I've been working on finding the distance to a target (of known real-world dimensions). The distance seems consistent enough when viewed from a forward-facing angle, but when I rotate the target (without translating it) the distance changes:
Here is the relevant code:
vector objectPoints;
objectPoints.push_back(Point3f(-1, -1, 0));
objectPoints.push_back(Point3f(-1, 1, 0));
objectPoints.push_back(Point3f(1, 1, 0));
objectPoints.push_back(Point3f(1, -1, 0));
Mat objectPointsMat(objectPoints);
Mat rvec;
Mat tvec;
solvePnP(objectPointsMat, inputContour, cameraMatrix, distortionCoeffs, rvec, tvec);
double tx = tvec.at(0, 0);
double ty = tvec.at(1, 0);
double tz = tvec.at(2, 0);
double dist = std::sqrt(tx*tx + ty*ty + tz*tz);
char *str;
asprintf(&str, "DIST = %7.3f", dist);
putText(original, str, Point(0, original.size().height), CV_FONT_HERSHEY_PLAIN,
3, Scalar(255, 255, 255), 3);
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