Compression - Run Length Encoding (AQA GCSE Computer Science)
Revision Note
Written by: Robert Hampton
Reviewed by: James Woodhouse
Run Length Encoding
What is run-length encoding?
Run-length encoding (RLE) is a form of data compression that condenses identical elements into a single value with a count
For a text file, "AAAABBBCCDAA" is compressed to "4A3B2C1D2A"
The string has four 'A's, followed by three 'B's, two 'C's, one 'D', and two 'A's
RLE is used in bitmap images to compress sequences of the same colour
For example, a line in an image with 5 red pixels followed by 3 blue pixels could be represented as "5R3B"
Represent Data in Frequency / Data Pairs
How do you represent data in frequency/data pairs?
Run-length encoding (RLE) uses frequency/data pairs to compress bitmap image data
For example, the following bitmap image with a colour depth of 1 bit would have the following binary bit pattern
Bitmap | Bit pattern |
---|---|
Using RLE we group pixel colours and can create frequency/data pairs as follows
30, 11, 20, 11, 20, 11, 50, 11, 30, 11, 10, 11, 60, 11......
3 x 0 = 3 x white, 1 x 1 = 1 x black
Data pairs can carry over on to the next line, e.g. end of first line and start of second line is 5 x 0 (5 x white)
How do you represent a bitmap image from frequency/data pairs?
To recreate a simple bitmap from frequency/data pairs you need to know what binary code is assigned to what colour and reverse the process above
If we assume 0 = white and 1 = black and have the following frequency/data pairs and the image size is 5x3 pixels
20, 11, 30, 31, 10, 51
The bitmap image would be
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