Game Development Reference
In-Depth Information
Here, xs and ys are the x , y values of the starting point for the line. If xs
is less than 0, then cs is given the value 1 shifted three places to the left,
which is 8. If xs is greater than or equal to the width of the canvas, then
4 is added to the code value. If the y value is less than 0, then 2 is added
to the code value, and finally if ys is greater or equal to the height of the
canvas, then 1 is added to the code value. This places the point in one of
the number sections of Figure 2.2, the number being the code value for a
point in that section. For example, if we have the point (-3, 16) on a
canvas that is 200 pixels square, then
cs = ((-3<0)<<3)|(-3>=200)<<2)|((16<0)<<1)|(16>=200)
cs = (1<<3)|(0<<2)|(0<<1)|0
cs = 8|0|0|0
cs = 8
From the code value, we know that the point (-3, 16) with respect to our
canvas is to the left in the section labelled 8 in the diagram.
The next step is to determine the slope of the line. This is done using
the y distance divided by the x distance. The x distance is the end x value
minus the start x value. The y distance is the end y value minus the start
y value. The slope of this line is a floating point value; since the values for
x and y are all integer values, we must remember to cast the integer
values as doubles to get a meaningful result for the slope. Now we have
a point location and a slope. By doing a bitwise And-ing of the start and
end locations, we determine whether the line remains off-screen
throughout its length.
Figure 2.2 Determining point location.
 
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