Page 47 - DCAP504_Computer Graphics

Computer Graphics

4.1 Methods-Analog and Digital

The process of scan conversion involves altering the picture’s data rate information and then inserting
the new picture within appropriate synchronization signals.
Two methods for changing a picture’s data rate are as follows:
1. Analog Method: This method is also known as non retentive, memory-less or real time method.
For conversion, this method uses a large number of delay cells. This method is mainly used for
analog video.
2. Digital Method: This method is also known as retentive or buffer method. In this method, a
picture is stored in a line or frame buffer with data rate speed (n1) and read with another data rate
speed (n2). To improve the picture quality and to prevent the conversion artifacts, this method
uses several picture processing techniques.

In a 120/140Hz analog TV, there is a scan converter circuit which converts the
vertical frequency (refresh rate) from standard 60/70Hz to 120/140Hz to
achieve a low level of flicker which is important in large screen (high inch) TVs.
4.2 Line

In computer graphics, a line refers to a line segment, which is a part of a straight line that extends
indefinitely in opposite directions. A line is defined by two endpoints and is represented by the line
equation y=mx+b. In the equation y=mx+b, m is the slope and b the intercept of the line.

Figure 4.1: Defining a Line

In figure 4.1, the two endpoints are described by P 1 (x 1 , y 1 ) and P 2 (x 2 , y 2 ). The line equation describes
the coordinates of all the points that lie between the endpoints.

Slope-intercept equation is not suitable for vertical lines.

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