Bitmap: Difference between revisions

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{{refimprove|date=April 2024}}

{{Other uses}}

{{Short description|Computing term}}

{{Use American English|date=September 2022}}

In [[computing]], a '''bitmap''' (also called '''raster''') graphic is an image formed from rows of different colored [[pixel]]s.<ref>{{cite web|url=https://kb.iu.edu/d/afmr|title=ARCHIVED: What are bitmap and vector graphics, and how are they different|publisher=University Information Technology Servivces|language=en-US|url-status=live|date=September 22, 2023|access-date=April 21, 2024|archive-date=April 21, 2024|archive-url=https://archive.today/20240421071909/https://kb.iu.edu/d/afmr}}</ref> A [[GIF]] is an example of a graphics image file that uses a bitmap.<ref>{{cite encyclopedia|url=https://www.britannica.com/technology/bitmap|title=bitmap|encyclopedia=[[Encyclopædia Britannica]]|author=Gregersen, Erik|language=en-GB|url-status=live|date=January 26, 2022|access-date=April 21, 2024|archive-date=April 21, 2024 |archive-url=https://archive.today/20240421072042/https://www.britannica.com/technology/bitmap}}</ref>

In [[computing]], a '''bitmap''' is a mapping from some ___domain (for example, a range of integers) to [[bit]]s. It is also called a [[bit array]] or [[bitmap index]].

As a noun, the term "bitmap" is very often used to refer to a particular bitmapping application: the '''pix-map''', which refers to a map of [[pixel]]s, where each pixel may store more than two colors, thus using more than one bit per pixel. In such a case, the ___domain in question is the array of pixels which constitute a digital graphic output device (a screen or monitor). In some contexts, the term ''bitmap'' implies one bit per pixel, whereas ''pixmap'' is used for images with multiple bits per pixel.<ref>{{cite book |author=James D. Foley |url=https://books.google.com/books?id=A4k29b0BdVMC&q=bitmap+pix+map+%22short+for+pixel+map%22&pg=PA13 |title=Computer Graphics: Principles and Practice |publisher=Addison-Wesley Professional |year=1995 |isbn=0-201-84840-6 |pages=13 |quote=The term bitmap, strictly speaking, applies only to 1-bit-per-pixel bilevel systems; for multiple-bit-per-pixel systems, we use the more general term pix-map (short for pixel map).}}</ref><ref>{{cite book |title= Comprehensive Computer Graphics: Including C++ |author= V.K. Pachghare |publisher= Laxmi Publications |pages= 93 |year= 2005 |isbn= 81-7008-185-8 |url= https://books.google.com/books?id=xIKK9RcSTR4C&q=bitmap+pixmap+one-bit+date:2004-2007&pg=PA93}}</ref>

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A bitmap is a type of [[computer storage|memory]] organization or [[image file format]] used to store [[digital image]]s. The term ''bitmap'' comes from the [[computer programming]] terminology, meaning just a ''map of bits'', a spatially mapped [[bit array|array of bits]]. Now, along with ''pixmap'', it commonly refers to the similar concept of a spatially mapped array of pixels. [[Raster graphics|Raster]] images in general may be referred to as bitmaps or pixmaps, whether synthetic or photographic, in files or memory.

Many [[graphical user interface]]s use bitmaps in their built-in graphics subsystems;.<ref>{{cite book |title= Cross-Platform GUI Programming with Wxwidgets |author1=Julian Smart |author2=Stefan Csomor |author3=Kevin Hock |name-list-style=amp |url= https://books.google.com/books?id=CyMsvtgnq0QC&q=bitmap+pixmap+gui&pg=PA265 |publisher= Prentice Hall |year= 2006 |isbn= 0-13-147381-6}}</ref> forFor example, the [[Microsoft Windows]] and [[OS/2]] platforms' [[Graphics Device Interface|GDI]] subsystem, where the specific format used isuses the ''Windows and OS/2 bitmap file format'', usually named with the [[file extension]] of <code>.BMP</code> (or <code>.DIB</code> for ''device-independent bitmap''). Besides [[BMP file format|BMP]], other file formats that store literal bitmaps include [[ILBM|InterLeaved Bitmap (ILBM)]], [[Portable pixmap|Portable Bitmap (PBM)]], [[X BitMap|X Bitmap (XBM)]], and [[Wireless Application Protocol Bitmap Format|Wireless Application Protocol Bitmap (WBMP)]]<!-- please keep this list short; detailed list should be below -->. Similarly, most other image file formats, such as [[JPEG]], [[TIFF]], [[Portable Network Graphics|PNG]], and [[GIF]], also store bitmap images (as opposed to [[vector graphics]]), but they are not usually referred to as ''bitmaps'', since they use [[Image compression|compressed]] formats internally.

