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phil67rpg

OpenGL asteroids and screen wrapping

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I am building an asteroids game using opengl.I am unsure of how to implement the wrapping of the space ship around the screen.also when I hit the up key initally nothing happens.

#include <stdlib.h>
#include <glut.h>

static GLfloat spin=0.0;

static GLfloat posY=0.0f;
static GLfloat posX=0.0f;
static GLint w = 800;
static GLint h = 600;

// Called to draw scene
void RenderScene(void)
{
// Clear the window with current clearing color
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glColor3f(1.0f,0.0f,0.0f);
glBegin(GL_LINE_LOOP);
glVertex3f(0.0f,-0.25f,0.0f);
glVertex3f(-0.25f,-0.5f,0.0f);
glVertex3f(-0.5f,-0.5f,0.0f);
glVertex3f(-0.0f,0.5f,0.0f);
glVertex3f(0.5f,-0.5f,0.0f);
glVertex3f(0.25f,-0.5f,0.0f);
glVertex3f(-0.0f,-0.25f,0.0f);
glEnd();
// Flush drawing commands
glutSwapBuffers();
}
void reshape(int w,int h)
{
glViewport(0,0,(GLsizei) w, (GLsizei) h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-10.0f,10.0f,-10.0f,10.0f,-1.0f,1.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void spinDisplay(void)
{
spin+=5;
glutPostRedisplay();
}
// Setup the rendering state
void SetupRC(void)
{
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
glShadeModel(GL_FLAT);
}

void mySpecialKeys(int key, int x, int y)
{
switch (key)
{
case GLUT_KEY_LEFT:
{
glRotatef(3.1415/1.0,0.0,0.0,1.0);
glutIdleFunc(spinDisplay);
break;
}
case GLUT_KEY_RIGHT:
{
glRotatef(-3.1415/1.0,0.0,0.0,1.0);
glutIdleFunc(spinDisplay);
break;
}
case GLUT_KEY_UP:
{
glTranslatef(0.0f,0.1f,0.0f);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glBegin(GL_LINE_STRIP);
glVertex3f(-0.25f,-0.5f,0.0f);
glVertex3f(0.0f,-1.0f,0.0f);
glVertex3f(0.25f,-0.5f,0.0f);
glEnd();
glutSwapBuffers();
break;
}
case GLUT_KEY_DOWN:
{
glTranslatef(0.0f,-0.1f,0.0f);
break;
}
default:
break;
}

}

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

}

// Main program entry point
void main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowPosition(100,100);
glutInitWindowSize(800,600);
glutCreateWindow("Asteroids");
glutDisplayFunc(RenderScene);
glutReshapeFunc(reshape);
glutKeyboardFunc(myKeyboard);
glutSpecialFunc(mySpecialKeys);
SetupRC();
glutMainLoop();
}


thanks in advance.

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Quote:
Original post by phil67rpg
I am building an asteroids game using opengl.I am unsure of how to implement the wrapping of the space ship around the screen.also when I hit the up key initally nothing happens.
*** Source Snippet Removed ***
thanks in advance.


you really shouldn't have glTranslate calls in your input handler, change the players velocity instead and use that to update the players position inside the game loop, do your translates and rotates in the render function based on the players current position and rotation (posX,posY,spin).

Thus you'd want to add some variables:
GLfloat velx=0.0f;
GLfloat vely=0.0f;
GLfloat acceleration=2.0f;
GLfloat maxSpeed=20.0f;
GLfloat turnSpeed=PI/64.0f;
GLfloat borderSize=20.0f;
bool isAccelerating = false;
int rotateDirection = 0; //(-1 for left, +1 for right)

then you get in mySpecialKeys

switch (key) {
case GLUT_KEY_LEFT:
rotateDirection=-1;
break;
case GLUT_KEY_RIGHT:
rotateDirection=1;
break;
case GLUT_KEY_UP:
isAccelerating=true;
break;
}








then you need to add mySpecialKeysUp

void mySpecialKeysUp(int key, int x, int y) {
switch(key) {
case GLUT_KEY_LEFT:
case GLUT_KEY_RIGHT:
rotateDirection=0;
break;
case GLUT_KEY_UP:
isAccelerating=false;
break;
}
}





and in glutMainLoop you do:

if (isAccelerating) {
velX+=cos(spin)*acceleration;
velY+=sin(spin)*acceleration;
float currentSpeed = sqrt(velX*velX+velY*velY);
if (currentSpeed>maxSpeed) {
velX = (velX / currentSpeed)*maxSpeed;
velY = (velY / currentSpeed)*maxSpeed;
}
}
if (rotateDirection!=0) {
spin+=rotateDirection*turnSpeed;
}

//update the ships positio based on current velocity
posX+=velX;
posY+=velY;

// to wrap the ship around the screen you simply do:

if (posX<0-borderSize) {
posX=w+borderSize;
} else if (posX>w+borderSize) {
posX=0-borderSize;
}

if (posY<0-borderSize) {
posY=h+borderSize;
} else if (posY>h+borderSize) {
posY=0-borderSize;
}
/*
If objects are bigger than the border you have to adjust your render function to render partially wrapped objects, while not that difficult it might be a bit too much right now.
*/





Note: The keyhandling isn't perfect in my example, it doesn't track the status of left/right keys so if you press and hold both the left and right key at the same time the game would turn in whatever direction you pressed last and would stop when either key is released, changing that is fairly trivial though.

Then finally in your render function

void RenderScene() {
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glColor3f(1.0f,0.0f,0.0f);
glLoadIdentity();
glRotatef(spin,0.0f,0.0f,1.0f);
glTranslatef(posX,posY,0.0f);
glBegin(GL_LINE_LOOP);
glVertex3f(0.0f,-0.25f,0.0f);
glVertex3f(-0.25f,-0.5f,0.0f);
glVertex3f(-0.5f,-0.5f,0.0f);
glVertex3f(-0.0f,0.5f,0.0f);
glVertex3f(0.5f,-0.5f,0.0f);
glVertex3f(0.25f,-0.5f,0.0f);
glVertex3f(-0.0f,-0.25f,0.0f);
glEnd();
glutSwapBuffers();
}




(There are a bunch of other changes that should be made, the code is still heavily reliant on global state and some structs or classes to encapsulate the player data wouldn't hurt but for now its probably not that important)

[Edited by - SimonForsman on December 4, 2010 1:19:15 AM]

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Always call glLoadIdentity before drawing your ship. Due to the nature of 3d rendering you can't achieve the wrap. You must have variables for the ship x,y. At any given time you personally don't know how much you have glTranslated your ship and thats why you want to do what the other guy suggested.

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These guys are right. You have to get used to using variables for tracking your objects, not 3d trasforms. You then use 3d transforms once a frame based on the varialbes you have used in order to render your objects.

On the bright side, that is the future for your games programming anyway. At some point, instead of having simple variables, you will have objects(with internal variables) that move themselves, render themselves, and update their own positions. For something simple as this, I wouldn't necesarily recommend trying that route, though it would be good for learning. But....for the future, you are better off getting used to this type of system because that is pretty much the way to go. It also makes for code organization, which will come in time, and will the need.

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