Tham khảo tài liệu 'mechanical engineers reference book 12e episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Computer graphics systems 5 31 processing. A digitizing tablet is used to extract information from existing documents rather than merely to scan the whole image and so this requires a human operator. The pointer usually a cross-hair device selects the major features of the document and the coordinates of these points are transferred to the processing system where the image feature may be reconstructed. Drawing The drawing operation takes the image parameters and converts them into a set of pixels in the frame buffer which define the display. The frame buffer contents are then a map of what is seen on the output device and this is therefore a bit-map or pixel-map of the image. Converting the image parameters into pixels is not always simple. The change from a continuous function such as a straight line to a discretized version pixels on a screen can create unusual effects which are discussed in Section . Graphics processors Processing graphical data requires considerable processing power. If this processing is performed in software then the range of processing operations is large limited only by the ability of the programmer. The more computing-intensive the operation the more the throughput suffers in terms of frames processed per second. One way of alleviating the problem is to perform some processing operations using dedicated hardware. Such devices include convolvers for filtering and masking images and SIMD or MIMD devices for post-processing images. Parallel-processing techniques are used to increase the speed of these computingintensive operations. In addition to the architectures mentioned above the transputer is often used for graphics applications. Colour look-up tables palettes If a display were to offer a realistic range of colours then the information that would need to be stored would require a very large frame buffer. Fortunately not all colours need to be available at once in a given image. For example a programmer may select 64 out of 4096 .
