CODEWITHSUNDEEP
c#xnainterpolation
Mathias Lykkegaard Lorenzen
Mathias Lykkegaard Lorenzen

Reputation: 15886

XNA interpolation on the CPU?

Interpolation on the CPU

I would like to be able to do interpolation on the CPU.

Consider the following example:

X1 | X2

-------

X3 | X4

X1, X2, X3 and X4 are all pixels (in Vector4 format). They all have the following coordinates.

Now, I want to be able to do interpolation between the pixels given a Vector2.

So let's say I want the color of the coordinates {X: 0.348, Y: 0.129}.

How would I interpolate properly between the pixels?

Why I want to do this

I know this sounds crazy, but it's what I want to do. I am trying to simulate a GPU shader algorithm on the CPU. It's for a Perlin Noise generator. I've already got the 2D terrain generated through a pixel shader that way in the game, and I want to be able to (at a given X, Y and Z coordinate) check if there's a wall present there or not, using the exact same algorithm on the CPU.

If you want to know more about what I am trying to do, see this question: https://gamedev.stackexchange.com/questions/15667/perlin-noise-copying-the-algorithm-on-the-cpu

Edit

I posted this the wrong place. It should have been in "gamedev.stackexchange.com". I hope you have the answer anyway.

Upvotes: 4

Views: 686

Answers (1)

Andrew Russell
Andrew Russell

Reputation: 27215

What you are looking for is bilinear interpolation (Wikipedia).

The XNA Lerp functions do linear interpolation - that is: one axis. Bi-linear is on two axes. Fortunately this is simply a matter of linearly interpolating on one axis (twice: once for each pair of inputs on that axis), and then linearly interpolating the two results.

So you'd do something like this:

Vector2 position = new Vector2(0.348f, 0.129f); // <- example data from question

Vector4 result = Vector4.Lerp(Vector4.Lerp(X1, X2, position.X),
                              Vector4.Lerp(X3, X4, position.X),
                              position.Y);

Upvotes: 6

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