Grating Scanning System
Interactive raster graphics Systems typically use several processing parts . In addition to the Central processing Unit (CPU), a video controller or display controller is used to control the operation of the display device. The simple grating system is shown in the organizational structure 2.24. Where the frame cache can be located anywhere in the system memory, the video controller refreshes the screen by accessing the frame cache. in addition to using video controllers, the more advanced raster systems use other processors as coprocessors and accelerators to perform various graphics operations.
video Controller
Figure 2.25 shows the commonly used grating system organization. The frame cache uses a fixed area of the system memory and is directly accessible by the video controller to
The location of the frame cache and the corresponding screen location using
Descartes (cartesian) coordinates. The application uses the command of the graphics package to set the coordinate position of the display object relative to the origin of the descartess coordinate system. Although we can set the origin to any convenient location in a particular system, in most cases the coordinate origin is defined in the lower-left corner of the screen. Figure 2.26 shows a two-dimensional Cartesian reference system with the origin in the lower left corner of the screen. The screen surface represents the first quadrant of the two-dimensional system, and the value of the
positive x increments to the right, and the bits of positive y increment from bottom to top. The pixel position is assigned with an integer x from 0 to the right of X (max) and integer y from the bottom of the screen to the 0 to the top of Y (max). but in hardware processing such as screen refreshes and in some software systems, the pixel location is referenced in the upper-left corner of the screen.
Figure 2.77 shows the basic refresh procedure for the video controller . The
has two registers to hold the coordinates of the screen pixels . At the beginning, set the X register to 0 for the top scan line, and the Y Register to Y (max). The value stored in the frame cache for the corresponding position of the pixel is taken out and used to set the intensity value of the CRT electron beam. Then the x register is incremented by 1, and the process repeats for the next pixel on the top scan line and repeats on each pixel along the scan line. after processing the last pixel of the top scan line, the X register resets to the O,y register minus 1, pointing to the next line of the top scan line. Then, the pixels along the scan line are processed sequentially, and the process repeats for each successive scan line. After the loop has finished processing all the pixels of the bottom scan line, The video controller resets the register to the position of the first pixel on the top line scan line, and the refresh process repeats itself.
because the screen must be refreshed at a minimum of 60 frames per second, the simple process shown in Figure 2.27 cannot use a RAM chip with too slow cycle times. To speed up pixel processing, the video controller extracts multiple pixel values from the refresh cache each time. These pixel intensities are stored in separate registers to control the intensity of the CRT electron beam for a set of neighboring pixels. When the group of pixels has been processed, the next pixel value is taken out of the frame cache.
The video controller can also perform some other operations. For multi-class applications, the video controller can remove pixel strength values from different storage areas during different refresh cycles.
For example, in high-performance systems, two frame caches are often provided, one cache is refreshed, and the other is populated with strength values. These two caches can then be swapped for roles. This approach provides a quick mechanism for generating real-time animations, because different views of the object being moved can be loaded into the refresh cache without interrupting the refresh cycle. Similarly, the video controller can complete the transformation of the pixel block.
during a refresh cycle, the screen area can be zoomed in, zoomed out, or moved from one location to another. In addition, a video controller often contains a
Lookup Table, the pixel values in the frame cache are used to access the lookup table instead of directly controlling the CRT electron beam strength. This provides a quick way to change the screen strength value
Find tables in more detail later. Finally,Some systems can be designed to allow video controllers to mix input images from TV cameras or other input devices with frame-cached images.
Raster Scan Display processor
Figure 2.28 shows a
method for establishing a grating system , which contains a separate
display processor (the processor), sometimes referred to as a
graphics Controller) or
show coprocessor (Display coprocessor). The purpose of the display processor is to free the CPU from the complex processing of graphics . In addition to the system memory, it is possible to provide a separate storage area for the display processor.
The main task of the display processor is to digitize the graphical definition given by the application into a set of pixel strength values , which are stored in the frame cache. This digital process is referred to as a
scanning conversion (scan conversion). The
Scan transformation transforms a graphical command of a given line and other geometry into a set of discrete points that correspond to the location of the screen pixel . For example, a scan transformation of a straight line segment means that the pixel position closest to the path of the straight segment must be determined, and the strength value of each position is stored in the frame cache. The scan transformation of other objects in the graphical definition alsouse a similar method. The character can be defined by a rectangular pixel lattice shown in 2.29, or by a curve outline shown in 2.30. The size of the character grid matrix can be 5x7 to 9 X12 and larger for high-quality displays. The dot-matrix display of a character is a coordinate position that attaches a rectangular lattice pattern to the specified coordinates in the frame cache. For characters defined using curve outlines, the character shape scan is converted to the frame cache by determining the pixel location closest to the contour.
The display processor can also perform some additional operations . These features include generating various linetypes (dashed, dotted, or solid), displaying colored areas, and performing certain transformations and management on the display object. The display processor generally has an interface with the mouse and other input devices.
in order to reduce the demand of storage in
grating system , the paper uses the method of organizing the frame buffer into a linked list and encoding the strength information. One
implementation is to store each line of scan lines as a set of integer pairs, one integer in each pair indicates the strength value, and the other integer sets the number of neighboring pixels with this intensity on the scan line.
This technique , called the
Stroke length encoding (run-length encoding) , can save a lot of storage space if the graph is made up of almost every single monochrome long stroke . When the pixel strength changes to linear, a similar approach can be used. Another method is to encode the raster by a block of rectangular area (
Unit Code , cell encoding).
The disadvantage of stroke coding is that the change of strength is difficult to record, and when the stroke length decreases, the storage cost increases sharply. In addition, when including many short strokes, it is difficult for the display controller to handle the grating. The size of the frame cache is no longer a major consideration due to the dramatic drop in memory costs. However, the coding method can be used in the transmission of digital memory and image information.
Computer graphics (i) video display equipment _7_ Grating scanning system
