Random Scan Device in Computer Graphics

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In the previous chapter, we covered the concept of raster scan displays as well as the basics of progressive and interlaced scanning. There is another method used for rendering images on screens; it is called Random Scan.

Random Scan Devices are used primarily for drawing vector-based images like lines, curves, and polygons. Unlike raster scan devices that display images by scanning pixels in a fixed order, random scan devices, directly draw lines and shapes based on the instructions they receive. Read this chapter to understand the basics of random scan devices and how they work.

Random Scan Device

A Random Scan Device are also known as vector displays or stroke-writing displays. This is a special type of display system used to render images by drawing graphics in the form of vectors.

In random scan displays, an electron beam is directed to draw shapes such as lines and curves in any order, based on the image's vector information. It does not follow a sequential scan of the screen, but instead moves to specific points to draw shapes directly.

Random scan devices are also called vector displays because they use vectors to define the images. Instead of scanning the screen line by line like in raster systems, random scan devices focus on drawing the geometric shapes necessary to display the image.

Components of a Random Scan Device

The key components of a random scan device include −

• Display Controller − This component interprets the instructions from the computer and directs the electron beam to draw lines or shapes on the screen.
• Electron Beam − The electron beam is responsible for "drawing" the shapes on the screen. It moves directly from one point to another to create the desired lines or shapes, as shown in the above figure.
• Phosphor-Coated Screen − Similar to raster scan display systems, random scan devices use a phosphor-coated screen that emits light when hit by the electron beam. However, the way the beam interacts with the screen is different compared to raster scan systems.
• Vector Generator − A vector generator converts instructions into a sequence of points, allowing the electron beam to accurately follow the necessary path to create the image.

Working of Random Scan Device

The working of random scanning display is interesting. A random scan device receives instructions from a computer, which are then processed by the display controller. The controller understands these instructions as coordinates and draws images on the screen accordingly.

Instead of scanning the entire screen line by line like raster scanning, the electron beam moves directly between specific points to draw lines, shapes, or polygons.

We can formalize these steps using a few set of points:

• Receiving Instructions − The computer sends vector-based instructions that define the image in terms of lines, curves, or shapes.
• Beam Movement − The electron beam moves to specific points on the screen based on the vector instructions. It then draws lines or shapes by moving between these points.
• Shape Rendering − The shapes are drawn one at a time, directly onto the screen. The entire image is formed by connecting the individual shapes.

Random scan devices focus on drawing lines and shapes. These are not for drawing images like TV screens. These type of devices are more efficient for applications that require high precision in rendering geometric diagrams.

Random scan displays generally produce images in monochrome, as color handling in vector displays is more complex.

Applications of Random Scan Devices

From the basic understanding it is quite clear about their applications. Random scan devices were widely used in the early days of computer graphics. Their ability to render precise geometric shapes made them special for many specialized applications.

Some common applications of random scan devices include the following −

• Engineering and CAD Systems − In engineering and Computer-Aided Design (CAD) systems, random scan devices were to display the blueprints, schematics, and other technical drawings. These devices generates precise lines and curves required for technical diagrams.
• Oscilloscopes − In laboratory settings, oscilloscopes often used random scan technology to display electrical waveforms. Since we know random scan displays generate accurate curves and lines. These made vector displays ideal for visualizing continuous signals and waveforms.
• Military Applications − Sometimes random scan devices are used for military applications, especially for radar displays. Vector displays provided clear and precise representations of radar data.

Advantages of Random Scan Devices

Throughout the discussion we have understood several advantages of random scan devices.

• High Precision − Random scan devices are precise at rendering lines and shapes. These are ideal for technical and scientific applications.
• Efficient Rendering of Geometric Shapes − For applications that rely heavily on vector graphics, such as CAD systems or oscilloscopes, random scan devices are more efficient than raster scan devices.
• No Jagged Edges − Since random scan devices directly draw lines and shapes, they avoid the "jagged edge" problem that can occur in raster scan systems when rendering lines.

Limitations of Random Scan Devices

There are some major disadvantages of random scan displays.

• Limited to Monochrome Displays − Most random scan devices are limited to monochrome displays, as color handling in vector-based systems is more complex.
• Not Suitable for Complex Images − Random scan devices are less efficient at displaying complex images or images with many colors, as they are designed primarily for vector-based graphics.

Conclusion

In this chapter, we explained the concept of random scan devices and how they work in computer graphics. We discussed how these devices use vector-based instructions to draw images directly onto the screen, which makes them ideal for applications that require high precision and accuracy.

We covered the components of a random scan device, its operation, and common applications, such as CAD systems, and oscilloscopes. We also discussed the advantages and limitations of random scanning displays. Random scan devices are excellent for drawing precise geometric shapes, however they are less suitable for complex images and have mostly been replaced by modern raster scan technologies.