# Issue with lone tiles or corner tiles in grid based movement

3 posts

 Like the topic says, the following code is based on top down grid style collision detection and movement… So I have this ball which is moving around tiles… let me just show you all the code first then I will mention the problem:- ``` if (isReady && isMovin) { //trace("speed is " + speed); speedX = Math.cos(angle) * speed * flagX; speedY = Math.sin(angle) * speed * flagY; x_pos = this.x + speedX; y_pos = this.y + speedY; left = Math.floor((x_pos - 9) / tile_size); bottom = Math.floor((y_pos + 9) / tile_size); top = Math.floor((y_pos - 9) / tile_size); right = Math.floor((x_pos + 9) / tile_size); top_left = level[top][left]; bottom_left = level[bottom][left]; top_right = level[top][right]; bottom_right = level[bottom][right]; //no wall encountered carry on if (top_left != 1 && bottom_left != 1 && top_right != 1 && bottom_right != 1) { this.x += speedX; this.y += speedY; } else //code for checking collision with walls { if (top_left == 1 && bottom_left == 1 && speedX < 0) { this.x = (left + 1) * tile_size + 10; //this.y += speedY; speed -= 0.2; flagX = -flagX; } else if (top_right == 1 && bottom_right == 1 && speedX > 0) { this.x = (right - 1) * tile_size + 8; //this.y += speedY; speed -= 0.2; flagX = -flagX; } else { this.x += speedX; } if (top_right == 1 && top_left == 1 && speedY < 0) { this.y = (top + 1) * tile_size + 9; //this.x += speedX; speed -= 0.2; flagY = -flagY; } else if (bottom_left == 1 && bottom_right == 1 && speedY > 0) { this.y = (bottom - 1) * tile_size + 10; //this.x += speedX; speed -= 0.2; flagY = -flagY; } else { this.y += speedY; } } speed -= fric; if (speed <= 0) isMovin = false; } ``` As you all can see I am only testing the top(similar for bottom, right and left) collision only if both the variables are blocked like top_right and top_left both are equal to one then only I will change the direction. The problem comes when only one of the variable is one (that is maybe top_right is 1 and top_left is not) then the ball goes straight or movement becomes jittery cause I havent added the code for checking with single variable. My issue is that for any given variable for eg if top_right is one then the ball can be colliding either on bottom of that tile or on the left hand side of that tile, I am not able to diagnose that properly… Here is pic of that issue:- So the ball can be moving in direction 1 and it hits left hand side of tile and direction 2 hits bottom of the tile… how to diagnose which direction it hits. Any help will be appreciated. Thanks for that. You are awesome. Its taking some time for my fuzzy brain to register what is going on in that code… I will check on that later…