带角度的爆发式桨叶碰撞

Question

I know there's so many stuff on the internet about this, and I have looked at quite a lot of it, but I just can't get it to work with my code. I kinda know the maths behind it, but again struggling to get it into code. I have 2 speed variables for the ball float xSpeed, ySpeed . Both are equal to 3 (or -3 depending on collision - I just have basic collision atm). I'm also using a rectangle for ball ballRect as well as the paddle paddleRect . I'm updating the position as so:

ballRect.X += xSpeed; ballRect.Y += ySpeed;

I've found bits of code, and tried doing it myself, but they just were really buggy. One would (I think) work on the first hit, but when it came to the next hit it would stick to the paddle.

double relativeBallPos = (paddleRect.X + ballRect.X);
        double ballVelx = xSpeed;
        double ballVely = ySpeed;

        double angleRads = Math.Tan((ballVelx / relativeBallPos));
        double angleInDeg = angleRads * (180 / Math.PI);
        double angleOfReflection = (angleInDeg * 2);

        ballVelx = ballVelx * angleOfReflection;


        if (ballRect.X + (ballRect.Width / 2) < paddleRect.X + (paddleRect.Width / 2))
        {
            xSpeed = (float)-ballVelx;
        }
        else if (ballRect.X + (ballRect.Width / 2) > paddleRect.X + (paddleRect.Width / 2))
        {
            xSpeed = (float)ballVelx;
        }

(This goes off at a (probably incorrect) angle and just goes straight up after the first hit) Thanks for any help :)

Answer 1

If your ball intersects any horizontal threshold (the horizontal sides of the bricks, or of the paddle), negate ySpeed . If it intersects any vertical threshold, negate xSpeed . There, now you have a functional breakout game.

You can add a bit more polish by changing the angle of the ball when it hits the paddle based on the position of the collision along it, with center being a full reflect (ie negate ySpeed ) and xSpeed getting a factor of the distance (positive or negative) of the distance from the center.

Answer 2

Yes, what you are seeing is completely normal, if your objects are moving very fast and game tick speed is slow, you may have glitches like sticking to the paddle or even missing the paddle completely, and just going straight through.

The solution to missing the paddle is increase game tick speed, ie process game moves more frequently.

Ball to paddle stickiness can be alleviated by doing a roll back of object movement in time. For example you have objects A and B, which are colliding at some point in time. If you simply reverse their X and Y speeds, you may end up with colliding again in the next point in time, so you would then reverse their speeds again, and so on, which appears as though it's stuck. It can sometimes get stuck on one only axis, so it will slide on the paddle, and then go straight down, which is just another side effect of the same issue.

How to do a roll back? You know where A is moving, and suppose B is a paddle, so it's static. You know that A is colliding with B at the moment. You can calculate a vector of A's movement and slide A back in time through that vector reversed. For example, if top left corner of the screen is (0,0) , and A was moving at the speed of X=+10,Y=+10 , you would move it back by whole or fraction of the step (depends on how complex you want to go with this), so by (-10,-10) or fraction thereof. You can use intersection rectangle to calculate precise values, and only move enough so the objects are barely touching.

Ideally, you would reflect the vector off the hit surface (use physics, ie hit angle = reflect angle) and before applying new speeds, move your ball in a way that it does not collide with paddle. To simplify, you can assume your hit surface is horizontal and is just a straight line, so your reflection formula is very simple.

This topic is well covered here (although using XNA, same principle applies):

• Stuck in Wall after rectangle bounding box collision