Calculating the size of an image file: resolution × colour depth (answer is in bits)
Calculating the size of a sound file: sample rate (Hz) × sample resolution × length (seconds)
What is Compression?
The reduction of FILE size
Why should we compress a file?
Less bandwidth will be required
Less storage will be required on the device
Shorter transmission time (less time to download/upload)
To make it small enough to attach to an email
Difference between Lossy and Lossless Compression
Lossy compression is permanent, lossless can be recovered to its original form
Lossy compression reduces the file size more than lossless compression
When to use Lossless or Lossy Compression?
Used when file quality is more important than file size
Used when file needs to be recoverable to original form (eg. text file or source code)
They want to be able to edit the original sound file
They want the highest sound quality for the file // They want the sound to be closest to the original recording using lossy would reduce the sound quality using lossy will permanently remove some of the data // no data will be permanently removed with lossless
Used when file size is more important than quality
Considerably smaller file size than lossless, quality may be adequate
Used for streaming as transmission of smaller files is faster
**Explain why lossy compression is not suitable for compressing a text
file**
Data will be permanently removed and that could be important/necessary data
Lossy is not suitable for text files as it will damage/corrupt the file
Lossless Compression of Files
How is a text file compressed with lossless?
Compression algorithm is used such as RLE (run length encoding)
Repeating words/characters/phrases are identified and indexed with the number of occurrences with their position
How is a video file compressed with lossless?
Compression algorithm is used such as RLE (run length encoding)
Repeating frames/pixels are identified and indexed
It records the changes between frames/pixels
No data is permanently removed
How is a sound file compressed with lossless?
Compression algorithm is used such as RLE
Repeating patterns in the music are identified (or repeated sounds/notes) and indexed
No data is permanently removed
How is an image file compressed with lossless?
Compression algorithm is used such as RLE
Repeating colours are identified and indexed with the number of occurrences