ulfben

Intersection tests (AABB and SAT)

Sep 23rd, 2016
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  1. #include <cmath>
  2.  
  3. //AABB intersection test
  4. bool isColliding(const Entity& lhs, const Entity& rhs) {
  5.     SDL_assert(&lhs != &rhs);
  6.     return !(lhs._x + lhs._width < rhs._x ||
  7.         rhs._x + rhs._width < lhs._x ||
  8.         lhs._y + lhs._height < rhs._y ||
  9.         rhs._y + rhs._height < lhs._y);
  10. }
  11.  
  12. //SAT intersection test. http://www.metanetsoftware.com/technique/tutorialA.html
  13. //Many thanks to Jerry Jonsson and Tommi Lipponen at Uppsala University, GAME,
  14. //for the original implementation of this function: http://pastebin.com/cbJWTfb9
  15. bool getOverlap(const Entity& lhs, const Entity& rhs, int& overlapX, int& overlapY) {
  16.     SDL_assert(&lhs != &rhs);
  17.     overlapX = 0;
  18.     overlapY = 0;
  19.     int centerDeltaX = static_cast<int>(lhs.centerX() - rhs.centerX());
  20.     int halfWidths = (lhs._width + rhs._width) / 2;
  21.  
  22.     if (std::abs(centerDeltaX) > halfWidths) return false; //no overlap on x == no collision
  23.    
  24.     int centerDeltaY = static_cast<int>(lhs.centerY() - rhs.centerY());
  25.     int halfHeights = (lhs._height + rhs._height) / 2;
  26.    
  27.     if (std::abs(centerDeltaY) > halfHeights) return false; //no overlap on y == no collision
  28.    
  29.     int dx = halfWidths - std::abs(centerDeltaX); //overlap on x
  30.     int dy = halfHeights - std::abs(centerDeltaY); //overlap on y      
  31.     if (dy < dx) {
  32.         overlapY = (centerDeltaY < 0) ? -dy : dy;          
  33.     } else if (dy > dx) {
  34.         overlapX = (centerDeltaX < 0) ? -dx : dx;      
  35.     } else {
  36.         overlapX = (centerDeltaX < 0) ? -dx : dx;
  37.         overlapY = (centerDeltaY < 0) ? -dy : dy;          
  38.     }
  39.     return true;
  40. }
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