Piecewise linear integer curve interpolation in C#/Unity3D
Solution 1:
I would use this interpolation cubic:
x=a0+a1*t+a2*t*t+a3*t*t*t
y=b0+b1*t+b2*t*t+b3*t*t*t
where a0..a3
are computed like this:
d1=0.5*(p2.x-p0.x);
d2=0.5*(p3.x-p1.x);
a0=p1.x;
a1=d1;
a2=(3.0*(p2.x-p1.x))-(2.0*d1)-d2;
a3=d1+d2+(2.0*(-p2.x+p1.x));
b0 .. b3
are computed in same way but use y
coordinates of course
p0..p3
are control points for cubic interpolation curve
t = < 0.0 , 1.0 >
is curve parameter from p1
to p2
This ensures that position and first derivation is continuous (c1). If you want to do this on integer math then just scale ai,bi
ant t
accordingly. You can also add as many dimensions as you need in the same manner
Now you need some parameter to go through your interpolation points for example u = <0 , N-1>
p(0..N-1)
are your control points list
u = 0
means start point p(0)
u = N-1
means end point p(N-1)
P0..P3
are control points used for interpolation
So you need to compute t
and select which points to use for interpolation
double t=u-floor(u); // fractional part between control points
int i=floor(u); // integer part points to starting control point used
if (i<1) { P0=p( 0),P1=p( 0),P2=p( 1),P3=p( 2); } // handle start edge case
else if (i==N-1) { P0=p(N-2),P1=p(N-1),P2=p(N-1),P3=p(N-1); } // handle end edge case
else if (i>=N-2) { P0=p(N-3),P1=p(N-2),P2=p(N-1),P3=p(N-1); } // handle end edge case
else { P0=p(i-1),P1=p(i ),P2=p(i+1),P3=p(i+2); }
(x,y) = interpolation (P0,P1,P2,P3,t);
If you want to do this on integer math then just scale u,t
accordingly. If N<3
then use linear interpolation ... or duplicate end points until N>=3
[edit1] linear interpolation approach
struct pnt { int x,y; };
pnt interpolate (pnt *p,int N,int x)
{
int i,j;
pnt p;
for (j=1,i=N-1;j<i;j<<=1); j>>=1; if (!j) j=1; // this just determine max mask for binary search ... can do it on p[] size change
for (i=0;j;j>>=1) // binary search by x coordinate output is i as point index with p[i].x<=x
{
i|=j;
if (i>=N) { i-=j; continue; }
if (p[i].x==x) break;
if (p[i].x> x) i-=j;
}
p.x=x;
p.y=p[i].y+((p[i+1].y-p[i].y)*(x-p[i].x)/(p[i+1].x-p[i].x))
return p;
}
add edge cases handling like x
is out of points bound or point list is too small