Bug in cv::warpAffine? - opencv

Question

I think the following examples shows a bug in warpAffine (OpenCV 3.1 with precompiled Win64 dlls):
Mat x(1,20, CV_32FC1);
for (int iCol(0); iCol<x.cols; iCol++) { x.col(iCol).setTo(iCol); }
Mat crop;
Point2d c(10., 0.);
double scale(1.3);
int cropSz(11);
double vals[6] = { scale, 0.0, c.x-(cropSz/2)*scale, 0.0, scale, c.y };
Mat map(2, 3, CV_64FC1, vals);
warpAffine(x, crop, map, Size(cropSz, 1), WARP_INVERSE_MAP | INTER_LINEAR);
float dx = (crop.at<float>(0, crop.cols-1) - crop.at<float>(0, 0))/(crop.cols-1);
Mat constGrad = crop.clone().setTo(0);
for (int iCol(0); iCol<constGrad.cols; iCol++) {
constGrad.col(iCol) = c.x + (iCol-cropSz/2)*scale;
}
Mat diff = crop - constGrad;
double err = norm(diff, NORM_INF);
if (err>1e-4) {
cout << "Problem:" << endl;
cout << "computed output: " << crop << endl;
cout << "expected output: " << constGrad << endl;
cout << "difference: " << diff << endl;
Mat dxImg;
Mat dxFilt(1, 2, CV_32FC1);
dxFilt.at<float>(0) = -1.0f;
dxFilt.at<float>(1) = 1.0f;
filter2D(crop, dxImg, crop.depth(), dxFilt);
cout << "x-derivative in computed output: " << dxImg(Rect(1,0,10,1)) << endl;
cout << "Note: We expect a constant difference of 1.3" << endl;
}
Here is the program output:
Problem:
computed output: [3.5, 4.8125, 6.09375, 7.40625, 8.6875, 10, 11.3125, 12.59375, 13.90625, 15.1875, 16.5]
expected output: [3.5, 4.8000002, 6.0999999, 7.4000001, 8.6999998, 10, 11.3, 12.6, 13.9, 15.2, 16.5]
difference: [0, 0.012499809, -0.0062499046, 0.0062499046, -0.012499809, 0, 0.012499809, -0.0062503815, 0.0062503815, -0.012499809, 0]
x-derivative in computed output: [1.3125, 1.28125, 1.3125, 1.28125, 1.3125, 1.3125, 1.28125, 1.3125, 1.28125, 1.3125]
Note: We expect a constant difference of 1.3
I create an image with entries 0, 1, 2, ...n-1, and cut a region around (10,0) with scale 1.3. I also create an expected image constGrad. However, they are not the same. Even more, since the input image has a constant derivative in x-direction and the mapping is affine, I expect also a constant gradient in the resulting image.
The problem is not a boundary stuff problem, the same happens at the inner of an image. It's also not related to WARP_INVERSE_MAP.
Is this a known issue? Any comments on this?