Calibrate coordinates on OpenGL? - c++

Question

I have a problem regarding the gluOrtho2D function. I want to resize the window size and calibrate its coordinates, but i have problems with the function. I think that the coordinates calculated
The problem is that if i click somewhere on the window, the point isn't on the cursor, but somewhere near it ( ~20 pixels). If i resize the window, i want the window points to be converted to the new window size, that's why the gluOrtho2D function had those parameters. Maybe i didn't figure it out the good way.
GLdouble mouseX = ( double )(2* widthValue * x / windowWidthSize) - widthSize ;
GLdouble mouseY = -( double )(2 * heightValue * y / windowHeightSize) + heightValue;
are good as i want them to take values from -30 to +30 (both x and y), but the
gluOrtho2D( -width / widthValue , width / widthValue , -height / heightValue , height / heightValue);
function doesn't convert them as i want. Any ideas to fix it?
#include<windows.h>
#include<GL/Glut.h> //includes the opengl, glu, and glut header files
#include<stdlib.h> //includes the standard library header file
#include<iostream>
#include<fstream>
#include<vector>
using namespace std;
vector<pair<GLdouble,GLdouble>>v;
GLdouble heightValue = 30.0; // the points X and Y coordinates would be [-30,30]
GLdouble widthValue = 30.0; // the points X and Y coordinates would be [-30,30]
void initGL()
{
glClearColor(1,1,1,0);
}
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glPointSize(5.0);
glBegin(GL_POINTS);
glColor3f(1.0f, 1.0f, 0.0f);
// prints the points
for(auto i : v)
glVertex2d(i.first, i.second);
glEnd();
glFlush();
}
void reshape(GLsizei width, GLsizei height) {
// Set the viewport to cover the new window
glViewport(0, 0, width, height);
// Set the aspect ratio of the clipping area to match the viewport
glMatrixMode(GL_PROJECTION); // To operate on the Projection matrix
glLoadIdentity(); // Reset the projection matrix
// here is the problem with the coordinates. Any help??
gluOrtho2D(-width/ widthValue, width/ widthValue,-height / heightValue ,height / heightValue);
}
void mouseClick(int button, int state, int x, int y)
{
if(button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) // left button is pressed
{
double windowHeightSize = glutGet(GLUT_WINDOW_HEIGHT); // gets window's size
double windowWidthSize = glutGet(GLUT_WINDOW_WIDTH); // gets window's size
// converts the click coordinate on the screen to the coordinate on the window
GLdouble mouseX = (double)(60.0 * x / windowWidthSize) - 30.0;
GLdouble mouseY = -(double)(60.0 * y / windowHeightSize) + 30.0;
v.push_back(make_pair(mouseX,mouseY) ); // insert the point's coordinates in the vector
display();
}
}
int main(int argc,char** argv)
{
glutInit(&argc,argv);
glutInitWindowSize(800, 600);
glutInitWindowPosition(50, 50);//sets the position of the window in pixels from top left corner
glutCreateWindow("Program Find Convex Hull");
glutDisplayFunc(display);
glutReshapeFunc(reshape); // functia este apelata cand redimensionam fereastra
glutMouseFunc(mouseClick);
// initGL();
glutMainLoop();//loops the current event
return 0;
}

Answer 1

If the Orthographic projection is set by:
gluOrtho2D(-ortho_x, ortho_x, -ortho_y, ortho_y);
then the transformation of the moue position (window coordinates) to view coordinates is:
GLdouble mouseX = (double)(x/windowWidthSize * 2*ortho_x) - ortho_x;
GLdouble mouseY = ortho_y - (double)(y/windowHeightSize * 2*ortho_y);
Apply that to your example:
GLdouble ortho_x = 30.0;
GLdouble ortho_y = -30.0;
void reshape(GLsizei width, GLsizei height) {
// Set the viewport to cover the new window
glViewport(0, 0, width, height);
// Set the aspect ratio of the clipping area to match the viewport
glMatrixMode(GL_PROJECTION); // To operate on the Projection matrix
glLoadIdentity(); // Reset the projection matrix
ortho_x = width / widthValue;
ortho_y = height / heightValue;
gluOrtho2D(-ortho_x, ortho_x, -ortho_y, ortho_y);
}
void mouseClick(int button, int state, int x, int y)
{
if(button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) // left button is pressed
{
double windowHeightSize = glutGet(GLUT_WINDOW_HEIGHT); // gets window's size
double windowWidthSize = glutGet(GLUT_WINDOW_WIDTH); // gets window's size
// converts the click coordinate on the screen to the coordinate on the window
GLdouble mouseX = (double)(x/windowWidthSize * 2*ortho_x) - ortho_x;
GLdouble mouseY = ortho_y - (double)(y/windowHeightSize * 2*ortho_y);
v.push_back(make_pair(mouseX,mouseY) ); // insert the point's coordinates in the vector
glutPostRedisplay();
}
}
Note, it is not necessary to change (ortho_x, ortho_y) in reshape. Since you mentioned in your question "i want them to take values from -30 to +30", it is even possible to keep (30.0, 30.0).