Programming a specific 3d (star-like) model in OpenGL?

Question

How can I create the following model:

by starting from the first drawing. Could it be programmed in OpenGL entirely or should I use other software like 3d Studio Max or Unity? Are there some specific algorithms that should be used?

Answer 1

Yes this can be done in C++/OpenGL

1. create random curves emitting from center

   simple 3D quadratic polynomial curve will fit the bill.

2. convert the curves to cones

   simply interpolate points along each curve and use it as a center for the cone slice. The direction is set by the previous or next point along the curve. Interpolate the cone slices and add their points to some point list. See:

   • Smoothly connecting circle centers

3. create faces

   simply connect the computed points to form the cones using any primitive ... I would suggest GL_QUADs...

4. core

   if you want to add also the core (nuclei?) it can be done a s a sphere with some noise added to its surface and probably some filtering to smooth it a bit...

Here simple curve generation C++ example:

List<double> pnt;
void spicule_init()
    {
    double t,tt,x,y,z;
    double a0[3],a1[3],a2[3];
    int ix0,ix,i,j;
    Randomize();
    for (i=0;i<20;i++)                      // cones
        {
        // random quadratic 3D curve coeff
        for (j=0;j<3;j++)
            {
            a0[j]=0.0;                      // center (0,0,0)
            a1[j]=2.0*(Random()-0.5);       // main direction
            a2[j]=1.0*(Random()-0.5);       // curvature
            }
        // curve interpolation
        ix0=pnt.num;
        for (t=0.0;t<=1.0;t+=0.04)
         for (tt=t*t,j=0;j<3;j++)
          pnt.add(a0[j]+(a1[j]*t)+(a2[j]*tt));
        }
    }

Preview of the generated points:

[Edit1] When added the cones,normals and faces it looks like this:

Its far from perfect but I think is a good start point. Just tweak the radius r and the curve coefficients a1[],a2[] to achieve desired shape ... and may be add the core and or check for self intersections too, I am too lazy to do that...

Here the updated C++/GL code:

//---------------------------------------------------------------------------
List<double> pnt,nor;   // points, normals
List<int> fac;          // QUAD faces
//---------------------------------------------------------------------------
void Circle3D(List<double> &pnt,List<double> &nor,double *p0,double *n0,double r,int N)
    {
    int i;
    double a,da=divide(pi2,N),p[3],dp[3],x[3],y[3];
    vector_ld(x,1.0,0.0,0.0); if (fabs(vector_mul(x,n0)>0.7)) vector_ld(x,0.0,1.0,0.0);
    vector_mul(x,x,n0); vector_one(x,x);
    vector_mul(y,x,n0); vector_one(y,y);
    for (a=0.0,i=0;i<N;i++,a+=da)
        {
        vector_mul( p,x,cos(a));
        vector_mul(dp,y,sin(a));
        vector_add(p,p,dp); nor.add(p[0]); nor.add(p[1]); nor.add(p[2]);
        vector_mul(p,p,r);
        vector_add(p,p,p0); pnt.add(p[0]); pnt.add(p[1]); pnt.add(p[2]);
        }
    }
//---------------------------------------------------------------------------
void spicule_init() // generate random spicule mesh
    {
    const int N=36;                         // points/circle
    const int N3=3*N;
    double t,tt,x,y,z,r;
    double a0[3],a1[3],a2[3];
    double p[3],n[3];
    int e,i,j,i00,i01,i10,i11;
    Randomize();
    pnt.num=0; nor.num=0; fac.num=0;
    for (i=0;i<20;i++)                      // cones
        {
        // random quadratic 3D curve coeff
        for (j=0;j<3;j++)
            {
            a0[j]=0.0;                      // center (0,0,0)
            a1[j]=2.0*(Random()-0.5);       // main direction and size
            a2[j]=1.0*(Random()-0.5);       // curvature
            }
        // curve interpolation
        vector_ld(n,0.0,0.0,0.0);
        for (e=0,t=0.05;t<=1.0;t+=0.05)
            {
            // points,normals
            for (tt=t*t,j=0;j<3;j++) p[j]=a0[j]+(a1[j]*t)+(a2[j]*tt);
            r=0.15*(1.0-pow(t,0.1));        // radius is shrinking with t
            vector_sub(n,p,n);              // normal is p(t)-p(t-dt)
            Circle3D(pnt,nor,p,n,r,N);      // add circle to pnt (N points)
            vector_copy(n,p);               // remember last point
            // faces
            if (!e){ e=1; continue; }       // ignore first slice of cone
            i00=pnt.num- 3; i10=i00-N3;
            i01=pnt.num-N3; i11=i01-N3;
            for (j=0;j<N;j++)
                {
                fac.add(i00);
                fac.add(i01);
                fac.add(i11);
                fac.add(i10);
                i00=i01; i01+=3;
                i10=i11; i11+=3;
                }
            }
        }
    }
//---------------------------------------------------------------------------
void spicule_draw() // render generated spicule
    {
    glEnable(GL_CULL_FACE);
    glFrontFace(GL_CCW);
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    int i,j;
    glColor3f(1.0,1.0,1.0);
    glBegin(GL_QUADS);
    for (i=0;i<fac.num;i++)
        {
        j=fac.dat[i];
        glNormal3dv(nor.dat+j);
        glVertex3dv(pnt.dat+j);
        }
    glEnd();
    }
//---------------------------------------------------------------------------

If you do not know how to compute vector operations like cross/dot products or absolute value see:

// cross product: W = U x V
W.x=(U.y*V.z)-(U.z*V.y)
W.y=(U.z*V.x)-(U.x*V.z)
W.z=(U.x*V.y)-(U.y*V.x)
// dot product: a = (U.V)
a=U.x*V.x+U.y*V.y+U.z*V.z
// abs of vector a = |U|
a=sqrt((U.x*U.x)+(U.y*U.y)+(U.z*U.z))

vector_mul(a[3],b[3],c[3]) is cross product a = b x c
a = vector_mul(b[3],c[3]) is dot product a = (b.c)
vector_one(a[3],b[3]) is unit vector a = b/|b|
vector_copy(a[3],b[3]) is just copy a = b
vector_add(a[3],b[3],c[3]) is adding a = b + c
vector_sub(a[3],b[3],c[3]) is substracting a = b - c
vector_neg(a[3],b[3]) is negation a = -b
vector_ld(a[3],x,y,z) is just loading a = (x,y,z)

Also some (if not all the) Vector math used can be found here:

• Understanding 4x4 homogenous transform matrices

I also use mine dynamic list template so:

List<double> xxx; is the same as double xxx[];
xxx.add(5); adds 5 to end of the list
xxx[7] access array element (safe)
xxx.dat[7] access array element (unsafe but fast direct access)
xxx.num is the actual used size of the array
xxx.reset() clears the array and set xxx.num=0
xxx.allocate(100) preallocate space for 100 items