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algorithmpseudocodelinear-interpolation

How to wrap last/first element making building interpolation?

I've this code that iterate some samples and build a simple linear interpolation between the points:

foreach sample:
   base = floor(index_pointer)
   frac = index_pointer - base
   out = in[base] * (1 - frac) + in[base + 1] * frac
   index_pointer += speed

   // restart
   if(index_pointer >= sample_length) 
   {
      index_pointer = 0
   }

using "speed" equal to 1, the game is done. But if the index_pointer is different than 1 (i.e. got fractional part) I need to wrap last/first element keeping the translation consistent.

How would you do this? Double indexes?

Here's an example of values I have. Let say in array of 4 values: [8, 12, 16, 20].

It will be:

1.0*in[0] + 0.0*in[1]=8
0.28*in[0] + 0.72*in[1]=10.88
0.56*in[1] + 0.44*in[2]=13.76
0.84*in[2] + 0.14*in[3]=16.64
0.12*in[2] + 0.88*in[3]=19.52
0.4*in[3] + 0.6*in[4]=8 // wrong; here I need to wrapper

the last point is wrong. [4] will be 0 because I don't have [4], but the first part need to take care of 0.4 and the weight of first sample (I think?).

Solution

  • Just wrap around the indices:

    out = in[base] * (1 - frac) + in[(base + 1) % N] * frac

    , where % is the modulo operator and N is the number of input samples.

    This procedure generates the following line for your sample data (the dashed lines are the interpolated sample points, the circles are the input values):

    Sample Data