![]() ![]() To move the beam across the CRT, the information about both magnitude and direction is required.If we do not want to display line from p1 to p2 and just move then we can blank the beam as we move it.If we want line between point p1 & p2 then we directly drive the beam deflection circuitry which focus beam directly from point p1 to p2.Random scan display directly traces out only the desired lines on CRT.Random scan also is known as a vector display.Each pixel can be specified by its row and column Numbers.Each screen point referred as pixel or Pel (Picture Element). ![]() That intensity is retrieved from frame buffer and display on screen one row at a time.Frame buffer holds set of intensity values for all the screen points.In raster scan displays a special area of memory is dedicated to graphics This memory is called Frame Buffer.This process is known as Refreshing of Screen. The screen image is maintained by repeatedly scanning the same image.It is made OFF and rapidly retraced back to the top left to start again and process of moving back to top is known as Vertical Retrace. When beam is reach at the bottom of the screen.When beam is moved from right to left it is OFF and process of moving beam from right to left after completion of row is known as Horizontal Retrace.When beam is moved from left to right it is ON.Here beam is swept back & forth from left to the right.In this method the horizontal and vertical deflection signals are generated to move the beam all over the screen in a pattern shown in Image.It will scan one line at a time from top to bottom & then back to the top.The video controller reads this refresh buffer and produces the actual image on screen.The display image is stored in the form of 1’s and 0’s in the refresh buffer.It consists of a display controller, CPU, video controller, refreshes buffer, keyboard, mouse, and CRT. Raster Scan Architecture of a raster display Random Scan system High Quality Images In SVG,the Size is Rescaleable like Logo in sites Raster Scan Mainly in photos like JPG,PNG,GIF File Format It is used for photos.That is Why photoshop is Raster editing program. Refreshing in Random scan display to draw all the components of lines of pictures 30 to 60 frames per second. Refreshing on Raster scan display is carryout at the Rate of 60 to 80 frames per seconds. Vector display only draws lines and characters. Raster display has ability to display areas,filled with solid colors or patterns. Mathematical functions are used to draw an image. Screen points/pixels are used to draw an image. These systems are designed for line- drawing and can’t display realistic shaded scenes. The capability of this system to store intensity values for pixel makes it well suited for the realistic display of scenes contain shadow and color pattern. Picture definition is stored as a set of line drawing instructions in a display file. Picture definition is stored as a set of intensity values for all screen points, called pixels in a refresh buffer area. Its resolution is good because this system produces smooth lines drawings because CRT beam directly follows the line path. Its resolution is poor because raster system in contrast produces zigzag lines that are plotted as discrete point sets. The electron beam is directed only to the parts of screen where a picture is to be drawn. The electron beam is swept across the screen, one row at a time, from top to bottom. Difference Between Raster Scan and Random Scan Comparison Chart Raster Scan ![]()
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