Computer graphics (i) video display equipment _2_ grating scanning display

Raster Scan display

The normal graphics monitor using the CRT is a TV-based raster scan display (Raster-scan display). In the gratingscanning system, the electron beam transverse scan screen, one line at a time, from top to bottom. Each row is called a scan line ( scanLine ). As the electron beam moves horizontally along each line, the intensity of the electron beam changes constantly, creating a pattern of bright spots. FigureThe shape definition is stored in memory called flush cache (refresh buffer) or frame buffer, where the frame refers to the entire screen range. This memory holds a set of intensity values for all points on the screen. The electron beam moves on the screen by point from therefreshes the intensity value of the cache check to control its strength. This way, 2.7 "paint" on the screen is one line at a time. Each canA screen point lit by an electron beam is called a pixel (pixel or PEL.) is a shorthand for the picture element. Because the refresh cache is used tothe screen color value is stored, so it is also called the color buffer. In addition to color, other information about the pixel is storedin the cache, so different cache areas are sometimes collectively referred to as "frame caches." Raster scanning system has storage for every point of the screenstrength information so that it works well for realistic displays of scenes with subtle shadows and color patterns. Home TV and Printers are another example of how raster scanning is used.

Grating system is commonly called resolution of the number of pixels as its characteristics. Another feature of the video display is the aspect ratio (aspect ratio), defined as the result of the number of pixel columns the system can display divided by the number of rows (sometimes the term aspect ratio is used to represent the result of the number of scanned rows divided by the number of pixel columns). It can also be described by the ratio of the number of points required to display the same length line in horizontal and vertical directions on the screen . Therefore, the aspect ratio of 4/3 means that the horizontal line plotted with 4 points is the same as the physical length of the vertical lines plotted with 3 points. Similarly, the aspect ratio of any rectangle, including the entire screen, is described by the result of its width divided by its height.

The color or grayscale level that a raster system can display depends on the type of phosphor used by the CRT and the number of bits in the frame cache corresponding to each pixel. For a simple black system, each screen point is either light or dark, so each pixel needs only one bit to control the brightness on the screen position. the bit is an L, indicating that the electron beam is opened at that position, and a value of 0 indicates that the electron beam is closed at that position. If you want the electron beam to have more strength levels in addition to the "on" and "off" states, you need to provide additional bits. up to 24 bits per pixel in a high-performance system , the screen with a resolution of 1024x768 x 1024 uses a refresh cache of 3 MB capacity. The number of bits per pixel is also sometimes referred to as the cache depth (depth) or bit plane (bit plane). Similarly, a per-pixel frame cache is often called a bitmap (bitmap), whereas a multi-bit frame cache per pixel is called a pixel graph (pixmap).
when the refresh frequency is not too low, we will feel that the content of two adjacent frames during the refresh process is smooth transition. At 24 frames per second below , we will feel a gap between the adjacent images on the screen , that is, the image flashes . For example, early silent movies were made in everythe rate of 16 frames in seconds, so there is flicker during the show. Movie rate has increased when developing sound systems in the 1920sto 24 frames per second, thus eliminating flicker and erratic movement of the actors. The early grating computer system was set at 30 frames per refresh .resulting in better results, but because the display technology on the monitor is fundamentally different from the film, the image qualityThe improvements also depend on a higher refresh rate on the monitor. A movie projector can be shown by continuously showing a frame until the next frame begins.maintain continuity of display results. On the video monitor, however, the fluorescence point begins to decay as soon as it is lit. Therefore, most sweeps now 60-80 frames per second , some systems have a refresh rate of 120 frames per second. Some graphics systems are designed toTo use a variable refresh rate. For example, select a high refresh rate for the stereoscopic display app to display two views of the scene alternatelywill not blink. This type of application typically uses multiple frame caching methods.

Sometimes, the refresh rate is described in how many cycles per second or hertz (Hz), where one period corresponds to one frame. Therefore, we can simply refer to the refresh rate of 60 frames per second as a single Hz. At the end of each scan line, the electron beam returns to the left side of the screen and then begins to display the next scan line. After each scan line is refreshed, the electron beam returns to the left end of the screen, which is called the horizontal sweep of the electron beam (horizontal retrace). And at the end of each frame (shown in 1/80 to 1/60 of a second), the electron beam returns ( vertical
Straight back , vertical retrace) to the left corner of the screen and start showing the next frame.
In some raster scanning systems and TVs, each frame is displayed in two times with interlaced (interlaced) refresh mode. For the first time, the electron beam is scanned from top to bottom, line by row. After the vertical sweep, the electron beam then scans the other half of the scan line (see Figure 2.8). This interlaced mode makes it possible to see the entire screen as half the time it takes to scan the line in a row. Interlaced scanning technology is mainly used for slower refresh rates. For example, for an old non-interlaced display of 30 frames per second, you might notice that it has flicker. However, with interlaced scanning, each of the two scans can be completed in 1/60 seconds, that is, the refresh rate is close to 60 frames per second. This is an effective technique for avoiding flicker and providing adjacent scan lines that contain similar display information.

Computer graphics (i) video display equipment _2_ grating scanning display