Game Development Reference

In-Depth Information

count++;

}

break;

case 3:

if (q2==0){

count-;

}else{

count++;

}

break;

}

}

}

return (count==4 || count==-4);

}

Another useful 2D technique is finding the nearest point on a line

segment to an arbitrary point.

The two triangles AP

P and BP

P define the position of the point P

on

the line segment AB. A simple technique to calculate the position of P

is

to use the old favourite the dot product. If we are trying to get the angle at

A then we can use two vectors. First, the line segment AB and, second,

the vector AP. If we set these two vectors to unit length then the dot

product AP

• AP returns the cosine of the angle between them. If this

value is negative then the angle is greater than 90° and so the nearest

point to the line segment AB is the point A itself. Similarly, at B, if the

returned cosine is negative then the point P must be to the right of the line

Figure 13.8 Finding the nearest point on a line segment to an arbitrary point.

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