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# Raster-Scan System In Computer Graphics

Interactive raster graphics systems typically employ several processing units. In addition to the Central Processing Unit (CPU), a special-purpose processor, called the video controller or display controller, is used to control the operation of the display device.

The raster scan system is well-suited for the realistic display of scenes containing shading and color patterns.

A raster-scan graphics system performs the following sequence of operations (figure 1.11):

### Architecture & Working of Raster Scan System

The raster scan systems are based on the use of a Cartesian coordinate system. Each point is represented by x and y coordinates, where the value of x-increases from left to right and y from top to bottom.

The Raster Scan System is also called Point Plotting Technique. In the Raster Scan System at the end of each scan line, the electron beam returns to the left side of the screen to begin displayers the next line, this return to the left of the screen after refreshing each scan line is called horizontal retrace and at the end of each frame the electron beam returns to the top corner of the screen to begin the next frame, this is called vertical retrace. Track of a refresh cycle. Suppose refresh rate is required as 60HZ. One cycle is completed in 1/60 sec.

Normally, refreshing on raster scan displays is carried out at the rate of 60 to 75 frames per second, although some systems are designed for higher refresh rates. Sometimes, refresh rates are described in units of cycles per second, or Hertz (Hz), where a cycle corresponds to one frame. Using these units, we would describe a refresh rate of 60 frames per second as simply 60 Hz.

In a simple black and white system, the cache screen point is either on or off, so only one bit per pixel is needed to control the intensity of screen positions. For a bit-level system, a bit value of 1 indicates that the electron beam is to be turned on at that position, and a value of 0 indicates that the beam intensity is to be off. Additional bits are needed when color and intensity variations can be displayed. Up to 24 bits per pixel are included in high-quality systems, which can require several megabytes of storage for the frame buffer, depending on the resolution of the system. A system with 24 bits per pixel and a screen resolution of 1024 by 1024 requires (1024 x 1024) x 24)/8 = 3 MB storage for the frame buffer.

On a black and white system with one bit per pixel, the frame buffer is commonly called a bitmap. For systems with multiple bits per pixel, the frame buffer is often referred to as a pixmap.

Refreshing on raster-scan displays is carried out at the rate of 60 to 80 frames per second, although some systems are designed for higher refresh rates. Sometimes, refresh rates are described in units of cycles per second, or Hertz (Hz), where a cycle corresponds to one frame.

### Raster-Scan Display Processor

The architecture of raster Scan with display processor is shown in figure 1.12. The major task of a display processor is digitizing a picture definition given in an application program into a set of pixel values for storage in the frame buffer.

A display processor for displaying data in one or more windows on a display screen. The display processor divides a display screen into a plurality of horizontal strips with each strip further subdivided into a plurality of tiles. Each tile represents a portion of a window to be displayed on the screen. Each tile is a tile defined by tile descriptors which include memory address locations of data to be displayed in that tile.

The descriptors need only be changed when the arrangement of the windows on the screen is changed or when the mapping of any of the windows into the bit-map is changed.

The display processor of the present invention does not require a bit map frame buffer to be utilized before displaying windowed data on a screen. Each horizontal strip may be as thin as 1 pixel, which allows for the formation of windows of irregular shapes, such as circles.

Display Video Monitor Frame Processor Buffer Controller Memory System Memory Display CPU Processor System Bus I/O Device Figure 1.12: Architecture of Raster Scan System with Display Processor

#### Interlacing

On some raster scan systems, each frame is displayed in two passes using an interlaced refresh procedure. In the first pass, the beam sweeps (odd scan lines) across every other scan line from top to bottom. Then after the vertical retrace, the beam sweeps out the remaining (even scan lines).

The interlacing of the scan lines in this way allows us to see the entire screen displayed in one-half the time it would have taken to sweep across all the scan lines at once from top to bottom. Interlacing is primarily used with slower refreshing rates.

Suppose we have a refresh rate 30 Hz for a non-interlaced display, some flicker is noticeable. But with interlacing, each of the two passes can be accomplished 1/60 of a second, which brings the refresh rate nearer to 60 frames per second as illustrated in figures 1.13(a) and 1.13(b)