problem with gravity

[tatsu0616]

im trying to make a program that simulates gravity as if your mouse pointer was a large object, and particles were gravitating around it. but when i run it, it crashes instantly, and i can't figure out why. i am relatively new to c++.

#include <iostream>
#include <SDL/SDL.h>
#include <vector>
#include <math.h>
#include <stdio.h>
#include <time.h>

using namespace std;

//set important variables
int screenWidth = 640;
int screenHeight = 480;
int mouseX = screenWidth / 2;
int mouseY = screenHeight / 2;

//class for the mouse pointer that everything gravitates around
class foci
{
public:
    int x, y;   //x and y coordinates
    double mass;    //mass of the object
    foci(int X, int Y, int Mass);
    void move();    //track mouse movement and move object to that location
};

foci::foci(int X, int Y, int Mass)
{
    x = X;
    y = Y;
    mass = Mass;
}

void foci::move()
{
    x = mouseX;
    y = mouseY;
}

//particles
class particle
{
public:
    int x, y, mass;    //x and y locations, and mass
    float xvel, yvel, xacc, yacc, distance, theta, gravForce;   //speed, acceleration, distance from foci, angle between foci and particle, and the force of gravity
    Uint32 color;     //color of the particle
    particle(int X, int Y, float Xvel, float Yvel,int Mass, Uint32 Color);
    void calcDistance(foci b);     //calculate distance from foci
    void calcGravForce(foci a);    //calculate the gravitational force foci has on particle
    void calcTheta(foci c);        //calculate the angle between foci and particle
    void calcAcceleration();       //calculate the acceleration of the particle
    void calcVelocity();           //calculate the velocity of the particle
    void move();                   //move the particle
    void show();                   //show the particle
};

particle::particle(int X, int Y, float Xvel, float Yvel,int Mass, Uint32 Color)
{
    x = X;
    y = Y;
    xvel = Xvel;
    yvel = Yvel;
    mass = Mass;
    color = Color;
}

void particle::calcDistance(foci b)
{
    distance = hypot((b.x - x), (b.y - y));
}

void particle::calcGravForce(foci a)
{
    gravForce = ((a.mass * mass) / (distance * distance));   //ignored gravitational constant(assumed its 1, for simplicity sake)
}

void particle::calcTheta(foci c)
{
    theta = atan2((c.y - y), (c.x - x));
}

void particle::calcAcceleration()
{
    //A = F/M
    xacc = (gravForce * cos(theta))/mass;
    yacc = (gravForce * sin(theta))/mass;
}

void particle::calcVelocity()
{
    //Vf(velocity final) = A*time(which is each frame) - Vi(velocity initial)
    xvel = xacc - xvel;
    yvel = yacc - yvel;
}

void particle::move()
{
    //move the particle
    x += xvel;
    y += yvel;

    //check boundaries
    if(x == 0)
        xvel = xvel * -1;
    if(y == 0)
        yvel = yvel * -1;
    if(y > screenHeight)
        yvel = yvel * -1;
    if(x > screenWidth)
        xvel = xvel * -1;
}

void particle::show()
{
    //show the particle
    Uint8* pixels=(Uint8*)SDL_GetVideoSurface()->pixels;
    Uint8* pixel=pixels+(int)y*SDL_GetVideoSurface()->pitch+(int)x;
    *pixel=color;
}




int main (int argc, char** argv)
{
    //initialize
    SDL_Init(SDL_INIT_EVERYTHING);
    SDL_Surface* screen;
    screen = SDL_SetVideoMode(screenWidth, screenHeight, 8, SDL_SWSURFACE);
    Uint32 start;

    //seed random # generator
    srand(time(NULL));
    const int FPS = 30;
    bool running = true;

    //max particles on screen
    int maxParticles = 5000;

    //make the mouse pointer foci
    foci Luna(screenWidth / 2, screenHeight, 999);

    //make the vector
    vector<particle*> particles;

