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I have two successive remap calls and I'm trying to merge them:

for (int j = 0; j < h; j++)
    for (int i = 0; i < w; i++) {
        float x, y;
        x = mapx2.at<float>(j, i);
        y = mapy2.at<float>(j, i);
        x = RANGE(x, 0, w - 1);
        y = RANGE(y, 0, h - 1);
        mapx1_2.at<float>(j, i) = mapx1.at<float>(y, x);
        mapy1_2.at<float>(j, i) = mapy1.at<float>(y, x);
    }

The overall result looks the same but details are different. Here is an example. The first extract with the two successive remaps shows a smooth line. The second extract with the single merged remap is not smooth.

I tried to cast x and y to int but with the same result.

I understand that "interpolation of interpolation" is not the same as a single merged interpolation. How could I fix the problem?

PS: I asked the question on Stackoverflow but here should be a better place to get an answer I think.