Encyclopedia of Computer Science and Technology

50 bitmapped imagesuch tasks as graphics tended to be slow, applications programmersoften bypassed the BIOS and dealt with devicesdirectly or created device drivers specific to a particularmodel of device. This made the life of the PC user morecomplicated because programs (particularly games) maynot work with some video cards, for example, or at leastrequired an updated device driver.While both the main BIOS and the auxiliary BIOS chipson devices such as video cards are still essential to theoperation of the PC, modern operating systems, such asMicrosoft Windows and applications written for them, generallydo not use BIOS routines and employ high performancedevice drivers instead. (By the mid-1990s BIOSesincluded built-in support for “Plug and Play,” a system forautomatically loading device drivers as needed. Thus, theBIOS is now usually of concern only if there is a hardwarefailure or incompatibility.)Further Reading“System BIOS Function and Operation.” Available online. URL:http://www.pcguide.com/ref/mbsys/bios/func.htm. AccessedApril 20, 2007.bitmapped imageA bitmap is a series of bits (within a series of bytes inmemory) in which the bits represent the pixels in an image.In a monochrome bitmap, each pixel can be represented byone bit, with a 1 indicating that the pixel is “on.” For grayscaleor color images several bits must be used to store theinformation for each pixel. The pixel value bits are usuallypreceded by a data structure that describes various characteristicsof the image.For example, in the Microsoft Windows BMP format,the file for an image begins with a BITMAPFILEHEADERthat includes a file type, size, and layout. This is followedby a BITMAPINFOHEADER that gives information aboutthe image itself (dimensions, type of compression, andcolor format). Next comes a color table that describes eachIn a monochrome bitmapped image, a 1 is used to represent a pixelthat is turned on, while the empty pixels are represented by zeroes.Color bitmaps must use many more bits per pixel to store colornumbers.color found in the image in terms of its RGB (red, green,blue) components. Finally comes the consecutive bytes representingthe bits in each line of the image, starting fromlower left and proceeding to the upper right.The actual number of bits representing each pixeldepends on the dimensions of the bitmap and the numberof colors being used. For example, if the bitmap has amaximum of 256 colors, each pixel value must use one byteto store the index that “points” to that color in the colortable. However, an alternative format stores the actual RGBvalues of each pixel in three consecutive bytes (24 bits),thus allowing for a maximum of 24 (16,777,216) colors (seecolor in computing).Shortcomings and AlternativesThe relationship between number of possible colors andamount of storage needed for the bitmap means that themore realistic the colors, the more space is needed to storean image of a given size, and generally, the more slowly thebitmap can be displayed. Various techniques have been usedto shrink the required space by taking advantage of redundantinformation resulting from the fact that most imageshave areas of the same color (see data compression).Vector graphics offer an alternative to bitmaps, particularlyfor images that can be constructed from a series of lines.Instead of storing the pixels of a complete image, vector graphicsprovides a series of vectors (directions and lengths) plusthe necessary color information. This can make for a muchsmaller image, as well as making it easy to scale the image toany size by multiplying the vectors by some constant.Further ReadingArtymiak, Jacek. Dynamic Bitmap Graphics with PHP and Gd. 2nded. Lublin, Poland: devGuide.net, 2007.“Microsoft Windows Bitmap File Format Summary.” FileFormat-Info. Available online. URL: http://www.fileformat.info/format/bmp/egff.htm. Accessed May 10, 2007.Slaybaugh, Matt. Professional Web Graphics. Boston: Course Technology,2001.bits and bytesComputer users soon become familiar with the use of bits(or more commonly bytes) as a measurement of the capacityof computer memory (RAM) and storage devices suchas disk drives. They also speak of such things as “16-bitcolor,” referring to the number of different colors that canbe specified and generated by a video display.In the digital world a bit is the smallest discernablepiece of information, representing one of two possible states(indicated by the presence or absence of something such asan electrical charge or magnetism, or by one of two voltagelevels). Bit is actually short for “binary digit,” and a bit correspondsto one digit or place in a binary (base 2) number.Thus an 8-bit value of11010101corresponds, from right to left, to (1 * 2 0 ) + (0 * 2 1 ) + (1 *2 2 ) + (0 * 2 3 ) + (1 * 2 4 ) + (0 * 2 5 ) + (1 * 2 6 ) + (1 * 2 7 ), or 213in terms of the familiar decimal system.