Page 141 - DCAP504_Computer Graphics

Computer Graphics

A region code is assigned to every point of a line depending on its position (x,y). Starting from the
leftmost bit,
Bit 1: sign(y-y max) = Endpoint is above the window

Bit 2: sign(y min-y) = Endpoint is below the window
Bit 3: sign(x-x max) = Endpoint is to the right of the window
Bit 4: sign(x min-x) = Endpoint is to the left of the window
Here, sign(a) is set to 1 if a is positive otherwise a is set to 0. The point with code 0000 is always inside
the window.

2. Step 2: If the endpoints of the line lie in the region with codes 0000 then the line is visible. The line
is not visible if the bitwise logical AND of the codes is not 0000. Thus, this line is considered as
clipping candidate.

Figure 9.5: Clipping Candidate

In case of a line in category 3 (Clipping candidate), you need to find the point that intersects with one of
the boundaries of the clipping window. First, you need to select the endpoint of the line (x 1, y 1) that is
outside the window whose region code is not 0000. Then, you need to select an extended boundary by
observing those boundary lines that are candidates for intersection. Here, the selected endpoint is
pushed across in order to change ‘1’ in its code to ‘0’. This indicates that:

If Bit 1 is 1, then intersect with line y = y max
If Bit 2 is 1, then intersect with line y = y min
If Bit 3 is 1, then intersect with line x = x max
If Bit 4 is 1, then intersect with line x = x min
In figure 9.5, the line LM falls under the clipping candidate category. If endpoint L is selected, then the
intersection point is computed by selecting the bottom boundary line y = y min. If endpoint M is selected,
then either the top boundary line y = y max or the right boundary line x = x max is selected for computing
intersection. The co-ordinates of the intersecting points are:

x i = x min or x max if the boundary line is vertical
y i = y 1 + m (x i + x 1)

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