OpenGL- C++ using mouse click to move shape to the clicked position

Question

I need help in understanding how to scale my coordinates. The instruction says, modify the code to move the triangle to the mouse click position. Can someone explain how this is done?

Here is the source code I'm working with:

#ifdef __APPLE__
#include <GLUT/glut.h> 
#else
#include <GL/glut.h> 
#endif

#include <stddef.h>
#include <iostream>

// values controlled by fast keys
float g_angle = 0.0f;
float g_xoffset = 0.0f;
float g_yoffset = 0.0f;
int x;
int y;

// increments
const float g_angle_step = 32.0f; // degrees
const float g_offset_step = 32.0f; // world coord units

// last cursor click
int g_cursor_x = 0;
int g_cursor_y = 0;

void draw_triangle()
{
    // in model cooridnates centred at (0,0)
    static float vertex[3][2] =
        {
            {-1.0f, -1.0f},
            {1.0f, -1.0f},
            {0.0f, 1.0f}
        };

    glBegin(GL_LINE_LOOP); 
        for (size_t i=0;i<3;i++)
            glVertex2fv(vertex[i]);
    glEnd();
}

void display()
{
    glClear(GL_COLOR_BUFFER_BIT); 
    glColor3f(1.0f, 1.0f, 1.0f); 
    glLineWidth(2.0f);
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
        glTranslatef(500.0f+g_xoffset, 500.0f+g_yoffset, 0.0f);
        glScalef(100.0f, 100.0f, 1.0f);
        glRotatef(g_angle, 0.0f, 0.0f, 1.0f); 
        draw_triangle();
    glPopMatrix(); // done with stack
    glutSwapBuffers(); 
}

// handles mouse click events
// button will say which button is presed, e.g. GLUT_LEFT_BUTTON, GLUT_RIGHT_BUTTON
// state will say if the button is GLUT_UP or GLUT_DOWN
// x and y are the poitner position 
void mouse_click(int button, int state, int x, int y)
{
    if (button==GLUT_LEFT_BUTTON)
    {       
        std::cerr << "\t left mouse button pressed!" << std::endl;

        if (state==GLUT_UP)
        {
            std::cerr << "\t button released...click finished" << std::endl;

            g_cursor_x = x;
            g_cursor_y = y;

            std::cerr << "\t cursor at (" << g_cursor_x << ", " << 
                                             g_cursor_y << ")" << std::endl;
        }

    }
    else
    if (button==GLUT_RIGHT_BUTTON)
    {
        std::cerr << "\t right mouse button pressed!" << std::endl;
    }

// Here is my attempt:
    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) 
    {
        float x_min = (-x+500)/512;
        float x_max = (x-500)/512;
        float y_min = (-y+500)/512;
        float y_max = (y-500)/512;
        gluOrtho2D(x_min, x_max, y_min, y_max);
        //glTranslatef(x/512, 1-y/512, 0.0f);
            std::cerr << x << ", " << y << std::endl;
    }
    glutPostRedisplay();
}

void mouse_motion(int x, int y)
{
    std::cerr << "\t mouse is at (" << x << ", " << y << ")" << std::endl;
    glutPostRedisplay();
}


// will get which key was pressed and x and y positions if required
void keyboard(unsigned char key, int, int)
{
    std::cerr << "\t you pressed the " << key << " key" << std::endl;

    switch (key)
    {
        case 'q': exit(1); // quit!

        // clockwise rotate
        case 'r': g_angle += g_angle_step; break;
    }

    glutPostRedisplay(); // force a redraw
}

// any special key pressed like arrow keys
void special(int key, int, int)
{
    // handle special keys
    switch (key)
    {
        case GLUT_KEY_LEFT: g_xoffset -= g_offset_step; break;
        case GLUT_KEY_RIGHT: g_xoffset += g_offset_step; break;
        case GLUT_KEY_UP: g_yoffset += g_offset_step; break;
        case GLUT_KEY_DOWN: g_yoffset -= g_offset_step; break;
    }

    glutPostRedisplay(); // force a redraw
}

void init()
{
    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity();
    gluOrtho2D(0, 1000, 0, 1000);
    glClearColor(0.0f, 0.0f, 1.0f, 0.0f); 

    g_cursor_x = g_cursor_y = 500; // middle of window
}

int main(int argc, char* argv[])
{
    glutInit(&argc, argv); 
    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGBA); 
    glutInitWindowSize(512, 512); 
    glutInitWindowPosition(50, 50); 
    glutCreateWindow("Mouse Test"); 
    glutDisplayFunc(display); 

    // handlers for keyboard input
    glutKeyboardFunc(keyboard); 
    glutSpecialFunc(special); 

    // mouse event handlers
    glutMouseFunc(mouse_click); 
    glutPassiveMotionFunc(mouse_motion); 

    init(); 
    glutMainLoop(); 

    return 0; 
}

Answer 1

This should solve your issue.replace your code(your attempt) with the code below.

 if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) 
 { 

    g_xoffset = x;
    g_yoffset = 1000-y;

   } glutPostRedisplay();