Bitmap
Encyclopedia
In computer graphics
, a bitmap or pixmap is a type of 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 array of bits. Now, along with pixmap, it commonly refers to the similar concept of a spatially mapped array of pixel
s. Raster
images in general may be referred to as bitmaps or pixmaps, whether synthetic or photographic, in files or memory.
In certain contexts, the term bitmap implies one bit per pixel, while pixmap is used for images with multiple bits per pixel.
Many graphical user interface
s use bitmaps in their built-in graphics subsystems; for example, the Microsoft Windows
and OS/2
platforms' GDI
subsystem, where the specific format used is the Windows and OS/2 bitmap file format, usually named with the file extension of
, Portable Bitmap (PBM)
, X Bitmap (XBM), and Wireless Application Protocol Bitmap (WBMP)
. Similarly, most other image file formats, such as JPEG
, TIFF, PNG, and GIF
, also store bitmap images (as opposed to vector graphics
), but they are not usually referred to as bitmaps, since they use compressed
formats internally.
bitmaps, image pixel
s are generally stored with a color depth
of 1, 4, 8, 16, 24, 32, 48, or 64 bits per pixel. Pixels of 8 bits and fewer can represent either grayscale
or indexed color
. An alpha channel (for transparency
) may be stored in a separate bitmap, where it is similar to a greyscale 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:
In the formula above, header size and 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
, additional bytes may be needed.
Here, "device independent" refers to the format, or storage arrangement, and should not be confused with device-independent color
.
uses a similar XBM format for black-and-white
images, and XPM (pixelmap) for color
images. Numerous other uncompressed bitmap file formats are in use, though most not widely. Much more common are the standardized compressed bitmap files such as GIF
, PNG, TIFF, and JPEG
. TIFF and JPEG have various options. JPEG is usually lossy compression. TIFF is usually either uncompressed, or lossless Lempel-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; such raw files are just bitmaps in files, often with no header or size information, and should not be confused with photographic raw image format
s, which store raw unprocessed sensor data in a structured container such as TIFF format along with extensive image metadata
.
Computer graphics
Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer with help from specialized software and hardware....
, a bitmap or pixmap is a type of memory
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
organization or image file format used to store digital image
Digital image
A digital image is a numeric representation of a two-dimensional image. Depending on whether or not the image resolution is fixed, it may be of vector or raster type...
s. The term bitmap comes from the computer programming
Computer programming
Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to the similar concept of a spatially mapped array of pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s. Raster
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
images in general may be referred to as bitmaps or pixmaps, whether synthetic or photographic, in files or memory.
In certain contexts, the term bitmap implies one bit per pixel, while pixmap is used for images with multiple bits per pixel.
Many graphical user interface
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
s use bitmaps in their built-in graphics subsystems; for example, the Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
and OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
platforms' GDI
Graphics Device Interface
The Graphics Device Interface is a Microsoft Windows application programming interface and core operating system component responsible for representing graphical objects and transmitting them to output devices such as monitors and printers....
subsystem, where the specific format used is the Windows and OS/2 bitmap file format, usually named with the file extension of
.BMP (or .DIB for device-independent bitmap). Besides BMP, other file formats that store literal bitmaps include InterLeaved Bitmap (ILBM)ILBM
ILBM is a subtype of the Interchange File Format used for storing picture data. ILBM stands for InterLeaved BitMap which refers to the way the pictures are stored. The image data is stored as a varying number of bitplanes, each storing one bit of data for each pixel in the image...
, Portable Bitmap (PBM)
Portable pixmap
The phrase Netpbm format commonly refers to any or all of the members of a set of closely related graphics formats used and defined by the Netpbm project....
, X Bitmap (XBM), and Wireless Application Protocol Bitmap (WBMP)
Wireless Application Protocol Bitmap Format
Wireless Application Protocol Bitmap Format is a monochrome graphics file format optimized for mobile computing devices....
