4. Raster Scan Displays
• In a raster-scan system the electron beam is swept across the screen one
row at a time from top to bottom. As the electron beam moves across each
row, the beam intensity is turned on and off to create a pattern of
illuminated spots.
• Each row is referred as a scan line.
5. Raster Scan Displays
• As the electron beam moves across each row, the beam intensity is turned
on and off to create a pattern of illuminated spots.
• Picture definition is stored in a memory area called the refresh buffer or
frame buffer.
7. Frame Buffer
• This memory area holds the set of intensity values for all the screen points.
• A frame buffer is characterized by size, x, y, and pixel depth.
• The resolution of a frame buffer is the number of pixels in the display. e.g.
1024x1024 pixels.
8. Display
• These stored intensity values are then retrieved from the
refresh buffer and used to control the intensity of the
electron beam as it moves from spot to spot across the
screen.
• They are painted on the screen one row (scan line) at a time.
9. Display
• Intensity range for pixel positions depends on the capability of the raster
system.
• In a black and white system each screen point is either on or off, so only one
bit per pixel is needed to control the intensity of screen positions. And this is
called a bitmap.
• For system with multiple bits per pixel, the frame buffer is called pixmap.
11. Refreshing and Retracing
• Refreshing: Refreshing on raster scan displays is carried out at the rate 60 to
80 frame per second.
• Horizontal retrace:The return to the left of the screen, after refreshing each
scan line.
• Vertical retrace: At the end of each frame the electron beam returns to the
top left corner of the screen to begin the next frame.
12. Raster Image Disadvantage
• Raster graphics cannot be scaled to a higher resolution without loss of
apparent quality.
• The reason is the pixels and if we try to open on bigger resolutions, the
number of pixels of raster image wont meet the display.
