I want to draw my current mouse cursor on the screen using opengl - so i convert the mouse window coordinates to world coordinates using gluUnProject.

But the resulting world coordinates are very small and does fit into the world space coordinates at all.

What am i doing wrong?

#include <string.h>
#include <stdio.h>
#include <iostream>
#include <GL/freeglut.h>
 
using std::cout;

float mouseX, mouseY;

void Draw()
{
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	glTranslatef(0.0f, 0.0f, -25.0f);

	glPointSize(40.0f);
	glColor3f(1.0f, 1.0f, 1.0f);
	glBegin(GL_POINTS);
	glVertex2f(mouseX, mouseY);
	glEnd();
	glPointSize(1.0f);

	glutSwapBuffers();
}

void Keyboard(unsigned char key, int x, int y)
{
	switch (key)
	{
	case 27:
		exit(0);
		break;
	}
}

void Mouse(int button, int state, int x, int y) {

}

void MouseMove(int x, int y) {
	double modelview[16], projection[16];
	int viewport[4];
	//get the projection matrix		
	glGetDoublev( GL_PROJECTION_MATRIX, projection );
	//get the modelview matrix		
	glGetDoublev( GL_MODELVIEW_MATRIX, modelview );
	//get the viewport		
	glGetIntegerv( GL_VIEWPORT, viewport );

	float z;
	double posX, posY, posZ;

	glReadPixels( x, viewport[3] - y - 1, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &z );

	gluUnProject( x, viewport[3] - y - 1, z, modelview, projection, viewport, &posX, &posY, &posZ );

	cout << x << ", " << y << ", " << z << " to " << posX << ", " << posY << ", " << posZ << "\n";

	mouseX = posX;
	mouseY = posY;
}

void Reshape(int width, int height)
{
	if (height == 0) {
		height = 1;
	}
	glViewport(0, 0, width, height);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(45.0, (float)width/(float)height, 0.1, 100.0);
	glMatrixMode( GL_MODELVIEW );
}

int main(int argc, char** argv)
{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE);
	glutInitWindowSize(800, 800);
	glutCreateWindow("GLUT Mouse!");

	glutReshapeFunc(Reshape);
	glutDisplayFunc(Draw);
	glutKeyboardFunc(Keyboard);
	glutMouseFunc(Mouse);
	glutMotionFunc(MouseMove);
	glutPassiveMotionFunc(MouseMove);
	glutIdleFunc(SimulationLoop);

	glClearColor( 0.0f, 0.0f, 0.0f, 1.0f );

	glEnable(GL_DEPTH_TEST);
	glDepthFunc(GL_LEQUAL);

	glutMainLoop();

	return 0;
}

first of all explain what do you expect, and what results you get.

secondly i see many errors here:

first you do not draw anything on the screen and yet you take the value from Depth buffer which is situated in your far plane (1.0 depth) (100.0 - in your case)

second thing you are not using ortographic projection but perspective projection, so you cant actually see the pixel (gl_point you try to draw) at the exact part of the screen you i think desire

so

cout << x << ", " << y << ", " << z << " to " << posX << ", " << posY << ", " << posZ << "\n";

should always show a point that is 100.0 units away from actual camera

heres a code that sets ortho projection (so you can draw things, with defining pixel position)

void __fastcall glPush2DDrawing()
{

 glPushAttrib(GL_DEPTH_TEST);
  glDisable(GL_DEPTH_TEST);


  glMatrixMode(GL_PROJECTION);
  glPushMatrix();
  glLoadIdentity();

  glOrtho(0, WINDOW_WIDTH, 0, WINDOW_HEIGHT, -1000.0f, 1000.0f);  // Change the projection matrix using an orthgraphic projection
  glMatrixMode(GL_MODELVIEW);
  glPushMatrix();
  glLoadIdentity();




  
}


void __fastcall glPop2DDrawing()
{
 glMatrixMode(GL_PROJECTION);
  glPopMatrix();
  glMatrixMode(GL_MODELVIEW);
  glPopMatrix();
  glPopAttrib();
}

usage is simple

glPush2DDrawing();

glBegin(GL_QUADS);

glVertex2f(0.0, 0.0);

glVertex2f(640.0, 0.0);

glVertex2f(640.0, 480.0);

glVertex2f(0.0, 480.0);

glEnd();

glPop2DDrawing();

this should draw a quad that is 640x480 pixels

first of all explain what do you expect, and what results you get.

secondly i see many errors here:

first you do not draw anything on the screen and yet you take the value from Depth buffer which is situated in your far plane (1.0 depth) (100.0 - in your case)

second thing you are not using ortographic projection but perspective projection, so you cant actually see the pixel (gl_point you try to draw) at the exact part of the screen you i think desire

...

apparently you didint show all the code because your was drawing exactly nothing.

yeah that ortho thing was not the best answer.