. Similarly, most other image file formats, such as JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
, TIFF, PNG, and GIF
GIF
The Graphics Interchange Format is a bitmap image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability....
, also store bitmap images (as opposed to vector graphics
Vector graphics
Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygon, which are all based on mathematical expressions, to represent images in computer graphics...
), but they are not usually referred to as bitmaps, since they use compressed
Image compression
The objective of image compression is to reduce irrelevance and redundancy of the image data in order to be able to store or transmit data in an efficient form.- Lossy and lossless compression :...
formats internally.
Pixel storage
In typical uncompressedImage compression
The objective of image compression is to reduce irrelevance and redundancy of the image data in order to be able to store or transmit data in an efficient form.- Lossy and lossless compression :...
bitmaps, image pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s are generally stored with a color depth
Color depth
In computer graphics, color depth or bit depth is the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel , particularly when specified along with the number of bits used...
of 1, 4, 8, 16, 24, 32, 48, or 64 bits per pixel. Pixels of 8 bits and fewer can represent either grayscale
Grayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
or indexed color
Indexed color
In computing, indexed color is a technique to manage digital images' colors in a limited fashion, in order to save computer memory and file storage, while speeding up display refresh and file transfers...
. An alpha channel (for transparency
Transparency (graphic)
Transparency is possible in a number of graphics file formats. The term transparency is used in various ways by different people, but at its simplest there is "full transparency" i.e. something that is completely invisible. Of course, only part of a graphic should be fully transparent, or there...
) may be stored in a separate bitmap, where it is similar to a greyscale 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
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s, can be calculated as:
- size = width • height • n/8, where height and width are given in pixels.
In the formula above, header size and 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
Word
In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...
, additional bytes may be needed.
Device-independent bitmaps and BMP file format
Microsoft has defined a particular representation of color bitmaps of different color depths, as an aid to exchanging bitmaps between devices and applications with a variety of internal representations. They called these device-independent bitmaps or DIBs, and the file format for them is called DIB file format or BMP file format. According to Microsoft support:
A device-independent bitmap (DIB) is a format used to define device-independent bitmaps in various color 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 with device-independent color
Color management
In digital imaging systems, color management is the controlled conversion between the color representations of various devices, such as image scanners, digital cameras, monitors, TV screens, film printers, computer printers, offset presses, and corresponding media.The primary goal of color...
.
Other bitmap file formats
The X Window SystemX Window System
The X window system is a computer software system and network protocol that provides a basis for graphical user interfaces and rich input device capability for networked computers...
uses a similar XBM format for black-and-white
Black-and-white
Black-and-white, often abbreviated B/W or B&W, is a term referring to a number of monochrome forms in visual arts.Black-and-white as a description is also something of a misnomer, for in addition to black and white, most of these media included varying shades of gray...
images, and XPM (pixelmap) for color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
images. Numerous other uncompressed bitmap file formats are in use, though most not widely. Much more common are the standardized compressed bitmap files such as GIF
GIF
The Graphics Interchange Format is a bitmap image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability....
, PNG, TIFF, and JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
. TIFF and JPEG have various options. JPEG is usually lossy compression. TIFF is usually either uncompressed, or lossless Lempel-Ziv-Welch compressed like GIF
GIF
The Graphics Interchange Format is a bitmap image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability....
. PNG uses deflate
DEFLATE
Deflate is a lossless data compression algorithm that uses a combination of the LZ77 algorithm and Huffman coding. It was originally defined by Phil Katz for version 2 of his PKZIP archiving tool and was later specified in RFC 1951....
lossless compression, another Lempel-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, and should not be confused with photographic raw image format
RAW image format
A camera raw image file contains minimally processed data from the image sensor of either a digital camera, image scanner, or motion picture film scanner. Raw files are so named because they are not yet processed and therefore are not ready to be printed or edited with a bitmap graphics editor...
s, which store raw unprocessed sensor data in a structured container such as TIFF format along with extensive image metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
.