Question

At the moment I am building a cloth physics app using OpenFrameworks. I’m new to C++, for a heads up.

In my app, two ‘neighbor’ particle objects are passed to a spring object as pointers. As a test, I have the spring object draw lines between the two particles (between their 3d vector positions). For some reason, these lines are different every time I run the program, even though no random values are involved. When I cout the values of the particle positions from the spring struct, they often are ridiculous values like -4.15301e-12. I’m following example code almost verbatim, so I’m not really sure where I’m going wrong.

Here is the example code I’m following:

https://sites.google.com/site/ofauckland/examples/17-cloth-physics

Here is my Spring struct:

#pragma once
#include "ofMain.h"
#include "Particle.h"

struct Spring {

    float k, restLength;
    Particle *a, *b;
    ofVec3f posA, posB;

    Spring(Particle *a, Particle *b, float k = .2) : a(a), b(b), k(k) {
        restLength = (b->pos - a->pos).length();
    }

    void update() {
        posA = a->pos;
        posB = b->pos;
    }
    void draw() {
        ofSetLineWidth(5);
        ofSetColor(0, 255, 0);
        ofLine(posA.x, posA.y, posB.x, posB.y);
    }
};                             

The particle struct:

#pragma once
#include "ofMain.h"

struct Particle {

    ofVec3f pos;

    Particle(ofVec3f pos) : pos(pos) {
    }

    void update() {
    }
    void draw() {
        ofSetColor(ofRandom(255), 0, 0);
        ofFill();
        ofCircle(pos.x, pos.y, 3); 
    }
};       

And this is where I pass the two particles to the spring as pointers:

#pragma once
#include "ofMain.h" 
#include "Particle.h"
#include "Spring.h"

struct Petal {

    float maxWidth, spacing;
    vector<Particle> particles;
    vector<Spring> springs;

    Petal(float maxWidth, float spacing) : maxWidth(maxWidth), spacing(spacing) {        
        setupPoints();
    }

    void setupPoints() {
        float x = 0;
        float y = 0;

        for(int r = 1; r <= maxWidth; r++) {
            x = (int)(r / 2) * -spacing;
            y += spacing;
            for(int c = 1; c <= r; c++) { 
                ofVec3f pos = ofVec3f(x, y, 0);
                Particle p(pos);
                particles.push_back(p); 
                x+=spacing;
            }
        }

        for(int r = maxWidth; r > 0; r--) {
            x = (int)(r / 2) * -spacing;
            y += spacing;
            for(int c = 1; c <= r; c++) { 
                ofVec3f pos = ofVec3f(x, y, 0);  
                Particle p(pos);
                particles.push_back(p); 
                x+=spacing;
            }
        }

        //find neighbors
        for(int i = 0; i < particles.size(); i++) {
            Spring s(&particles[i], &particles[findNeighbor(i)]);
            springs.push_back(s);
        }
    }

    int findNeighbor(int pIndex) {
        float leastDist = 0;
        float leastDistIndex = 0;
        for(int i = 0; i < particles.size(); i++) {
            if(i != pIndex) {
                float distance = particles[pIndex].pos.distance(particles[i].pos);
                if(abs(distance) < abs(leastDist) || leastDist == 0) {
                        leastDist = distance;
                        leastDistIndex = i;
                }
            }
        }
        return leastDistIndex;

    }

    void update() {   
        for(int i = 0; i < particles.size(); i++) {
            particles[i].update();   
        }
        for(int s = 0; s < springs.size(); s++) {
            springs[s].update();
        }
    }
    void draw() {
        for(int i = 0; i < particles.size(); i++) {
            particles[i].draw();   
        }
        for(int s = 0; s < springs.size(); s++) {
            springs[s].draw();
        }
    }
};

This is what happens. What’s strange is that some of the springs seem to be in the correct position.

Please let me know if I can clarify something.

Thanks!

Answer 1

Your particles vector holds Particles by value, and the vector can copy and move these values around. When you pass Particles as pointers to the Spring, you are passing the address of something that might not be there at some point in the future. I am not sure if this is the problem, but it certainly is something that needs fixing.