contourArea function
ContourArea calculates a contour area.
For further details, please see: https:///docs.opencv.org/3.3.0/d3/dc0/group__imgproc__shape.html#ga2c759ed9f497d4a618048a2f56dc97f1
Implementation
double contourArea(VecPoint contour) {
final p = calloc<ffi.Double>();
cvRun(() => cimgproc.cv_contourArea(contour.ref, p, ffi.nullptr));
final rval = p.value;
calloc.free(p);
return rval;
}