shooting bullets opengl - opengl

Question

I have the following code which draws me a "spacecraft" which I'm able to ctrol with my arrow keys to spin clockwise and anti-clockwise. I'm trying to get the spacecraft shoot bullets when another key is pressed (it can be any key). I am so new to OpenGL that I'm confused on how to do it. I have tried so many methods but none of them work. Below is the code where I draw my spaceship and how I control it. Can someone help me with the bullets please?
struct
{
float rotateSpaceCraft;
} scene;
void SpaceCraft (){
glBegin(GL_TRIANGLE_FAN);
//specify the vertices to draw Ship in 2d space
glColor3f(1.0, 0.0, 1.0);
glVertex2f( X_CENTRE + LENGTH * 0, Y_CENTRE + LENGTH * 12);
glColor3f(1.0, 1.0, 0.0);
glVertex2f( X_CENTRE - LENGTH * 8, Y_CENTRE - LENGTH * 8);
glColor3f(0.0, 1.0, 1.0);
glVertex2f( X_CENTRE - LENGTH * 0, Y_CENTRE - LENGTH * 0);
glColor3f(1.0, 0.0, 0.0);
glVertex2f( X_CENTRE + LENGTH * 8, Y_CENTRE - LENGTH * 8);
glEnd();
}
void display(void)
{
glClear (GL_COLOR_BUFFER_BIT); /* clear window */
glColor3f(1.0, 1.0, 1.0); /* white drawing objects */
/* define object to be drawn as a square polygon */
glPushMatrix();
glRotatef(scene.rotateSpaceCraft, 0.0, 0.0, 1.0);
SpaceCraft();
glPopMatrix();
glutSwapBuffers();
glFlush(); /* execute drawing commands in buffer */
}
static void specialKey(int key, int x, int y)
{
switch(key)
{
case GLUT_KEY_LEFT:
scene.rotateSpaceCraft = fmod(scene.rotateSpaceCraft + 7, 360);
break;
case GLUT_KEY_RIGHT:
scene.rotateSpaceCraft = fmod(scene.rotateSpaceCraft - 7, 360);
break;
default:
break;
}
glutPostRedisplay();
}
int main(int argc, char** argv)
{
/* window management code ... */
/* initialises GLUT and processes any command line arguments */
glutInit(&argc, argv);
/* use single-buffered window and RGBA colour model */
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
/* window width = 400 pixels, height = 400 pixels */
glutInitWindowSize (600, 600);
/* window upper left corner at (100, 100) */
glutInitWindowPosition (250, 50);
/* creates an OpenGL window with command argument in its title bar */
glutCreateWindow ("Asteroids");
glutKeyboardFunc(key);
glutSpecialFunc(specialKey);
init();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}

Answer 1

Step by Step you could do it this way:
Create a spaceship object which holds rotation of the spaceship
Create a bullet object ( struct or class ), which holds the vector of the bullet, the start position ( and the time at which the bullet has been fired, you will need it later )
Create a scene object which holds a list of all bullets fired and the spaceship
Catch Key Events from Windows or any other platform you are using
When the bullet is fired, use the rotation and position of your spaceship to determine the position of the new bullet and the vector of it.
Some pseudo code for the structure:
struct Bullet {
Vector3 Position;
Vector3 Rotation;
}
struct {
struct {
Vector3 Position;
Vector3 Rotation;
} Ship;
Vector<Bullet*> Bullets;
} Scene;
....:
void Update(void) {
if (Key.IsPressed(Space)) {
CreateNewBullet();
}
}
void UpdateBullets(void) {
for (Bullet bullet in Scene.Bullets)
{
// Delete bullets here if not longer used
// and move all others
}
}
void Draw(void) {
// Draw spaceship here
....
// Draw bullets
for (Bullet bullet in Scene.Bullets) {
DrawBullet(bullet);
}
}

Answer 2

I also recommend using VBO but if you want to do it with the deprecated set then what you are looking for is to render a bullet similar to your SpaceCraft() function. A bullet will need to travel so you will need to translate its position every frame.
glTranslatef(x, 0.0f, 0.0f);
This will translate the bullet x amount in the x axis. Therefore you will need a variable that increments certain amount per frame and passes it to your new Bullet() function. You can store the translation X amount just like you did with struct
{
float rotateSpaceCraft;
} scene;