Question

I have an image, and a selection given by a list of points (a poly). My goal is to get another image where everything except the selection is transparent.

There are two parts to this question:
* how to create the mask from the list of points?
* what’s the best way of creating the new image using the mask? I was thinking of cvCopy but how can I handle the alpha channel?

Answer 1

I had a simillar issue and I solved it in a strange (not very fast) way:

1. Extracted red, green and blue channels of an image (using extractChannel).
2. Do a bitwise_and operation between each of the channels and poygon mask image.
3. Merged them back (using merge function).

Note that if you have one-channel image than you just need to do a bitwise_and.

C++ code:

void createPOI(const Mat& src, const Mat& polyMat, Mat& dst)
{
    vector<Mat> array(3);

    for (int i=0; i<3; i++)
    {
        extractChannel(src, array[i], i);
        bitwise_and(array[i], polyMat, array[i]);
    }

    merge(array, dst);
}

You can create polyMat mask using fillPoly/fillConvexPoly functions (depends on your polygon). For example:

Mat polyMat = Mat::zeros(size, CV_8UC1);
fillConvexPoly(polyMat, polygon, Scalar(255));