December 1999
Discussed here are a number of interpolation methods, this is by no means an exhaustive list but the methods shown tend to be those in common use in computer graphics. The main attributes is that they are easy to compute and are stable. Interpolation as used here is different to "smoothing", the techniques discussed here have the characteristic that the estimated curve passes through all the given points. The idea is that the points are in some sense correct and lie on an underlying but unknown curve, the problem is to be able to estimate the values of the curve at any position between the known points.
Linear interpolation is the simplest method of getting values at positions in between the data points. The points are simply joined by straight line segments. Each segment (bounded by two data points) can be interpolated independently. The parameter mu defines where to estimate the value on the interpolated line, it is 0 at the first point and 1 and the second point. For interpolated values between the two points mu ranges between 0 and 1. Values of mu outside this range result in extrapolation. This convention is followed for all the subsequent methods below. As with subsequent more interesting methods, a snippet of plain C code will server to describe the mathematics. double LinearInterpolate( double y1,double y2, double mu) { return(y1*(1mu)+y2*mu); } Linear interpolation results in discontinuities at each point. Often a smoother interpolating function is desirable, perhaps the simplest is cosine interpolation. A suitable orientated piece of a cosine function serves to provide a smooth transition between adjacent segments. double CosineInterpolate( double y1,double y2, double mu) { double mu2; mu2 = (1cos(mu*PI))/2; return(y1*(1mu2)+y2*mu2); } Cubic interpolation is the simplest method that offers true continuity between the segments. As such it requires more than just the two endpoints of the segment but also the two points on either side of them. So the function requires 4 points in all labeled y0, y1, y2, and y3, in the code below. mu still behaves the same way for interpolating between the segment y1 to y2. This does raise issues for how to interpolate between the first and last segments. In the examples here I just haven't bothered. A common solution is the dream up two extra points at the start and end of the sequence, the new points are created so that they have a slope equal to the slope of the start or end segment. double CubicInterpolate( double y0,double y1, double y2,double y3, double mu) { double a0,a1,a2,a3,mu2; mu2 = mu*mu; a0 = y3  y2  y0 + y1; a1 = y0  y1  a0; a2 = y2  y0; a3 = y1; return(a0*mu*mu2+a1*mu2+a2*mu+a3); } Paul Breeuwsma proposes the following coefficients for a smoother interpolated curve, which uses the slope between the previous point and the next as the derivative at the current point. This results in what are generally referred to as CatmullRom splines. a0 = 0.5*y0 + 1.5*y1  1.5*y2 + 0.5*y3; a1 = y0  2.5*y1 + 2*y2  0.5*y3; a2 = 0.5*y0 + 0.5*y2; a3 = y1; Hermite interpolation like cubic requires 4 points so that it can achieve a higher degree of continuity. In addition it has nice tension and biasing controls. Tension can be used to tighten up the curvature at the known points. The bias is used to twist the curve about the known points. The examples shown here have the default tension and bias values of 0, it will be left as an exercise for the reader to explore different tension and bias values. /* Tension: 1 is high, 0 normal, 1 is low Bias: 0 is even, positive is towards first segment, negative towards the other */ double HermiteInterpolate( double y0,double y1, double y2,double y3, double mu, double tension, double bias) { double m0,m1,mu2,mu3; double a0,a1,a2,a3; mu2 = mu * mu; mu3 = mu2 * mu; m0 = (y1y0)*(1+bias)*(1tension)/2; m0 += (y2y1)*(1bias)*(1tension)/2; m1 = (y2y1)*(1+bias)*(1tension)/2; m1 += (y3y2)*(1bias)*(1tension)/2; a0 = 2*mu3  3*mu2 + 1; a1 = mu3  2*mu2 + mu; a2 = mu3  mu2; a3 = 2*mu3 + 3*mu2; return(a0*y1+a1*m0+a2*m1+a3*y2); } While you may think the above cases were 2 dimensional, they are just 1 dimensional interpolation (the horizontal axis is linear). In most cases the interpolation can be extended into higher dimensions simply by applying it to each of the x,y,z coordinates independently. This is shown on the right for 3 dimensions for all but the cosine interpolation. By a cute trick the cosine interpolation reverts to linear if applied independently to each coordinate. For other interpolation methods see the Bezier, Spline, and piecewise Bezier methods here. 
Linear
Cosine Cubic Hermite 3D linear 3D cubic 3D Hermite 
Trilinear Interpolation
Written by Paul BourkeJuly 1997
Trilinear interpolation is the name given to the process of linearly interpolating points within a box (3D) given values at the vertices of the box. Perhaps its most common application is interpolating within cells of a volumetric dataset.
Consider a unit cube with the lower/left/base vertex at the origin as shown here on the right. 
The value at position (x,y,z) within the cube will be denoted V_{xyz} and is given by
V_{xyz} =  V_{000} (1  x) (1  y) (1  z) + V_{100} x (1  y) (1  z) + V_{010} (1  x) y (1  z) + V_{001} (1  x) (1  y) z + V_{101} x (1  y) z + V_{011} (1  x) y z + V_{110} x y (1  z) + V_{111} x y z 
In general the box will not be of unit size nor will it be aligned at the origin. Simple translation and scaling (possibly of each axis independently) can be used to transform into and then out of this simplified situation.
