Line Collision Detection

Found this excellent explanation of line-to-line collision and many others.

Boolean function

```boolean linesTouching(float x1, float y1, float x2, float y2, float x3, float y3, float x4, float y4) {
float denominator = ((x2 - x1) * (y4 - y3)) - ((y2 - y1) * (x4 - x3));
float numerator1 = ((y1 - y3) * (x4 - x3)) - ((x1 - x3) * (y4 - y3));
float numerator2 = ((y1 - y3) * (x2 - x1)) - ((x1 - x3) * (y2 - y1));

// Detect coincident lines (has a problem, read below)
if (denominator == 0) return numerator1 == 0 && numerator2 == 0;

float r = numerator1 / denominator;
float s = numerator2 / denominator;

return (r >= 0 && r <= 1) && (s >= 0 && s <= 1);
}```

Returns point of contact

```PVector lineIntersection(float x1, float y1, float x2, float y2, float x3, float y3, float x4, float y4) {

// calculate the distance to intersection point
float uA = ((x4-x3)*(y1-y3) - (y4-y3)*(x1-x3)) / ((y4-y3)*(x2-x1) - (x4-x3)*(y2-y1));
float uB = ((x2-x1)*(y1-y3) - (y2-y1)*(x1-x3)) / ((y4-y3)*(x2-x1) - (x4-x3)*(y2-y1));

// if uA and uB are between 0-1, lines are colliding
if (uA >= 0 && uA <= 1 && uB >= 0 && uB <= 1) {
return new PVector(x1 + (uA * (x2-x1)), y1 + (uA * (y2-y1)));
}
return null;
}```
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