Explain comparison of various methods of compression
Answers
Explanation:
There are two categories of compression techniques used with digital graphics, lossy and lossless. Whilst each uses different techniques to compress files, both have the same aim. To look for duplicate data in the graphic and use a much more compact data representation.
Lossy and Lossless each have various methods which are used by different file formats and achieve different results. Therefore not all lossy or lossless formats will use the same methods. It is beyond the scope of this Unit to look at these methods in detail so you will not be assessed on them. The Unit entitled Digital Imaging: Bitmaps covers compression methods in more detail. If you are a bit unclear about this, the following may help:
Lossy compression methods include DCT (Discreet Cosine Transform), Vector Quantisation and Huffman coding
Lossless compression methods include RLE (Run Length Encoding), string-table compression, LZW (Lempel Ziff Welch) and zlib.
Compression methods are otherwise known as algorithms, which are calculations that are used to compress files. Organisations that create file formats create their own algorithms and compete with each other to create the best format. You will look at this in more detail later in the 'File Format' section of this course.
Bitmaps consist of pixels that have the same values. Think of a photograph of a setting. There will be areas of pixels in the photo that all have the same value. There is no need to store every single pixel. Algorithms look for redundant data, ie repeat values that aren't needed, in the graphic and create a new representation of the data. This representation creates a smaller file size but keeps the relevant data for displaying the graphic. Essentially when you look at a compressed file you should see no or little difference in the graphic