    //fill the vector
    for(int i = 0; i < maxParticles; ++i)
        particles.push_back(new particle((rand()%screenWidth) - 5, (rand()%screenHeight) - 5, 0, 0, 100, SDL_MapRGB(screen->format, rand()%255, rand()%255, rand()%255)));

    while(running)
    {
        start = SDL_GetTicks();
        SDL_Event event;
        while(SDL_PollEvent(&event))
        {
            switch(event.type)
            {
                case SDL_QUIT:
                    running = false;
                    break;
                case SDL_MOUSEMOTION:
                    mouseX = event.motion.x;
                    mouseY = event.motion.y;

            }
        }
        //check mouse pointer location
        Luna.move();

        //make screen black
        SDL_FillRect(screen, &screen->clip_rect, SDL_MapRGB(screen->format, 0, 0, 0));

        for(int j = 0; j < maxParticles; ++j)
        {
            //make all the calculations for the variables and move the particles
            particles[j]->calcDistance(Luna);
            particles[j]->calcGravForce(Luna);
            particles[j]->calcTheta(Luna);
            particles[j]->calcAcceleration();
            particles[j]->calcVelocity();
            particles[j]->move();
            particles[j]->show();
        }

        SDL_Flip(screen);


        //regulate fps
        if(1000/FPS>SDL_GetTicks()-start)
                SDL_Delay(1000/FPS-(SDL_GetTicks()-start));


    }
    SDL_Quit();
    return 0;
}

[Smac89]

I would suggest perhaps adding a break to line 163.

[greenleaf800073]

Is that necessary Smac?

Um tatsu, would an arrow key controlled cursor be easier to handle? And what if particles spawn on each other?

[ne555]

$ gdb a.out
> run
Program received signal SIGSEGV, Segmentation fault.
0x0000000000401432 in particle::show (this=0x71aff0) at foo.cpp:117
117 *pixel=color;

> print pixel
$1 = (Uint8 *) 0x77ee09 <Address 0x77ee09 out of bounds>



Also, you are leaking memory with your `particles' vector. ¿is there a good reason to use dynamic allocation there?

[tatsu0616]

ne555,
as i said, im new to c++, the only way i learned to make something hold all the particles was to make the vector in that way. i was also told before that my way of printing the pixels to the screen was not the best, but i have never found a better way to do it. any help would be appreciated. also, i use codeblocks, which babys me very much in terms of error finding lol, so I cant understand the error message you posted. sorry X(

greenleaf800073,
i think that setting the x and y to the mouses x and y was pretty easy in itself, but if it would cause any disturbance to the program then i could easily make it keyboard controlled

thanks everyone :)

[ne555]

void particle::show()
{
    //show the particle
    Uint8* pixels=(Uint8*)SDL_GetVideoSurface()->pixels;
    Uint8* pixel=pixels+(int)y*SDL_GetVideoSurface()->pitch+(int)x;
    *pixel=color;
}

The values of `x' and `y' are referring to a point that it is outside the screen.
You try to access something that you don't own, producing the crash.


About the vector

    vector<particle> particles; //holding objects instead of pointers

    //fill the vector
    for(int i = 0; i < maxParticles; ++i)
        particles.push_back(particle(/**/));

    particles[j].calcDistance(Luna); //accessing 

When the vector goes out of scope (the function ends), all the particles will be destroyed.

[ne555]

I was playing a little.

void particle::show()
{
    if(x<0 or y<0 or x>=screenWidth or y>=screenHeight) //out of bounds
      return;
   //...
}

so you can see it.

It looks awful. I found two mistakes

void particle::calcVelocity()
{
    //Vf(velocity final) = A*time(which is each frame) - Vi(velocity initial)
    //nope, A = (Vf-Vi)/t so Vf = A*t + Vi

    xvel += xacc;
    yvel += yacc;
}

//particles
class particle
{
public:
    /*int*/double x, y;

The force is really small, by using integers for the coordinates, you'll have artifacts in the movements.
Of course, you'll need integers in `show()' as you use pointer arithmetic.


Another thing, `cos(theta)' is equivalent to `(f.x-x)/distance' where `f' is the foci

[greenleaf800073]

Can you use scroll wheel or up and down buttons to alter gravity?

and colored particles would be beautiful

[tatsu0616]

Thanks ne555, I'll try it next time I can get to my computer. And I can't believe I got the formula wrong X(

And greenleaf, I could possibly use the scroll wheel, but I want to get this working first lol, and the particles are colored. The color is randomly generated :)

[greenleaf800073]

yay