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Incomputing, abitmap (also calledraster) graphic is an image formed from rows of different coloredpixels.[1] AGIF is an example of a graphics image file that uses a bitmap.[2]
As a noun, the term "bitmap" is very often used to refer to a particular bitmapping application: thepix-map, which refers to a map ofpixels, 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 termbitmap implies one bit per pixel, whereaspixmap is used for images with multiple bits per pixel.[3][4]
A bitmap is a type ofmemory organization orimage file format used to storedigital images. The termbitmap comes from thecomputer programming terminology, meaning just amap of bits, a spatially mappedarray of bits. Now, along withpixmap, it commonly refers to the similar concept of a spatially mapped array of pixels.Raster images in general may be referred to as bitmaps or pixmaps, whether synthetic or photographic, in files or memory.
Manygraphical user interfaces use bitmaps in their built-in graphics subsystems.[5] For example, theMicrosoft Windows andOS/2 platforms'GDI subsystem uses theWindows and OS/2 bitmap file format, usually named with thefile extension.BMP (or.DIB fordevice-independent bitmap). BesidesBMP, other file formats that store literal bitmaps includeInterLeaved Bitmap (ILBM),Portable Bitmap (PBM),X Bitmap (XBM), andWireless Application Protocol Bitmap (WBMP). Similarly, most other image file formats, such asJPEG,TIFF,PNG, and GIF, also store bitmap images (as opposed tovector graphics), but they are not usually referred to asbitmaps, since they usecompressed formats internally.
In typicaluncompressed bitmaps, imagepixels are generally stored with a variable number of bits per pixel which identify its color (thecolor depth). Pixels of 8 bits and fewer can represent eithergrayscale orindexed color. Analpha channel (fortransparency) 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 bepacked 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 leastn/8 bytes, wheren is the bit depth.
For an uncompressed, packed-within-rows bitmap, such as is stored in Microsoft DIB orBMP file format, or in uncompressedTIFF format, a lower bound on storage size for an-bit-per-pixel (2n colors) bitmap, inbytes, can be calculated as:
where width and height are given in pixels.
In the formula above, header size andcolor 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 aword, additional bytes may be needed.
Microsoft has defined a particular representation of color bitmaps of differentcolor depths, as an aid to exchanging bitmaps between devices and applications with a variety of internal representations. They called these device-independent bitmaps "DIBs", and the file format for them is called DIB file format orBMP file format. According to Microsoft support:[6]
A device-independent bitmap (DIB) is a format used to define device-independent bitmaps in variouscolor resolutions. The main purpose of DIBs is to allow bitmaps to be moved from one device to another (hence, the device-independent part of the name). A DIB is an external format, in contrast to a device-dependent bitmap, which appears in the system as a bitmap object (created by an application...). A DIB is normally transported in metafiles (usually using the StretchDIBits() function), BMP files, and the Clipboard (CF_DIB data format).
Here, "device independent" refers to the format, or storage arrangement, and should not be confused withdevice-independent color.
TheX Window System uses a similarXBM format forblack-and-white images, andXPM (pixelmap) for color images. Numerous other uncompressed bitmap file formats are in use, though most not widely.[7] For most purposes, standardized compressed bitmap files such asGIF,PNG,TIFF, andJPEG are used.Lossless compression in particular provides the same information as a bitmap in a smaller file size.[8] TIFF and JPEG have various options. JPEG is usuallylossy compression. TIFF is usually either uncompressed, or losslessLempel–Ziv–Welch compressed likeGIF. PNG usesdeflate lossless compression, anotherLempel-Ziv variant.
There are also a variety of "raw" image files, which store raw bitmaps with no other information. Such raw files are just bitmaps in files, often with no header or size information (they are distinct from photographicraw image formats, which store raw unprocessed sensor data in a structured container such asTIFF format along with extensive imagemetadata).
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).
