Persisting shapes

Table of Contents

• Write and read of the user Polygon

What if your user created the shapes you needed from him, and you want to save that setup?

No problem! shapes can be saved to a file and read back from that file.

Currently only the shapes of the entire Canvas can be written to a file and read from it.

Write and read of the user Polygon

This is the same program as from the previous tutorial (Creating a polygon from the GUI)

Adding only these lines to screen text:

s <<  "Use s to save.\n";
s <<  "Use l to load.";

And these lines to the switch:

case 's':
    c.writeShapesToFile("canvas.xml");
    break;
case 'l':
    c.readShapesFromFile("canvas.xml");
    break;

Which gives:

#include <canvascv/canvas.h>
#include <canvascv/shapes/polygon.h>

#include <opencv2/highgui.hpp>

using namespace std;
using namespace cv;
using namespace canvascv;

void handlePolyPoints(Canvas &c, Polygon &poly)
{
    vector<Point> polyPoints;
    poly.getPoints(polyPoints);
    stringstream s;
    s <<  "polygon points are at :\n[";
    for (auto &v : polyPoints)
    {
        s << " " << v << " ";
    }
    s <<  "]\n\n";
    s <<  "Use q to exit.\n";
    s <<  "Use s to save.\n";
    s <<  "Use l to load.";
    c.setScreenText(s.str());
}

int main(int argc, char **argv)
{
    --argc;
    ++argv;
    if (! argc)
    {
        Canvas::fatal("Must get a path to an image as a parameter" , -1);
    }

    Mat image = imread(argv[0]);
    if (image.empty())
    {
        Canvas::fatal(string("Cannot load image ") + argv[0], -2);
    }

    Canvas c("Persistent Shapes", image.size());
    c.enableScreenText();

    c.enableStatusMsg();
    c.setDefaultStatusMsg("Click-Release-Drag to create a single polygon.\n"
                          "Select/Unselect with the mouse.\n"
                          "Drag selected polygon with the mouse.\n"
                          "DEL deleted the selected shape.");

    c.setShapeType(Polygon::type); // default shape type for direct GUI creation
    c.notifyOnShapeCreate([&c](Shape* shape)
    {
        handlePolyPoints(c, *(Polygon*) shape); // it can only be a Polygon
        c.setShapeType(""); // default shape type for direct GUI creation
    });
    c.notifyOnShapeModify([&c](Shape* shape)
    {
        handlePolyPoints(c, *(Polygon*) shape);
    });
    c.notifyOnShapeDelete([&c](Shape*)
    {
        c.setShapeType(Polygon::type); // default shape type for direct GUI creation
    });

    namedWindow("Persistent Shapes", WINDOW_AUTOSIZE);
    c.setMouseCallback(); // optional for mouse usage see also (example_selectbox.cpp)

    int key = 0;
    Mat out; // keeping it out of the loop is a little more efficient
    do
    {
        switch (key)
        {
        case 's':
            c.writeShapesToFile("canvas.xml");
            break;
        case 'l':
            c.readShapesFromFile("canvas.xml");
            break;
        case 65535:
            c.deleteActive();
            c.setScreenText("");
            break;
        }

        c.redrawOn(image, out);
        c.imshow(out);
        key = c.waitKeyEx(); // GUI and callbacks happen here
    } while (key != 'q');

    destroyAllWindows();
    return 0;
}

Notes:

• The canvascv::Canvas::readShapesFromFile() will clear itself from shapes before reading the shapes from the file.
• When executed with a path to an image, this gives you (depends on your image):
  
  
  

That's all for this tutorial