Fix: Detect collision if one line is almost vertical (#5510) (#5531)

Author: bielern

src/rshapes.c

@@ -2365,30 +2365,30 @@ bool CheckCollisionCircleRec(Vector2 center, float radius, Rectangle rec)
 // Check the collision between two lines defined by two points each, returns collision point by reference
 bool CheckCollisionLines(Vector2 startPos1, Vector2 endPos1, Vector2 startPos2, Vector2 endPos2, Vector2 *collisionPoint)
 {
-    bool collision = false;
+    // According to https://en.wikipedia.org/wiki/Line–line_intersection#Given_two_points_on_each_line_segment
+    float rx = endPos1.x - startPos1.x;
+    float ry = endPos1.y - startPos1.y;
+    float sx = endPos2.x - startPos2.x;
+    float sy = endPos2.y - startPos2.y;
 
-    float div = (endPos2.y - startPos2.y)*(endPos1.x - startPos1.x) - (endPos2.x - startPos2.x)*(endPos1.y - startPos1.y);
+    float div = rx * sy - ry * sx;
 
-    if (fabsf(div) >= FLT_EPSILON)
-    {
-        collision = true;
+    if (fabsf(div) < FLT_EPSILON) {
+        return false;
+    }
 
-        float xi = ((startPos2.x - endPos2.x)*(startPos1.x*endPos1.y - startPos1.y*endPos1.x) - (startPos1.x - endPos1.x)*(startPos2.x*endPos2.y - startPos2.y*endPos2.x))/div;
-        float yi = ((startPos2.y - endPos2.y)*(startPos1.x*endPos1.y - startPos1.y*endPos1.x) - (startPos1.y - endPos1.y)*(startPos2.x*endPos2.y - startPos2.y*endPos2.x))/div;
+    float s12x = startPos2.x - startPos1.x;
+    float s12y = startPos2.y - startPos1.y;
 
-        if (((fabsf(startPos1.x - endPos1.x) > FLT_EPSILON) && (xi < fminf(startPos1.x, endPos1.x) || (xi > fmaxf(startPos1.x, endPos1.x)))) ||
-            ((fabsf(startPos2.x - endPos2.x) > FLT_EPSILON) && (xi < fminf(startPos2.x, endPos2.x) || (xi > fmaxf(startPos2.x, endPos2.x)))) ||
-            ((fabsf(startPos1.y - endPos1.y) > FLT_EPSILON) && (yi < fminf(startPos1.y, endPos1.y) || (yi > fmaxf(startPos1.y, endPos1.y)))) ||
-            ((fabsf(startPos2.y - endPos2.y) > FLT_EPSILON) && (yi < fminf(startPos2.y, endPos2.y) || (yi > fmaxf(startPos2.y, endPos2.y))))) collision = false;
+    float t = (s12x * sy - s12y * sx) / div;
+    float u = (s12x * ry - s12y * rx) / div;
 
-        if (collision && (collisionPoint != 0))
-        {
-            collisionPoint->x = xi;
-            collisionPoint->y = yi;
-        }
+    if (0.0f <= t && t <= 1.0f && 0.0f <= u && u <= 1.0f) {
+        collisionPoint->x = startPos1.x + t * rx;
+        collisionPoint->y = startPos1.y + t * ry;
+        return true;
     }
-
-    return collision;
+    return false;
 }
 
 // Check if point belongs to line created between two points [p1] and [p2] with defined margin in pixels [threshold]