SFML 1.6 Collision Issues

Question

I am currently in the process of coding a game in my spare time using C++ and SFML 1.6. When it got around to be time to put collision detection in, I ran into an annoying bug where the program detects collision as if the object was shifted to the upper left by an amount differing based on the size of the stationary sprite I am using to test this, usually by about half its size. I can't seem to find what the bug is in my code, but perhaps you can. My code is listed below, thanks in advance.

#include <SFML/Graphics.hpp>
#include <math.h>

bool collide_rec(sf::Sprite object_1, sf::Sprite object_2);
bool collide_point(sf::Vector2f point, sf::Sprite object);
void collide(sf::Sprite& movable, object& movable_data, sf::Sprite& sationary);

void collide(sf::Sprite& movable, object& movable_data, sf::Sprite& stationary)
{
//Handle Collision NOTE: Results in infinate loop if started on colliding object
for(;collide_rec(movable, stationary);)
{
if((movable_data.move.spd.x<0)||(movable_data.move.left==true||movable_data.direction==LEFT )) movable.Move( 1, 0);
if((movable_data.move.spd.x>0)||(movable_data.move.right==true||movable_data.direction==RIGHT)) movable.Move(-1, 0);
if((movable_data.move.spd.y<0)||(movable_data.move.up   ==true||movable_data.direction==UP   )) movable.Move(0,  1);
if((movable_data.move.spd.y>0)||(movable_data.move.down ==true||movable_data.direction==DOWN )) movable.Move(0, -1);
  }
}

bool collide_point(sf::Vector2f point, sf::Sprite object)
{

}

bool collide_rec(sf::Sprite object_1, sf::Sprite object_2)
{
  bool hit=true;
  sf::Vector2f tl_1, br_1, tl_2, br_2;//Top-Left Coner, Botom Right Corner

  //Assign the corners proporly
  tl_1= object_1.GetPosition();
  tl_2= object_2.GetPosition();

   br_1= (object_1.GetPosition()+object_1.GetSize());
   br_2= (object_2.GetPosition()+object_2.GetSize());

  if((tl_1.x<tl_2.x)&&(br_1.x<tl_2.x) || (tl_1.x<br_2.x)&&(br_1.x<br_2.x)) //if both points are to the left or right on the x demtion
  hit=false;

  if((tl_1.y<tl_2.y)&&(br_1.y<tl_2.y) || (tl_1.y<br_2.y)&&(br_1.y<br_2.y)) //if both points are to the left or right on the y demtion
  hit=false;

  return hit;
}

Rohit Rohit · Accepted Answer · 2012-10-30T04:37:43

Not sure if you still need an answer to this, but I might as well try and help.

A couple things you might want to check: - Whether object_1.getPosition() and object_2's position is the top left corner of the object. - If the objects are not squares then the bottom right corners will need slightly different equations to compute. It would be (Pos.x + Width, Pos.y + Height) for the (x, y) coords.

Once you're sure of that, a couple things I noticed in your hit detection algorithm:

1)

if((tl_1.x<tl_2.x)&&(br_1.x<tl_2.x) || (tl_1.x<br_2.x)&&(br_1.x<br_2.x))

The second part of this condition:

(tl_1.x < br_2.x)&&(br_1.x < br_2.x))

seems like it should be

(tl_1.x > br_2.x)&&(br_1.x > br_2.x))

2)

Likewise:

(tl_1.y < tl_2.y)&&(br_1.y < tl_2.y)

seems like it should be

(tl_1.y > tl_2.y)&&(br_1.y > tl_2.y)

3)

Finally, your conditions have a certain amount of redundancy to them, which is unneeded.

Once you have your topLeft1, BottomRight1 and topLeft2, BottomRight2 you can simply check for collision like this:

Assuming a cartesian coordinate system, with the origin (0, 0) being the bottom left.

X Stage Collision:

if ( topLeft1.x > BottomRight2.x || BottomRight1.x < TopLeft2.x )

// If the left side of box1 > the right side of box2 OR // If the right side of box1 < the left side of box2 then - the boxes dont collide on the X axis.

Y Stage Collision:

if ( topLeft1.y < BottomRight2.y || BottomRight1.y > TopLeft2.y )

// If the top side of box1 < the bottom side of box2 OR // If the bottom side of box1 > the top side of box2 then - the boxes dont collide on the Y axis.

That reduces it from 8 conditions to 4. Hope that helps.

SFML 1.6 Collision Issues

1 Answers