Game mechanic player following path looks very roughly

Question

I want my player entity to follow a path, which works, but it looks very roughly. Like it would stuck everytime.

    bool arrivedX = false;
    bool arrivedY = false;

    float speed = 0.06f;

    if(bestway.empty()) {
        //arrived
    } else {

        if(entity->x < bestway.front()->x * gridSize) {
            entity->x += speed * machine->getDeltaTime();
        } else if(entity->x > bestway.front()->x * gridSize) {
            entity->x -= speed * machine->getDeltaTime();
        } else {
            arrivedX = true;
        }

        if(entity->y < bestway.front()->y * gridSize) {
            entity->y += speed * machine->getDeltaTime();
        } else if(entity->y > bestway.front()->y * gridSize) {
            entity->y -= speed * machine->getDeltaTime();
        } else {
            arrivedY = true;
        }

        if(arrivedX && arrivedY) {
            bestway.pop_front();
        }
    }

the vector contains grid coords from 0 to WIDTH or HEIGHT / gridSize. And the entity has pixels coords. I have tried already:

entity->x / gridSize < bestway.front()->x

which runs smoothly, but when it goes up or right it will omit one grid tile. I think it is because of when the player is at pixel 19 and the gridSize is 20 it would calculate 0 . 19/20 = 0... I tried to ceil and floor it, but it didn't work out. Hope you guys can help me.

edit:

        int ix = int( x + 0.5f );
        int iy = int( y + 0.5f );

        if(ix < bestway.front()->x * gridSize) {
            x += speed * machine->getDeltaTime();
        } else if(ix > bestway.front()->x * gridSize) {
            x -= speed * machine->getDeltaTime();
        } else {
            arrivedX = true;
        }

Video

http://youtu.be/-DugHbMdODg

Answer 1

I believe your error arises from rounding errors.

I believe the problem could be solved by using float for pixel-coordinate storage. This way you will always get the exact increment needed for the given delta time, which is usually very small and thus the resulting per update increment is less than 1, and truncated to 0.

Don't forget to use proper rounding when converting back for rendering or comparison. For rendering I would use: int ix = (int)( entity->x + 0.5 ) Which will yield 'proper' rounding: up from 0.5 and down with anything less.

---

Edit:

From the video it seems you are overshooting the position you are aiming for. There are three ways to solve this:

• After moving, check if you overshot and correct for it:

  entity->x += speed * dt;
  if( entity->x > movelocation->x * gridSize )
      entity->x = movelocation->x * gridSize;
  

• Lower the speed you are moving with.

• Use a higher update frequency.

The most solid option seems to be checking for overshoot and correcting. Note: The simple code I provided doesn't account for the movement speed lost when arriving at each tile.