Draw a polygon in OpenGL GLUT with mouse

Question

i want to draw a polygon in openGL Glut with mouse interaction, every left click that will be made will be a vertex and a line will be drawn between every vertex. when the r

Answer 1

You have to separate the mouse events and the drawing function.

In the mouse event you should just collect the inputs. I Suggest to use a std::vector for this. The following function adds a point to the std::vector if the left mouse button is pressed. If the right button is pressed the polygon is marked closed. If the left button is pressed again, the polygon is cleared and the process restarts.

#include <vector>
#include <array>

int vp_width = 640;
int vp_height = 480;

std::array<int, 2> currentPt;
std::vector<std::array<int, 2>> pts;
bool closed = false;

void draw_polygon(int button, int state, int x, int y)
{
    currentPt = std::array<int, 2>{x, vp_height-y}; 

    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
    {
        if ( closed )
            pts.clear(); // restart if last action was close
        closed = false;
        pts.push_back( currentPt );
    }
    if ( button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN )
        closed = true;
}

In a mouse move event function you can track the current mouse position:

void mouse_move(int x, int y)
{
    currentPt = std::array<int, 2>{x, vp_height-y};
    glutPostRedisplay();
}

In your main loop you can continuously draw the lines between the current points. The fllowing function draw lines betwwen a list of points. If the "cloesd" flag is set, then the polygon is closed. Else a line from the last point in the list to the current mouse position is drawn.

void display(void)
{
    glClearColor(0, 0, 0, 0);
    glClear(GL_COLOR_BUFFER_BIT);

    if ( !pts.empty() )
    {
        glBegin(GL_LINE_STRIP);
        for ( auto &pt : pts )
            glVertex2f( (float)pt[0], (float)pt[1] );
        auto &endPt = closed ? pts.front() : currentPt;
        glVertex2f( (float)endPt[0], (float)endPt[1] );
        glEnd();
    }

    //glFlush();
    glutSwapBuffers();
}

int main()
{
    ..... 
    glutDisplayFunc(display);
    glutPassiveMotionFunc (mouse_move);
    glutMouseFunc(draw_polygon);

    glMatrixMode( GL_PROJECTION );
    glOrtho(0.0f, (float)vp_width, 0.0f, (float)vp_height, -1.0, 1.0);
    .....
}

Preview: