Photographs are often saved as JPG files.
Describe how lossy compression reduces the size of a photograph.
Mark scheme
Award up to two marks for a linked description such as:
- Some data is removed permanently (1) choosing detail that people are least likely to notice (1)
- Small changes in colour/shade are discarded (1) so fewer different values need to be stored (1)
- Similar pixels are stored as one value (1) so there are fewer values in the file (1)
- Fine detail is removed (1) so fewer bits are needed to store each area (1)
Do not accept answers that say repeated patterns are stored once / no data is lost.
Example answers and how they were marked
Full marks2/2
It permanently removes some of the pixel data (1) that the human eye is unlikely to notice, such as small colour changes (1).
Why: Linked point: data removed permanently (1) and development: detail least noticeable (1).
Some marks1/2
Some of the data in the photograph is removed permanently.
Why: States permanent removal of data (1); no linked development is given (1).
No marks0/2
It finds repeated pixels and stores them once, so no data is lost.
Why: Describes lossless compression, not lossy; no credit.