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you can use a mask with copyTo().

so, first make a black image:

Mat mask = new Mat(submat.size(), CvType.CV_8U, new Scalar(0));
// then draw a filled white star into that,
// e.g. with drawContours(), and linesize=-1
//
// now use that mask, to copy only where the mask is white:
dst.copyTo(submat, mask);