==Pixel storage==

In typical [[image compression|uncompressed]] bitmaps, image [[pixel]]s are generally stored with a variable number of bits per pixel which identify its color, (the [[color depth]]). Pixels of 8 bits and fewer can represent either [[grayscale]] or [[indexed color]]. An [[alpha channel]] (for [[transparency (graphic)|transparency]]) may be stored in a separate bitmap, where it is similar to a grayscale bitmap, or in a fourth channel that, for example, converts 24-bit images to 32 bits per pixel.

The bits representing the bitmap pixels may be [[packed]] or unpacked (spaced out to byte or word boundaries), depending on the format or device requirements. Depending on the color depth, a pixel in the picture will occupy at least ''n''/8 bytes, where {{Var|n}} is the bit depth.

For an uncompressed, packed -within -rows, bitmap, such as is stored in Microsoft DIB or [[BMP file format]], or in uncompressed [[TIFF]] format, a lower bound on storage size for a ''n''-bit-per-pixel (2ⁿ colors) bitmap, in [[byte]]s, can be calculated as:

:<math>\text{size} = \text{width} \cdot \text{height} \cdot n/8</math>

where width and height are given in pixels.

In the formula above, header size and [[Palette (computing)|color palette]] size, if any, are not included. Due to effects of row padding to align each row start to a storage unit boundary, such as a [[word (computer architecture)|word]], additional bytes may be needed.

==Device-independent bitmaps and BMP file format==

{{Main|BMP file format}}

[[Microsoft]] has defined a particular representation of color bitmaps of different [[color depth]]s, as an aid to exchanging bitmaps between devices and applications with a variety of internal representations. They called these device-independent bitmaps as "DIBs", and the file format for them is called DIB file format or [[BMP file format]]. According to Microsoft support:<ref name=DIBhelp>{{cite web |url= http://support.microsoft.com/kb/q81498/ |title= DIBs and Their Uses |work= Microsoft Help and Support |date= 2005-02-11}}</ref>

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The [[X Window System]] uses a similar [[X Bitmap|XBM]] format for [[black-and-white]] images, and [[X PixMap|XPM]] (''pixelmap'') for color images. Numerous other uncompressed bitmap file formats are in use, though most not widely.<ref>{{cite web |url= http://www.file-extensions.org/filetype/extensions/name/Bitmap+image/

|title= List of bitmap file types |work= File-Extensions.org}}</ref> For most purposes, standardized compressed bitmap files such as [[GIF]], [[Portable Network Graphics|PNG]], [[TIFF]], and [[JPEG]] are used;. [[losslessLossless compression]] in particular provides the same information as a bitmap in a smaller file size.<ref>{{cite book |title= Communicating Science Effectively: a practical handbook for integrating visual elements |author1=J. Thomas |author2=A. Jones |publisher= IWA Publishing |year= 2006 |isbn= 1-84339-125-2 |url= https://books.google.com/books?id=xrgkojGgwDYC&q=gif+png+tiff+jpeg+common&pg=PA26}}</ref> TIFF and JPEG have various options. JPEG is usually [[lossy compression]]. TIFF is usually either uncompressed, or lossless [[Lempel-Ziv-WelchLempel–Ziv–Welch]] compressed like [[GIF]]. PNG uses [[deflate]] lossless compression, another [[Lempel-Ziv]] variant.

There are also a variety of "raw" image files, which store raw bitmaps with no other information;. suchSuch raw files are just bitmaps in files, often with no header or size information (they are distinct from photographic [[raw image format]]s, which store raw unprocessed sensor data in a structured container such as [[TIFF]] format along with extensive image [[metadata]]).

==See also==