Compression PDF

# Compression ## Compression "Reduction of file size" ## Why do we need to compress a file? - The data files are very large. - It would take a long time to send the compressed file // compressed files will download faster - A higher bandwidth would be needed to transmit the uncompressed file...

# Compression ## Compression "Reduction of file size" ## Why do we need to compress a file? - The data files are very large. - It would take a long time to send the compressed file // compressed files will download faster - A higher bandwidth would be needed to transmit the uncompressed file ## Types Of Compression - **Lossless File Compression**: In this technique the data is not lost, and the compressed file can be decompressed as the original. - **Lossy File Compression:** In this technique the data is lost and the decompressed file is not same as the original. ## Application Of Lossy and Lossless - **Lossy:** accuracy of data is not important - Smaller file size needed => Lossy reduces file size more than Lossless - **Lossless:** accuracy of data is important ## Scenario Based Questions **Question:** Abdullah wants to compress the source code Identify the most appropriate compression technique he should use. - **Lossless** - In this technique data is not lost. - Any lost data will mean the program will not work. **Question:** A photograph is to be emailed to a friend. - **Lossy** - All the data is not required - The number of colors can be reduced without the user noticing. - Email requires a significantly smaller file size. - Takes a shorter time to transmit. - **Lossless** - A high quality image may be needed. - All of the data is needed and cannot afford to lose any data. ## How an bitmap image can be compressed? - **RLE** (Run Length Encoding) - lossless compression ## 6) A black and white bitmap image is shown. | Colour | Code | |---|---| | Black | 1A | | White | 3B | ## 3B9 1A3 3B3 1A2 3B1 1A2 3B2 1A1 3B3 1A1 3B2 1A2 3B1 1A2 3B3 1A3 3B9 ## Explaination: - Looks for runs of consecutive pixels of the same colour - Store the color value once and the number of times it occurs - Lossless method of compression - Reference to the given image (depends on the question) ## JPEG - lossy Compression - Removes the parts of image which are not noticeable to human eye - **Crop:** removes pixels => lossy compression - **RLE:** stores color code and count of repetition => lossless compression. - **JPEG:** removes part of images which are ignored by human eye => lossy compression. - **Fewer Colors:** uses fewer bits per pixel => lossy compression ## Describe lossy methods that can be used to compress The image - **Reduces Bit depth:** - reduces the number of bits per pixel which means each pixel has fewer bits. - **Reduce number of colors:** - fewer colors means fewer bits needed to store each color. - **Reduce image resolution:** - fewer pixels per unit measurement means less binary to store. ## How a vector image can be compressed? - By using .svg (Scalable vector graphic) which are defined in XML text files which allows them to be compressed ## How a sound can be compressed? - MP3 uses technology known as audio compression - Sound file can be compressed by using the perceptual music shaping algorithm ## Perceptual Music shaping removes certain sounds - (lossy Compression) - Frequencies that are outside the human hearing range (20 Hz to 20000 Hz) - If two sounds are played at the same time, only the louder one can be heard. - By the ear, so the softer sound is removed. ## Describe lossy methods that can be used to compress a sound? (Not from Marking Scheme) - **Reduce Sampling resolution:** - reduces the number of bits per sample which means each sample has fewer bits. - **Reduce Sampling rate:** - reduces the number of samples per unit time which means less bits to store. - **Perceptual Music Shaping:** - removing parts of sound which are not noticeable to human ear ## (c) When storing music tracks in a computer, the MP3 format is often used. This reduces file size by about 90%. Explain how the music quality is apparently retained. - MP3 is a lossy compressed format - uses psycho-acoustic modelling - and perceptual music shaping - removes sounds that the human ear can't hear - only keeps sounds human ear can hear better than others - discards softer sound if two sounds played together. ## How a text file can be compressed? - **RLE** - a a a a a a a a b b bb bb bbcccccc - ASCII a - 97 b - 98 c - 99 - 05 97 08 98 06 99