Bitmap: Difference between revisions

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{{Short description|Data structure for mapping from some ___domain (for example, a range of integers) to bits}}

{{Other uses}}

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 one 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, while ''pixmap'' is used for images with multiple bits per pixel.<ref>{{cite book |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). |title= Computer Graphics: Principles and Practice |author= James D. Foley |pages= 13 |publisher= Addison-Wesley Professional |year= 1995 |isbn= 0-201-84840-6 |url= https://books.google.com/books?id=A4k29b0BdVMC&q=bitmap+pix map+%22short+for+pixel+map%22&pg=PA13}}</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> for example, the [[Microsoft Windows]] and [[OS/2]] platforms' [[Graphics Device Interface|GDI]] subsystem, where the specific format used is 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 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==