Linear Regression
Written by Paul BourkeOctober 1998
Linear regression is a method to best fit a linear equation (straight line) of the form y(x) = a + b x to a collection of N points (x_{i},y_{i}). Where b is the slope and a the intercept on the y axis.
The result will be stated below without derivation, that requires minimisation of the sum of the squared distance from the data points and the proposed line. This function is minimised by calculating the derivative with respect to a and b and setting these to zero. For a more complete derivation see the "Numerical Recipes in C".
The solution is clearer if we define the following
Then
and
And finally the regression coefficient is
This is 0 if there is no linear trend, 1 for perfect linear fit.
Note
 This discussion assumes there is no known variance for the x and y values. There are solutions which can take this into account, this is particularly important if some values are known with less error than others.
 The solution above requires that the slope is not infinite, S_{xx} is not zero.
Example
The following example shows the points and the best fit line as determined using the techniques discussed here.Source
Clinregress.c
C++ contributed by Charles Brown
RegressionLine.cpp
RegressionLine.hpp
Curve Fit Through Arbitrary Points
Written by Paul BourkeAugust 1991
The following introduces a method of immediately deriving a polynomial that passes through an arbitrary number of points. That is, find a polynomial f(x) that passes through the N points
The key to this solution is that we want an exact fit at the points given and we don't care what happens in between those points. The general solution is
To see how this works, consider the product term. When x = x_{i} the product term has the same denominator and numerator and thus equals 1 and therefore contributes y_{i} to the sum. All other terms in the summation contribute 0 since there exists a (x_{j}  x_{j}) in the numerator. Thanks to Simon Stegmaier for pointing out that this is known as a Lagrange Polynomial.
For a numerical example consider the polynomial that passes through the following points
(1,1) (3,3) (4,0) (6,5) 

The function using the above formula is
f(x) = 2 * (x1) * (x3) * (x4) * (x6) / [ (01) * (03) * (04) * (06) ] + 1 * (x0) * (x3) * (x4) * (x6) / [ (10) * (13) * (14) * (16) ] + 3 * (x0) * (x1) * (x4) * (x6) / [ (30) * (31) * (34) * (36) ] + 0 * (x0) * (x1) * (x3) * (x6) / [ (40) * (41) * (43) * (46) ] + 5 * (x0) * (x1) * (x3) * (x4) / [ (60) * (61) * (63) * (64) ] f(x) = (x1) * (x3) * (x4) * (x6) / 36  (x0) * (x3) * (x4) * (x6) / 30 + (x0) * (x1) * (x4) * (x6) / 6 + (x0) * (x1) * (x3) * (x4) / 36 f(x) = 17*x^{4}/90  181*x^{3}/90 + 563*x^{2}/90  163*x/30 + 2
By substituting the values of x for the points the function must pass through (x=0,1,3,4,6) it is easy to see that the expression above achieves the result, namely y=2,1,3,0,5 respectively.
What happens at other points?All bets are off regarding the behavior between the fixed points. The polynomial is of degree N and could violently fly off anywhere. The continuous curve for the numerical example above is shown below.
A competition in the Mathematics news groups in October 1998
From: "John Santos" <santos_john@hotmail.com> Newsgroups: alt.sci.math.probability,alt.tv.mathnet,aus.mathematics Subject: $100.00 prize for the solution Date: Tue, 15 Sep 1998 20:56:50 0700 XNewsreader: Microsoft Outlook Express 4.72.3110.1 XMimeOLE: Produced By Microsoft MimeOLE V4.72.3110.3 NNTPPostingHost: 209.239.197.111 XNNTPPostingHost: 209.239.197.111 MessageID: <35ff36d1.0@blushng.jps.net> XNNTPPostingHost: 209.63.114.134 Hi everyone, My name is John Santos and I am willing to pay anyone $100.00 for the first person to solve this problem. I am providing some hints. This works as follows: you will see nine digits  the tenth digit or letter is the answer. What I need is the formula or mathematical equation to get there... Number Answer  749736637 1 713491024 8 523342792 D 749236871 P 727310078 E 746261832 4 733237527 L 743510589 9 715240338 K 722592910 1 739627071 R The first one with the answer and emails it to me wins the $100.00 Email address: santos_john@hotmail.com Good Luck !!!!
They refused to pay up for this solution !!!!
