Editing Raster graphics (section)

====Compression====
{{main | Image compression}}

High-resolution raster grids contain a large number of pixels, and thus consume a large amount of memory. This has led to multiple approaches to compressing the data volume into smaller files. The most common strategy is to look for patterns or trends in the pixel values, then store a parameterized form of the pattern instead of the original data. Common raster compression algorithms include [[run-length encoding]] (RLE), [[JPEG]], [[LZ77 and LZ78|LZ]] (the basis for [[Portable Network Graphics|PNG]] and [[Zip (file format)|ZIP]]), [[Lempel–Ziv–Welch]] (LZW) (the basis for [[GIF]]), and others.

For example, Run length encoding looks for repeated values in the array, and replaces them with the value and the number of times it appears. Thus, the raster above would be represented as:
{{aligned table|cols=12|col1header=y|class=wikitable|leftright=on
| values | 1| 3| 0| 1|12| 8| 0| 1| 4| 3 |...
| lengths| 1| 1| 2| 1| 1| 1| 1| 1| 1| 2 |...
}}
This technique is very efficient when there are large areas of identical values, such as a line drawing, but in a photograph where pixels are usually slightly different from their neighbors, the RLE file would be up to twice the size of the original.

Some compression algorithms, such as RLE and LZW, are ''lossless'', where the original pixel values can be perfectly regenerated from the compressed data. Other algorithms, such as JPEG, are ''lossy'', because the parameterized patterns are only an approximation of the original pixel values, so the latter can only be estimated from the compressed data.