Editing Raster graphics (section)

===Image storage===
{{see also |Bitmap}}
[[File:The use of a raster data structure to summarize a point pattern.gif|thumb|Using a raster to summarize a point pattern]]

Most computer images are stored in [[Image file formats#Raster formats|raster graphics formats]] or compressed variations, including [[GIF]], [[JPEG]], and [[Portable Network Graphics|PNG]], which are popular on the [[World Wide Web]].<ref name="MSDN_bitmapTypes" /><ref name="RasterVsVector">{{cite web |url=https://vector-conversions.com/vectorizing/raster_vs_vector.html |title=Raster vs Vector |publisher=Gomez Graphics Vector Conversions |access-date=1 January 2019 |quote=Raster images are created with pixel-based programs or captured with a camera or scanner. They are more common in general such as jpg, gif, png, and are widely used on the web. |archive-date=5 January 2019 |archive-url=https://web.archive.org/web/20190105151547/http://vector-conversions.com/vectorizing/raster_vs_vector.html |url-status=live }}</ref> A '''raster data''' structure is based on a (usually rectangular, square-based) [[tessellation]] of the 2D [[plane (geometry)|plane]] into cells, each containing a single value. To store the data in a file, the two-dimensional array must be serialized. The most common way to do this is a ''row-major'' format, in which the cells along the first (usually top) row are listed left to right, followed immediately by those of the second row, and so on. 

In the example at right, the cells of tessellation A are overlaid on the point pattern B resulting in an array C of quadrant counts representing the number of points in each cell. For purposes of visualization a [[lookup table]] has been used to color each of the cells in an image D. Here are the numbers as a serial row-major array:

1 3 0 0 1 12 8 0 1 4 3 3 0 2 0 2 1 7 4 1 5 4 2 2 0 3 1 2 2 2 2 3 0 5 1 9 3 3 3 4 5 0 8 0 2 4 3 2 8 4 3 2 2 7 2 3 2 10 1 5 2 1 3 7

To reconstruct the two-dimensional grid, the file must include a ''header'' section at the beginning that contains at least the number of columns, and the pixel datatype (especially the number of bits or bytes per value) so the reader knows where each value ends to start reading the next one. Headers may also include the number of rows, [[georeferencing]] parameters for geographic data, or other [[metadata]] tags, such as those specified in the [[Exif]] standard.

====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.

====Raster–vector conversion====
Vector images (line work) can be [[rasterisation|rasterized]] (converted into pixels), and raster images [[image tracing|vectorized]] (raster images converted into vector graphics), by software. In both cases some information is lost, although certain vectorization operations can recreate salient information, as in the case of [[optical character recognition]].