# Corner Collision Detection Problems in a 2D Platformer

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Hey guys, I am working on a 2D-Platformer and I having some troubles with 2D collision detection and response. The problem is with happens when the player collides with a corner, the result being the player sticks to the corners for a brief amount of time. He also snaps to corners as well.

The player is one tile in size, which means he can only collide with 4 tiles at one time. I check the four tiles within his vicinity minus the one he started in, then we change his position to the closest possible tile without making a collision. Here is the code:

void update(float dt)
{
float tempx = x + velx * dt;//The player's temporary X
float tempy = y + vely * dt;//The player's temporary y
int xdir = velx > 0 ? 1 : 0; // get x-sign using ternary operator
int ydir = vely < 0 ? 1 : 0; // get y-sign using ternary operator
velx *= 0.995;
vely += 4.9*dt;

bool q1 = world->tiles[int(tempx + xdir) + int(tempy + ydir) * 20] != 0;//Where the first quadrant is
bool q2 = world->tiles[int(tempx + xdir) + int(tempy + 1 - ydir) * 20] != 0;//Where the second quadrant is
bool q3 = world->tiles[int(tempx + 1 - xdir) + int(tempy + 1 - ydir) * 20] != 0;//Where the third quadrant is

float ttempx = tempx, ttempy = tempy;

if(velx)
{
if(q1 || (q2 && !q3)) //Checks for collision in the first quadrant
{
ttempx = xdir ? floor(tempx) : ceil(tempx);
}
}

if(vely)
{
if(q3 || (q2 && !q1))//Checks for collision in the third quadrant
{
vely = 0;
ttempy = ydir ? ceil(tempy) : floor(tempy);
}
}

float xpen = abs(ttempx - tempx);
float ypen = abs(ttempy - tempy);

if(ypen > xpen)
{
tempy = ttempy;
}
else if(ypen < xpen)
{
tempx = ttempx;
}

x = tempx;
y = tempy;
}


Can someone please tell me what is wrong with this code. Thank you.

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I'm not sure what's the problem (maybe it's easier to understand with an image), but there are suspicious parts...

First, you have xdir and ydir that, for your comment, store the sign of the velocity. The sign should be + or - (or +1 and -1 respectively), but your code set it as 1 or 0. Also, this is minor, but velx = 0 is being considered as negative in your code, while it makes more sense to count 0 as a positive number. And ydir stores exactly the opposite as the velocity's sign.

Later on you use xdir and ydir to access the tiles of your world, but the sums you make look weird too (the " + 1 - xdir" part). What tiles should you get while moving only right for example? Are you getting the 3 tiles correctly for every combination of x and y direction?

For the stick and snap to the corners... you're checking for collisions in every update call, but you're objects move from one tile to another (correct me if I'm wrong). Shouldn't you check for collisions only when you try to move to a tile instead of checkin during all the transition? I guess you're detecting a collision before it actually happens (it snaps) or you're detecting one even when you shouldn't (it sticks).

Sorry if this doesn't make much sense, I'm making a lot of assumptions.

Edited by DiegoSLTS

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