Raster scan display
- 1. introduction
- 2. Raster scan display is the most common type of display used in CRT televisions and monitors. In a raster-scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the beam is swept across the row, the beam intensity is turned on and off to create a pattern of illuminated spots.
- 3. The picture definition that is to be drawn is stored in a memory area called the refresh buffer or frame buffer. The refresh buffer holds the set of intensity values for all the screen points. Stored intensity values are the retrieved from the refresh buffer and “painted” on the screen one row at a time .
- 4. In a simple black & white raster scan display, the intensity value is either is only 1 bit per pixel as it’s either on or off. But in color raster scan display up to 24 bits per pixel are included in high-quality systems. In a black and white raster scan display system with 1 bit per pixel the frame buffer is called “BITMAP”. Whereas is color system/multiple bit per pixel system the frame buffer is called “PIXMAP”
- 5. Refresh rate of raster scan display scan can be 60 to 80 frames per second. At the end of each scan line, the electron beam returns to the left side of the screen to begin displaying the next scan line which is known as “horizontal retrace” At the end of each frame the electron beam returns to the top left corner of the screen to display new frame. This process is called “ vertical retrace”.
- 6. We can display real life images which have different shades. The color range available is way bigger than random scan display.
- 7. The resolution is comparatively lower than random scan display. High memory is required. Data about intensities of all pixel over the screen has to be stored rather than only the area where picture is displayed.
- 8. In random scan display the electron beam is directed to the area on the screen only where the picture is to be drawn. Random scan display are also known as vector display or stroke-writing or calligraphic display. The reason its also called vector display is because random scan display draws the picture one line at a time.
- 9. Refresh rate on a random-scan system depends on the number of lines to be displayed. The picture definition is stored as a set of line drawing commands in an area of memory called the refresh display file. The refresh display file is also called the display list or simply the refresh buffer.
- 10. To display a specified picture, the system cycles through the set of commands in the display file, drawing each component line in turn. After all line-drawing commands have been processed, the system cycles back to the first line command in the list. The refresh rate of random scan display is 30 to 60 frames per second (fps)
- 11. For example if we want to display and maintain a picture of a triangle ABC on the screen then the display list will have commands like this: Draw a line from A to B. Draw a line from B to C. Draw a line from C to A.
- 12. Random scan display have higher resolution compared to raster scan display as the picture is stored as line drawing instructions rather than intensity values. It produces smooth line drawing. It also requires less memory as we storing information about only the picture to be drawn rather than information about full screen.
- 13. We cannot draw realistic images like with different shades There is limitation to the colors that can be drawn for example when we draw images with beam penetration display its actually random scan display and we can draw only maximum 4 colors which are based on how deep the beam penetrates the phosphor layer.