The following is the solution to the posted problem, although it probably doesn't offer the insight you are seeking, it certainly falls within the scope of competition. To illustrate my solution I will use the following symbols instead of the numbers you used simply to save space. For your second column with letters and numbers use their ASCII values instead, this has no loss of generality as it is a simple 1 to 1 mapping. x1 y1 x2 y2 x3 y3 x4 y4 etc The following is a general method of making a function that passes through any pair of values (xi,yi). y1 (xx2) (xx3) (xx4) f(x) =  (x1x2) (x1x3) (x1x4) y2 (xx1) (xx3) (xx4) +  (x2x1) (x2x3) (x2x4) y3 (xx1) (xx2) (xx4) +  (x3x1) (x3x2) (x3x4) y4 (xx1) (xx2) (xx3) +  (x4x1) (x4x2) (x4x3) etc etc. As you can see, at x=x1 all the terms disappear except the first which equals y1, at x=x2 all the terms disappear except the second which equals y2, etc etc. X Y Number Answer ASCII  749736637 1 49 713491024 8 56 523342792 D 68 749236871 P 80 727310078 E 69 746261832 4 52 733237527 L 76 743510589 9 57 715240338 K 75 722592910 1 49 739627071 R 82The "lovely" expression in this case is
f(x) = + 49 ((x  713491024) / 36245613) ((x  523342792) / 226393845) ((x  749236871) / 499766) ((x  727310078) / 22426559) ((x  746261832) / 3474805) ((x  733237527) / 16499110) ((x  743510589) / 6226048) ((x  715240338) / 34496299) ((x  722592910) / 27143727) ((x  739627071) / 10109566) + 56 ((x  749736637) / 36245613) ((x  523342792) / 190148232) ((x  749236871) / 35745847) ((x  727310078) / 13819054) ((x  746261832) / 32770808) ((x  733237527) / 19746503) ((x  743510589) / 30019565) ((x  715240338) / 1749314) ((x  722592910) / 9101886) ((x  739627071) / 26136047) + 68 ((x  749736637) / 226393845) ((x  713491024) / 190148232) ((x  749236871) / 225894079) ((x  727310078) / 203967286) ((x  746261832) / 222919040) ((x  733237527) / 209894735) ((x  743510589) / 220167797) ((x  715240338) / 191897546) ((x  722592910) / 199250118) ((x  739627071) / 216284279) + 80 ((x  749736637) / 499766) ((x  713491024) / 35745847) ((x  523342792) / 225894079) ((x  727310078) / 21926793) ((x  746261832) / 2975039) ((x  733237527) / 15999344) ((x  743510589) / 5726282) ((x  715240338) / 33996533) ((x  722592910) / 26643961) ((x  739627071) / 9609800) + 69 ((x  749736637) / 22426559) ((x  713491024) / 13819054) ((x  523342792) / 203967286) ((x  749236871) / 21926793) ((x  746261832) / 18951754) ((x  733237527) / 5927449) ((x  743510589) / 16200511) ((x  715240338) / 12069740) ((x  722592910) / 4717168) ((x  739627071) / 12316993) + 52 ((x  749736637) / 3474805) ((x  713491024) / 32770808) ((x  523342792) / 222919040) ((x  749236871) / 2975039) ((x  727310078) / 18951754) ((x  733237527) / 13024305) ((x  743510589) / 2751243) ((x  715240338) / 31021494) ((x  722592910) / 23668922) ((x  739627071) / 6634761) + 76 ((x  749736637) / 16499110) ((x  713491024) / 19746503) ((x  523342792) / 209894735) ((x  749236871) / 15999344) ((x  727310078) / 5927449) ((x  746261832) / 13024305) ((x  743510589) / 10273062) ((x  715240338) / 17997189) ((x  722592910) / 10644617) ((x  739627071) / 6389544) + 57 ((x  749736637) / 6226048) ((x  713491024) / 30019565) ((x  523342792) / 220167797) ((x  749236871) / 5726282) ((x  727310078) / 16200511) ((x  746261832) / 2751243) ((x  733237527) / 10273062) ((x  715240338) / 28270251) ((x  722592910) / 20917679) ((x  739627071) / 3883518) + 75 ((x  749736637) / 34496299) ((x  713491024) / 1749314) ((x  523342792) / 191897546) ((x  749236871) / 33996533) ((x  727310078) / 12069740) ((x  746261832) / 31021494) ((x  733237527) / 17997189) ((x  743510589) / 28270251) ((x  722592910) / 7352572) ((x  739627071) / 24386733) + 49 ((x  749736637) / 27143727) ((x  713491024) / 9101886) ((x  523342792) / 199250118) ((x  749236871) / 26643961) ((x  727310078) / 4717168) ((x  746261832) / 23668922) ((x  733237527) / 10644617) ((x  743510589) / 20917679) ((x  715240338) / 7352572) ((x  739627071) / 17034161) + 82 ((x  749736637) / 10109566) ((x  713491024) / 26136047) ((x  523342792) / 216284279) ((x  749236871) / 9609800) ((x  727310078) / 12316993) ((x  746261832) / 6634761) ((x  733237527) / 6389544) ((x  743510589) / 3883518) ((x  715240338) / 24386733) ((x  722592910) / 17034161) f( 749736637) = 49 (1) f( 713491024) = 56 (8) f( 523342792) = 68 (D) f( 749236871) = 80 (P) f( 727310078) = 69 (E) f( 746261832) = 52 (4) f( 733237527) = 76 (L) f( 743510589) = 57 (9) f( 715240338) = 75 (K) f( 722592910) = 49 (1) f( 739627071) = 82 (R)
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