Angles

Cyclone_S

I got two questions. First I'm making a brick game. I want the ball to bounce off the paddle at different angles depending on where the ball hits the paddle. If the angle variable is specified directly it bounces off in the correct angle but I'm not sure how to calculate the angle depending on where the ball hits the paddle.

double a = 135;
double angle = (Math::PI / 180) * a;
Ball_List[b]->xVel = speed*Math::Cos(angle);
Ball_List[b]->yVel = -speed*Math::Sin(angle);

Second. I want to animate a box going in a circle but I'm Not sure on the math. Thanks in advance.

Luc Pattyn

Cyclone_S wrote:

I want the ball to bounce off the paddle at different angles depending on where the ball hits the paddle

That is pretty vague, and probably not according to normal physics. Maybe what you want is: the outgoing angle equals the supplement of the incoming angle plus some delta, which is zero in the center and grows when the hit point is away from the center; so maybe calculate that distance and use it to add to or multiply the outgoing angle.

Cyclone_S wrote:

going in a circle

the equations for a circle in two dimensional space are:

(x - xc)^2 + (y - yc)^2 = r^2

or

x = xc + r * cos(a)
y = yc + r * sin(a)

where (x,y) is a point on the circle, (xc,yc) is the center, r the radius, a an angle in radians. I cannot believe you would not know that. Look at the equations, they say the point(x,y) is at a fixed distance r from a fixed point (xc,yc). :)

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modified on Tuesday, May 31, 2011 9:17 PM

David Crow · modified on Tuesday, May 31, 2011 9:17 PM

Luc Pattyn wrote:

I cannot believe you would not know that.

Just out of curiosity, why would you assume he would know?

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Luc Pattyn

Assuming he is adolescent or older, he would have learned this and much more at school, both as formula's for describing a circle, and as a geometric illustration for explaining what a sine and cosine actually are. He already was using angles, sine and cosine, in his post, so it puzzles me how he would not come up with the equations if he had ever seen and understood them. And then there are books, and Google, and Wolfram, etc. :)

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Stefan_Lang

You might want to check out this site[Physics engines for dummies] for an easy introduction into the basics of vector algebra used to emulate quasi-physical simulations. You will find that in stead of angles and sinus/cosinus it explains the calculations needed for reflections with the help of vector algebra. Although the code is not C/C++ it should be easy enough to translate the relevant code for your problems.

Cyclone_S

Thanks for the replies. Both problems are mostly solved. I have the paddle/ball equation figured out but I'm having a problem where any value less then 1 is ignored... any ideas? I need finer precision. Thanks.

Luc Pattyn

Cyclone_S wrote:

I need finer precision

then scale it all up in integers; or use floating-point arithmetic throughout. :)

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Cyclone_S

I am using doubles in the function. The problem is I can't move the panel(ball) anything less then 1 pixel. Maybe move the ball 1pixel over a couple of frames? I'm not sure how to do that. Here is the code.

						int speed=Ball\_List\[b\]->speed;
						// Paddle center.
						int paddle\_center = Player->Location.X + Player->Width / 2;

						// Ball center.
						int ball\_center = Ball\_List\[b\]->gameBall->Location.X + Ball\_List\[b\]->gameBall->Width / 2;

						// Find the location on the paddle that the ball hit
						int paddle\_location = ball\_center - paddle\_center;

						// Increase X speed according to distance from center of paddle.
						double a=(90-paddle\_location);
						double angle = (Math::PI / 180) \* a; // angle in radians.
						Ball\_List\[b\]->xVel = speed\*Math::Cos(angle);
						Ball\_List\[b\]->yVel = -speed\*Math::Sin(angle);

						Ball\_List\[b\]->gameBall->Top = Player->Location.Y - 16;

Thanks

Luc Pattyn

All of that could have been floats or doubles; it is only when painting that it eventually needs to be rounded to pixels, hence integers. :)

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Cyclone_S

so there's no solution then?