gravity.cpp

/*##############################################
##                                            ##
##  Program: Gravity simulation               ##
##  Author: Suraj Kumar Jana        ##
##  E-mail: surajjana2@gmail.com           ##
##  WebSite: http://surajjana.com

key s: switch between simple tiny particles and accelerated particles
left_mouse_button: add tiny particles, also accelerated particles in "s" mode
right_mouse_button: add Huge yellow particle
middle_mouse_button: remove all particles
##                                            ##
##############################################*/

#include <GL/glut.h>
#include <vector>
#include <cmath>

struct Particle {
	float x;
	float y;
	float r;
	float vx;
	float vy;
	float m;
	float color[3];
};

struct Line {
	float x1;
	float y1;
	float x2;
	float y2;
} line;

void timer(int = 0);
void display();
void mouse(int, int, int, int);
void mouseMotion(int, int);
void addParticle(float, float, bool = true, float = 0, float = 0);
void removeParticles();
void keyboard(unsigned char, int, int);

int Mx, My, WIN;
bool PRESSED_LEFT = false, PRESSED_RIGHT = false,
     PRESSED_MIDDLE = false, SPEED_PARTICLES = false;

std::vector<Particle> particles;

int main(int argc, char **argv)
{
	Particle p;
	//initial centered Huge mass particle
	p.x = 0;
	p.y = 0;
	p.vx = p.vy = 0;
	p.m = 10000;
	p.r = 10;
	p.color[0] = 1;
	p.color[1] = 1;
	p.color[2] = 0;
	particles.push_back(p);

	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
	glutInitWindowSize(500, 500);
	glutInitWindowPosition(50, 50);
	WIN = glutCreateWindow("Gravity");

	glClearColor(0, 0, 0, 1);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	glOrtho(-250.0, 250.0, 250.0, -250.0, 0, 1);

	glutDisplayFunc(display);
	glutMouseFunc(mouse);
	glutMotionFunc(mouseMotion);
	glutKeyboardFunc(keyboard);
	timer();

	glutMainLoop();
	return 0;
}

void timer(int)
{
	display();
	if(PRESSED_LEFT && !SPEED_PARTICLES)
	{
		addParticle(10, 3); //add tiny particle
		PRESSED_LEFT = false;
	}

	if(PRESSED_RIGHT)
	{
		addParticle(10000, 10, 0); //add huge particle
		PRESSED_RIGHT = false;
	}

	if(PRESSED_MIDDLE)
		removeParticles(); //remove all particles

	for(int i = 0; i < particles.size(); i++)
	{
		Particle &p = particles[i];
		bool not_fall = true;
		for(int j = 0; j < particles.size(); j++)
		{
			if(j == i || p.m >= 10000) // we consider the 10000 as infinit (big mass) so this particles won't move
				continue;

			const Particle &p1 = particles[j];

			float d = sqrt((p1.x - p.x)*(p1.x - p.x) + (p1.y - p.y)*(p1.y - p.y));

			if(d > p1.r)
			{
				p.vx += 0.03 * p1.m / (d*d) * (p1.x - p.x)/d; //f = ma => a = f/m
				p.vy += 0.03 * p1.m / (d*d) * (p1.y - p.y)/d;
			}
			else
				not_fall = false;
		}

		if(not_fall)
		{
			p.x += p.vx;
			p.y += p.vy;
		}
		else
			particles.erase(particles.begin()+i);
	}

	glutTimerFunc(1, timer, 0);
}

void display()
{
	glClear(GL_COLOR_BUFFER_BIT);

	//draw the drag line
	glColor3f(0, 0, 1);
	glBegin(GL_LINES);
		glVertex2f(line.x1, line.y1);
		glVertex2f(line.x2, line.y2);
	glEnd();

	//draw particles
	for(int i = 0; i < particles.size(); i++)
	{
		Particle &p = particles[i];
		glColor3f(p.color[0], p.color[1], p.color[2]);
		glBegin(GL_POLYGON);
		for(float a = 0; a < 2*M_PI; a+=0.2)
			glVertex2f(p.r*cos(a) + p.x, p.r*sin(a) + p.y);
		glEnd();
	}

	glFlush();
	glutSwapBuffers();
}

void mouse(int button, int state, int x, int y)
{
	//set the coordinates
	Mx = x - 250;
	My = y - 250;

	//add speed particles by line draging
	if(SPEED_PARTICLES)
	{
		if(line.x2 != 0 && line.y2 != 0 && state == GLUT_UP && PRESSED_LEFT)
			addParticle(100, 5, 1, line.x1 - line.x2, line.y1 - line.y2); //add a speed particle
		else
		{
			line.x1 = line.x2 = Mx;
			line.y1 = line.y2 = My;
		}
	}

	//check which button is pressed
	if(button == GLUT_LEFT_BUTTON)
		PRESSED_LEFT = state == GLUT_DOWN;
	else if(button == GLUT_RIGHT_BUTTON)
		PRESSED_RIGHT = state == GLUT_DOWN;
	else if(button == GLUT_MIDDLE_BUTTON)
		PRESSED_MIDDLE = state == GLUT_DOWN;
}

void mouseMotion(int x, int y)
{
	Mx = x - 250;
	My = y - 250;

	//end of line with draging
	if(SPEED_PARTICLES && PRESSED_LEFT)
	{
		line.x2 = Mx;
		line.y2 = My;
	}
}

void addParticle(float m, float r, bool randColor, float vx, float vy)
{
	Particle p;
	p.x = Mx;
	p.y = My;
	p.vx = vx / 30; // /30 in case it is a speed particle,
	p.vy = vy / 30; // slow down the speed a little
	p.m = m;
	p.r = r;
	if(randColor)
	{
		p.color[0] = rand()%200 / 200.0;
		p.color[1] = rand()%200 / 200.0;
		p.color[2] = rand()%200 / 200.0;
	}
	else // if is huge particle make it yellow
	{
		p.color[0] = 1;
		p.color[1] = 1;
		p.color[2] = 0;
	}
	particles.push_back(p);

	if(line.x1 != 0)
		line.x1 = line.x2 = line.y1 = line.y2 = 0;
}

void removeParticles()
{
	for(int i = 0; i < particles.size(); i++)
		particles.pop_back();
}

void keyboard(unsigned char key, int x, int y)
{
	switch(key)
	{
		case 's':
			SPEED_PARTICLES = !SPEED_PARTICLES;
			break;
		case 27:
			removeParticles();
			glutDestroyWindow(WIN);
			exit(0);
			break;
	}
}