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Asked: 2026-06-14T17:06:12+00:00

I’ve got the following code to do a biliner interpolation from a matrix of

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I’ve got the following code to do a biliner interpolation from a matrix of 2D vectors, each cell has x and y values of the vector, and the function receives k and l indices telling the bottom-left nearest position in the matrix

// p[1]                     returns the interpolated values
// fieldLinePointsVerts     the raw data array of fieldNumHorizontalPoints x fieldNumVerticalPoints
//                          only fieldNumHorizontalPoints matters to determine the index to access the raw data
// k and l                  horizontal and vertical indices of the point just bellow p[0] in the raw data

void interpolate( vertex2d* p, vertex2d* fieldLinePointsVerts, int fieldNumHorizontalPoints, int k, int l ) {

    int index = (l * fieldNumHorizontalPoints + k) * 2;

    vertex2d p11;
    p11.x = fieldLinePointsVerts[index].x;
    p11.y = fieldLinePointsVerts[index].y;

    vertex2d q11;
    q11.x = fieldLinePointsVerts[index+1].x;
    q11.y = fieldLinePointsVerts[index+1].y;

    index = (l * fieldNumHorizontalPoints + k + 1) * 2;

    vertex2d q21;
    q21.x = fieldLinePointsVerts[index+1].x;
    q21.y = fieldLinePointsVerts[index+1].y;

    index = ( (l + 1) * fieldNumHorizontalPoints + k) * 2;

    vertex2d q12;
    q12.x = fieldLinePointsVerts[index+1].x;
    q12.y = fieldLinePointsVerts[index+1].y;

    index = ( (l + 1) * fieldNumHorizontalPoints + k + 1 ) * 2;

    vertex2d p22;
    p22.x = fieldLinePointsVerts[index].x;
    p22.y = fieldLinePointsVerts[index].y;

    vertex2d q22;
    q22.x = fieldLinePointsVerts[index+1].x;
    q22.y = fieldLinePointsVerts[index+1].y;

    float fx = 1.0 / (p22.x - p11.x);
    float fx1 = (p22.x - p[0].x) * fx;
    float fx2 = (p[0].x - p11.x) * fx;

    vertex2d r1;
    r1.x = fx1 * q11.x + fx2 * q21.x;
    r1.y = fx1 * q11.y + fx2 * q21.y;

    vertex2d r2;
    r2.x = fx1 * q12.x + fx2 * q22.x;
    r2.y = fx1 * q12.y + fx2 * q22.y;

    float fy = 1.0 / (p22.y - p11.y);
    float fy1 = (p22.y - p[0].y) * fy;
    float fy2 = (p[0].y - p11.y) * fy; 

    p[1].x = fy1 * r1.x + fy2 * r2.x;
    p[1].y = fy1 * r1.y + fy2 * r2.y;
}

Currently this code needs to be run every single frame in old iOS devices, say devices with arm6 processors

I’ve taken the numeric sub-indices from the wikipedia’s equations http://en.wikipedia.org/wiki/Bilinear_interpolation

I’d accreciate any comments on optimization for performance, even plain asm code

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1 Answer

  1. Editorial Team

    This code should not be causing your slowdown if it’s only run once per frame. However, if it’s run multiple times per frame, it easily could be.

    I’d run your app with a profiler to see where the true performance problem lies.

    There is some room for optimization here: a) Certain index calculations could be factored out and re-used in subsequent calculations), b) You could dereference your fieldLinePointsVerts array to a pointer once and re-use that, instead of indexing it twice per index…

    but in general those things won’t help a great deal, unless this function is being called many, many times per frame. In which case every little thing will help.

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