How to extract a polygon from an image using imagemagick or magick++

Question

I would like to us Magick++ or the imagemagick command line to extract an n-gon from an image. The n-gon is specified by a list the vertices. So, for example I would like to be able to extract an n-gon defined by the points a,b,c,d,e,f,g,h which define a region that looks like:

      a---------h
      |         |
      |         g-f
      |           |
      b---c       |
          |       |
          |       |
          |       |
          d-------e

out of, for example, a png image. I assume I have to do some kind of composite operation with an image mask, or if using Magick++ define a DrawableClipPath. But, I can't find any documentation for this. Any help would be appreciated.

p.s. My preference is to do this using C++ and Magick++ because I have other processing to do, but am happy to do it with the commandline too.

Answer 1

You would use a combination of Magick::DrawablePolygon and Magick::Image.composite.

Create a new mask image, and draw the n-gon shape

Magick::Image mask;
mask.draw( Magick::DrawablePolygon( std::list<Magick::Coordinate> ) );

Then just apply the mask to a destination image, and compose the existing source.

Magick::Image dest;
dest.composite( Magick::Image, Magick::Geometry, Magick::CompositeOperator );

Example:

#include <iostream>
#include <Magick++.h>

int main(int argc, const char ** argv)
{
    Magick::InitializeMagick(*argv);

    Magick::Image mask( Magick::Geometry(120,120), Magick::Color("white"));
    Magick::Image dest( Magick::Geometry(120,120), Magick::Color("white"));

    // Example source image
    Magick::Image source;
    source.read("rose:");
    source.resize(Magick::Geometry(200,120)); // Resize for fun

    mask.fillColor("black");

    // Define points
    std::list<Magick::Coordinate> points;
    points.push_back(Magick::Coordinate(10, 10)); // a
    points.push_back(Magick::Coordinate(10, 50)); // b
    points.push_back(Magick::Coordinate(30, 50)); // c
    points.push_back(Magick::Coordinate(30,100)); // d
    points.push_back(Magick::Coordinate(75,100)); // e
    points.push_back(Magick::Coordinate(75, 30)); // f
    points.push_back(Magick::Coordinate(60, 30)); // g
    points.push_back(Magick::Coordinate(60, 10)); // h

    // Draw Polygon "n-gon"
    mask.draw( Magick::DrawablePolygon(points) );

    // Extract n-gon from source image to destination image
    dest.clipMask(mask);
    Magick::Geometry offset(0,0,0,0);
    dest.composite( source, offset, Magick::OverCompositeOp );

    dest.write("n-gon.png"); // Output
}