![]() Each entry in the pixmap occupies a number of bits to represent the color of the pixel. Raster can be explained as a rectangular collection of dots or points plotted. On color systems, the frame buffer storing the values of the pixels is called a pixmap. Each entry in the bitmap is a 1-bit data which determine the ON (1) and OFF (0) of the intensity of the pixel. It is Used With Raster Scan System to display color. On black and white systems, the frame buffer storing the values of the pixels is called a bitmap. Beam Penetration method Where Used Colors Color Dependency Shadow Mask method It is used with Random Scan System to display color. Each screen point is called a pixel (picture element). This frame buffer stores the intensity values for all the screen points. Picture definitions is stored in a memory area called the frame buffer. You can call it a refresh raster or refresh scan or a raster scan it is different from the Calligraphic or Random Scan display system. The refreshing rate, called the frame rate, is normally 60 to 80 frames per second. The beam is OFF, when it is moved from the right to the left as shown by dotted line in Fig. when the beam is moved from the left to the right, it is ON. Here, the beam is swept back and forth from the left to the right across the screen. In this method, the horizontal and vertical deflection signals are generated to move the beam all over the screen in a pattern shown in the Fig. Raster scan is the most common method of displaying images on the CRT screen. Each complete scanning of a screen is normally called a frame. This scanning process is called refreshing. As it moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Raster Scan Display in this technique the electron beam is swept across the screen one row at a time from top to bottom. The refresh rate depends on the number of lines to be displayed. In this the picture can be draw in terms of line, one at a time. In vector displays beam is deflected from end point to end Point, hence this technique is also called random scan. This information is generated with the help of vector graphics generator. Picture definition is stored as a set of line. It is also called vector display, stroke-writing display, or calligraphic display. To move the beam across the CRT, the information about both, magnitude and direction is required. Random Scan (Vector Scan) In this technique, the electron beam is directed only to the part of the screen where the picture is to be drawn rather than scanning from left to right and top to bottom as in raster scan. If we want to move the beam from point A to Point B without showing a line between points, we can blank the beam as we move it. ![]() If we want a line connecting Point A with Point B on the vector graphics display, we simply drive the beam deflection circuitry, which will cause beam to go directly from Point A to B. It is also called stroke-writing display, or calligraphic display.Īs shown in Fig., vector scan CRT display directly traces out only the desired lines on CRT i.e. Vector Scan or Random Scan Display in this technique, the electron beam is directed only to the part of the screen where the picture is to be drawn rather than scanning from left to right and top to bottom as in raster scan.
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