Using ImageMagick.NET to extract a polygon

Question

I'm using ImageMagick.NET to crop a polygon from an image using coordinates.

Most image cropping questions rely on cropping an image using a rectangle using 4 points, however I need to crop a polygon using between four to eight points instead.

I'm using the following code to create a mask and a drawable polygon to apply to the source image, however the result is that I'm getting is the inverse of what I would expect.

According to http://www.imagemagick.org/Usage/compose/tables/ the In operator should get me my result but instead its getting the remainder of the image

        MagickImage image = new MagickImage(imagePath);
        MagickImage dest = new MagickImage(new MagickColor(Color.White), 1120, 450);
        MagickImage mask = new MagickImage(new MagickColor(Color.White), 1120, 450);

        mask.FillColor = new MagickColor(Color.Black);

        List<Coordinate> coordinates = new List<Coordinate>();
        coordinates.Add(new Coordinate(0, 0));
        coordinates.Add(new Coordinate(0, 300));
        coordinates.Add(new Coordinate(100, 300));
        coordinates.Add(new Coordinate(100, 450));
        coordinates.Add(new Coordinate(1120, 450));
        coordinates.Add(new Coordinate(1120, 150));
        coordinates.Add(new Coordinate(850, 150));
        coordinates.Add(new Coordinate(850, 0));

        mask.Draw(new DrawablePolygon(coordinates));


        MagickGeometry offset = new MagickGeometry(0,0,0,0);
        dest.Alpha(AlphaOption.Transparent);
        dest.ClipMask = mask;

        dest.Composite(image, offset, CompositeOperator.Out);

        dest.Write(@"C:\temp\test.png");

Any help would be greatly appreciated

Answer 1

It looks like you are creating the mask incorrectly for the solution that you want. You should switch the black and white colors:

List<Coordinate> coordinates = new List<Coordinate>();
coordinates.Add(new Coordinate(0, 0));
coordinates.Add(new Coordinate(0, 300));
coordinates.Add(new Coordinate(100, 300));
coordinates.Add(new Coordinate(100, 450));
coordinates.Add(new Coordinate(1120, 450));
coordinates.Add(new Coordinate(1120, 150));
coordinates.Add(new Coordinate(850, 150));
coordinates.Add(new Coordinate(850, 0));

using (MagickImage image = new MagickImage(imagePath))
{
  using (MagickImage mask = new MagickImage(Color.Black, 1120, 450))
  {
    mask.FillColor = new MagickColor(Color.White);

    mask.Draw(new DrawablePolygon(coordinates));

    using (MagickImage dest = new MagickImage(Color.White, 1120, 450))
    {
      dest.Alpha(AlphaOption.Transparent);
      dest.Mask = mask;

      MagickGeometry offset = new MagickGeometry(0, 0, 0, 0);
      dest.Composite(image, offset, CompositeOperator.Out);

      dest.Write(@"D:\test.png");
    }
  }