File: Planet.c

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/* Copyright (c) 2007 Scott Lembcke
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */
 
#include <stdlib.h>
#include <math.h>

#include "chipmunk.h"
#include "drawSpace.h"
#include "ChipmunkDemo.h"

static cpSpace *space;
static cpBody *planetBody;

static cpFloat gravityStrength = 5.0e6f;

static void
update(int ticks)
{
	int steps = 1;
	cpFloat dt = 1.0f/60.0f/(cpFloat)steps;
	
	for(int i=0; i<steps; i++){
		cpSpaceStep(space, dt);
		
		// Update the static body spin so that it looks like it's rotating.
		cpBodyUpdatePosition(planetBody, dt);
	}
}

static void
planetGravityVelocityFunc(cpBody *body, cpVect gravity, cpFloat damping, cpFloat dt)
{
	// Gravitational acceleration is proportional to the inverse square of
	// distance, and directed toward the origin. The central planet is assumed
	// to be massive enough that it affects the satellites but not vice versa.
	cpVect p = body->p;
	cpFloat sqdist = cpvlengthsq(p);
	cpVect g = cpvmult(p, -gravityStrength / (sqdist * cpfsqrt(sqdist)));
	
	cpBodyUpdateVelocity(body, g, damping, dt);
}

static cpVect
rand_pos(cpFloat radius)
{
	cpVect v;
	do {
		v = cpv(frand()*(640 - 2*radius) - (320 - radius), frand()*(480 - 2*radius) - (240 - radius));
	} while(cpvlength(v) < 85.0f);
	
	return v;
}

static void
add_box()
{
	const cpFloat size = 10.0f;
	const cpFloat mass = 1.0f;
	
	cpVect verts[] = {
		cpv(-size,-size),
		cpv(-size, size),
		cpv( size, size),
		cpv( size,-size),
	};
	
	cpFloat radius = cpvlength(cpv(size, size));

	cpBody *body = cpSpaceAddBody(space, cpBodyNew(mass, cpMomentForPoly(mass, 4, verts, cpvzero)));
	body->velocity_func = planetGravityVelocityFunc;
	body->p = rand_pos(radius);

	// Set the box's velocity to put it into a circular orbit from its
	// starting position.
	cpFloat r = cpvlength(body->p);
	cpFloat v = cpfsqrt(gravityStrength / r) / r;
	body->v = cpvmult(cpvperp(body->p), v);

	// Set the box's angular velocity to match its orbital period and
	// align its initial angle with its position.
	body->w = v;
	cpBodySetAngle(body, cpfatan2(body->p.y, body->p.x));

	cpShape *shape = cpSpaceAddShape(space, cpPolyShapeNew(body, 4, verts, cpvzero));
	shape->e = 0.0f; shape->u = 0.7f;
}

static cpSpace *
init(void)
{
	planetBody = cpBodyNew(INFINITY, INFINITY);
	planetBody->w = 0.2f;
	
	cpResetShapeIdCounter();
	
	space = cpSpaceNew();
	cpSpaceResizeActiveHash(space, 30.0f, 10000);
	space->iterations = 20;
	
	for(int i=0; i<30; i++)
		add_box();
	
	cpShape *shape = cpSpaceAddShape(space, cpCircleShapeNew(planetBody, 70.0f, cpvzero));
	shape->e = 1.0f; shape->u = 1.0f;
	shape->layers = NOT_GRABABLE_MASK;
	
	return space;
}

static void
destroy(void)
{
	cpBodyFree(planetBody);
	cpSpaceFreeChildren(space);
	cpSpaceFree(space);
}

chipmunkDemo Planet = {
	"Planet",
	NULL,
	init,
	update,
	destroy,
};