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How do I interpolate within a 2d dataset?
Hi, thanks in advance for the help!
For those of you familiar with matlab, I'm simply trying to perform an interp1, but I can't seem to find a similar function in javascript. I'm sure I'm just overlooking it, despite hours of research! Here's my problem:
I'm importing a text file of data into a 2d array (though I'm open to suggestions otherwise) and I would like to input an x value and return an interpolated y value.
The text file looks something like this:
5 20
6 21
7 25
etc, etc.
In matlab I would simply write:
y = interp1(xdata,ydata,x)
where x is my input and y is my result. Any ideas on how to perform this in js?
Thanks again!!
Answer by RedFive · Oct 03, 2012 at 03:25 AM
I didn't really like the AnimationCurve solution Mike posted below. I suppose it could have worked, but it was not intended for what I was attempting to do -- evaluate data. So I wrote a simple function that essentially mimics what Matlab's interp1 does.
function InterpX(x){ //linearly interpret a Y value given an X input
arraylength = dataPairs.length - 1; // get # of rows of my array
//Debug.Log(arraylength);
for (var i = 0; i < arraylength; i++){ //start for loop to look through each data point
var index = i; //save current value of i for use later since using i directly fucks it up
//get x and y value for i position
var NumA = parseFloat(dataPairs[i][0]);
var NumC = parseFloat(dataPairs[i][1]);
//get x and y value from data for i + 1 position
if(index < arraylength){
var NumB = parseFloat(dataPairs[index + 1][0]);
var NumD = parseFloat(dataPairs[index + 1][1]);}
//find which data points our x input falls between
if (NumA < x && x < NumB){
var DeltaAB = Mathf.Abs(NumB - NumA); //Get difference between the two x values
var DeltaxAB = x - Mathf.Min(NumA,NumB); //Our input minus whichever value is lowest
var Ratio = DeltaxAB/DeltaAB; // ratio of our input to the span between A and B
Debug.Log(x + ' is between ' + NumA + ' and ' + NumB);
var DeltaCD = Mathf.Abs(NumD - NumC); //Get difference between the two y values
var y = (DeltaCD * Ratio) + Mathf.Min(NumC,NumD); //Linear interpolation of y
Debug.Log('y:' + y);
break; //end i loop
}
}
} //end function InterpX
There may be more efficient ways to have written this function -- and I'm open to suggestions to improve it -- but all that really matters is that I can now process some data :D
I wanted to post this in case anybody else is ever looking for a similar function. Of course the prerequisite prior to calling the function is that you have imported data from a file to an array which is called dataPairs here.
Thanks again for the help, Mike! Much appreciated.
You should tick this as correct to get it off the unanswered list.
I should point out that AnimationCurves are usable to evaluate data - I use this technique frequently in game balancing algorithms, however it would not have produced exactly the same results as matlib.
Answer by whydoidoit · Sep 29, 2012 at 04:55 PM
So import the data into an array of float - use float.Parse(someStringValue) to convert it.
Then use Mathf.Lerp(value1, value2, aNumberBetween0and1) to get your result.
No, I don't think that's what I want. I read a lot about lerp, but it simply interpolates between two points based on a percentage. I.e. if your value1 = 1, value2 = 100, and aNumberBetween0and1 = 0.5, the result is 50.
I don't see how that helps me get a y value...maybe I'm missing something?
Yeah you are quite right :) Not what you want at all.
Ok so I now think that you are probably after adding those coordinates to an AnimationCurve. Using the provided samples of X as time and the value as the Y from your input data. You can then interpolate to a point of any arbitrary X (or time).
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