colour depth in order to save more space. All in all, resolution is reduced and space is saved.
→	Correct
	Lossy file compression
→	False

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