Page 42 - DCAP504_Computer Graphics

Unit 3: Overview of Graphics I/O Devices

all even-numbered scan lines. The effective refresh rate to produce the picture becomes greater than 30
Hertz.
3.6.1 Types of Frame Buffers

Frame buffers use different kinds of memory. Initially, frame buffers made use of drums and disks with
rotational frequency compatible to the rate of refresh. However, the rotating-memory frame buffers are
now replaced by integrated-circuit shift registers due to lower costs.
A number of shift registers are used to construct a frame buffer. Each shift register uses one column of
pixels on the screen. Every shift register contributes one bit per horizontal scan line. However, it is not
very easy to change a given spot on the screen using shift registers. Therefore, they are suitable for
applications that are interactive.
Random-scan integrated circuits are used in modern frame buffers where the pixel intensities are
represented by 1, 2, 4, 8, 16 or 24 bits. Nearly, 8 bit per pixel is required for encoding text and simple
images. However, to produce a good quality colored image 24 bits are required.
A color map is one of the best methods used to encode colored pictures. The pixel values in the frame
buffer are considered as addresses of a look-up-table with entries for every pixel’s red, green and blue
components. The entry value is used to control the intensity or color on the CRT. Every color
components can be defined to high precision offering accurate control over the colors displayed.

A multiple-plane frame buffer is another type of frame buffer where the frame buffer can be treated as
consisting of several frames or planes. Each contains information (intensity and/or color) values of a
separate image. An 8-bit per pixel frame buffer can be made to represent a single image with 8-bit of
intensity precision, or it can represent two images, each of 4-bit intensity precision or eight black and
white images with 1-bit intensity precision each. A variety of image mixing can be done.
In animation systems, several moving objects can be displayed as separate
planes.

3.7 Logical Functioning of I/O Devices

The I/O devices are capable of performing different logical functions. The functional capabilities of I/O
devices are divided in four types. They are:
1. Locator Function: The locator function provides coordinating information in two or three
dimensions. Basically, the coordinates are returned as normalized coordinates and may be either
relative or absolute. The locator functions are performed by devices such as track ball, joystick,
tablet, mouse, touch panel, and so on.

The locator functions are performed by devices such as track ball, joystick,
tablet, mouse, touch panel, and so on.
2. Valuator Function: The valuator function provides only a single value as a real number. This may
be bound or unbound. A bound valuator has mechanical or programmed ends within a fixed
range, whereas an unbounded valuator has an infinite range.
3. Choice or Button Function: The choice function selects and activates events or procedures which
control the interactive flow or change the underlying task. It generally provides binary
information.

The keyboard is a specific example of collection of buttons or choice functions.

4. Pick Function: The pick function selects objects within the displayed picture. It picks up the
selected objects.

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