# Angles and Collision Detection

Lets say i have a ball, and it hits a wall at a certian angle and i want it to leave the wall at the exact opposite angle. any clue how this would be achieved? i think that it is the tangent of the angle but im not really sure. Any help would be appreciated! thanx!

Tangent, no no no. You need to brush up on your trig.

If you think of an X axis lying along your surface of impact, with the Y axis going straight out:

NewAngle=180-OldAngle

You can work out the cosine of the angle by finding the dot product of the surface its hitting and the velocity vector.

The easiest way however, for a vertical wall and a ball moving with velocity V(x,y) is NV(-x, y). So the ball just changes direction. Same deal with a horizontal surface NV(x, -y).

Hmmm. That sounds similar to what was discussed in this post ( http://www.opengl.org/discussion_boards/ubb/Forum2/HTML/008162.html ). Only difference is you want angles where we converted it to vectors.

Here is an example of how I am handling the bouncing off the wall’s:

#include <GL/glut.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

static int player_score = 0;

static float ball_pos_x = 100, ball_pos_y = 100, ball_state = 1, ball_rate = 4, ball_direction_x = 1, ball_direction_y = 1, ball_angle = 90;

void init(void)
{

glClearColor (0.0, 0.0, 0.0, 0.0);

}

void display(void)
{

glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glMatrixMode(GL_MODELVIEW);

glBegin(GL_LINES); // draw border
glColor3f(1.0, 1.0, 1.0);
glVertex2f( 0.0, 0.0);
glVertex2f( 499.0, 0.0);
glVertex2f( 499.0, 0.0);
glVertex2f( 499.0, 249.0);
glVertex2f( 499.0, 249.0);
glVertex2f( 0.0, 249.0);
glVertex2f( 0.0, 249.0);
glVertex2f( 0.0, 0.0);
glEnd();

glColor3f( 1.0, 1.0, 1.0);
glScalef( 1.0, 4.0, 1.0);
glutSolidCube( 10 );
glPopMatrix();

glPushMatrix(); // draw ball
glTranslatef( ball_pos_x, ball_pos_y, -10);
glColor3f( 0.5, 0.5, 0.5);
glutSolidSphere(5, 16, 8);
glPopMatrix();

glColor3f( 1.0, 0.0, 0.0);
glRasterPos3i(5, 24, 0);
glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, ‘S’);
glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, ‘C’);
glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, ‘O’);
glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, ‘R’);
glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, ‘E’);

glutSwapBuffers();
}

void reshape (int w, int h)
{

if (( w != 500 ) | | ( h != 250 ))
{
glutReshapeWindow( 500, 250);
w = 500;
h = 250;
}

glViewport (0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode (GL_PROJECTION);
glOrtho(0.0f, (GLfloat) w,(GLfloat) h, 0.0f,-10.0f,10.0f);
glMatrixMode(GL_MODELVIEW);
}

void keyboard (unsigned char key, int x, int y)
{
switch (key) {
case 27:
exit(0);
break;
default:
break;
}

}

void Special_keys(unsigned char key, int x, int y)
{
switch (key) {
case GLUT_KEY_DOWN:
break;
case GLUT_KEY_UP:
break;
default:
break;
}

}

void My_event_manager(void)
{

{
}

if (ball_state > 0) // Manage ball if in motion
{
ball_pos_x = ball_pos_x + 0.5 * ball_direction_x;
ball_pos_y = ball_pos_y + 0.5 * ball_direction_y;
}

if ((ball_pos_x > 490) | | (ball_pos_x < 10)) ball_direction_x = ball_direction_x * -1;
if ((ball_pos_y > 240) | | (ball_pos_y < 10)) ball_direction_y = ball_direction_y * -1;

if ((ball_pos_x < paddle_pos + 20) && (ball_pos_x > paddle_pos - 20))
{
if ((ball_pos_y < 35 ) && (ball_pos_y > 30))
{
ball_direction_x = 1;
ball_direction_y = 1;
}

if ((ball_pos_y < 20 ) && (ball_pos_y > 25))
{
ball_direction_x = -1;
ball_direction_y = -1;
}
}

glutPostRedisplay();

}

int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize (500, 250);
glutInitWindowPosition (10, 10);
glutCreateWindow (argv[0]);
glutSetWindowTitle(“Glutplanets”);
init ();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutSpecialFunc(Special_keys);
glutIdleFunc(My_event_manager);
glutMainLoop();
return 0;
}

Originally posted by DragonXTC:
Lets say i have a ball, and it hits a wall at a certian angle and i want it to leave the wall at the exact opposite angle. any clue how this would be achieved? i think that it is the tangent of the angle but im not really sure. Any help would be appreciated! thanx!