Using gluUnProject() to draw a quad at the cursor position

Question

I am using SDL with OpenGL and up until now I have done everything in 2d (Using glOrtho())

Now I want to try drawing things in 3D but of course since I have now thrown a 3rd dimension into the mixture, things are getting more complicated.

What I want to do is, take the screen co-ordinates of the cursor, translate them to (?)world co-ordinates (Co-ordinates I can use in OpenGL since I now have perspective etc.), and draw a quad at that position (The mouse cursor being at the center of the quad)

I have read a few tutorials on gluUnProject() and I sort of understand what I am supposed to do, but since I am learning by myself it is very easy to get confused and not properly understand what I am doing.

As such, I have mainly copied from examples I have found.

Below is my code (I took out error checking etc in an attempt to cut it down a bit)

As you can see, I have mouseX, mouseY and mouseZ variables, of which mouseX and mouseY get their values from SDL.

I tried to use glReadPixel() to get mouseZ but I am not sure if I am doing it right.

The problem is that when I click, the quad is not drawn in the correct position (I guess partly due to the fact that I don't know how to get mouseZ properly, so I just replaced it in gluUnProject() with 1.0?)

I am using the new co-ordinates (wx, wy, wz) in the glTranslatef() function but once again because I don't know how to get mouseZ properly I am guessing this is the reason it doesn't work properly. If I replace wz with -499 it seems to work ok, but I need to know how to get accurate results without manually finding the correct (Or almost correct) numbers

If anybody can tell me what I am doing wrong, or give me any advice it would be greatly appreciated.

#include <SDL/SDL.h>
#include <SDL/SDL_opengl.h>

SDL_Surface* screen = 0;
SDL_Event event;
bool isRunning = true;

int mouseX, mouseY, mouseZ = 0;

GLint viewport[4];
GLdouble mvmatrix[16], projmatrix[16];
GLint realY; /* OpenGL y coordinate position */
GLdouble wx, wy, wz; /* returned world x, y, z coords */

int main(int argc, char **argv) {
    SDL_Init(SDL_INIT_EVERYTHING);

    screen = SDL_SetVideoMode(800, 600, 32, SDL_OPENGL);

    glEnable(GL_DEPTH_TEST);
    glDepthMask(GL_TRUE);

    glViewport(0, 0, 800, 600);

    glClearDepth(1.f);
    glClearColor(0, 0, 0, 0);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    gluPerspective(90.f, 1.f, 1.f, 500.f);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    while(isRunning) {
        while(SDL_PollEvent(&event)) {
            if(event.type == SDL_QUIT) {
                isRunning = false;
            }

            if(event.type == SDL_MOUSEBUTTONDOWN) {
                if(event.button.button == SDL_BUTTON_LEFT) {
                    SDL_GetMouseState(&mouseX, &mouseY);

                    glGetIntegerv(GL_VIEWPORT, viewport);
                    glGetDoublev(GL_MODELVIEW_MATRIX, mvmatrix);
                    glGetDoublev(GL_PROJECTION_MATRIX, projmatrix);
                    realY = viewport[3] - (GLint) mouseY - 1;
                    glReadPixels(mouseX, realY, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &mouseZ);

                    gluUnProject((GLdouble)mouseX, (GLdouble)realY, 1.0, mvmatrix, projmatrix, viewport, &wx, &wy, &wz);
                }
            }
        }

        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        glPushMatrix();

        glTranslatef(wx, wy, wz);

        glBegin(GL_QUADS);
            glColor4f(1.f, 0.f, 0.f, 1.f);
            glVertex3f(-20.f, -20.f, 0.f);

            glColor4f(0.f, 1.f, 0.f, 1.f);
            glVertex3f(-20.f, 20.f, 0.f);

            glColor4f(0.f, 0.f, 1.f, 1.f);
            glVertex3f(20.f, 20.f, 0.f);

            glColor4f(1.f, 0.f, 1.f, 1.f);
            glVertex3f(20.f, -20.f, 0.f);
        glEnd();

        glPopMatrix();

        SDL_GL_SwapBuffers();
    }

    SDL_FreeSurface(screen);

    return 0;
}

Answer 1

I found that if I added the following:

GLfloat depth[2];

then changed the glReadPixels() from:

glReadPixels(mouseX, realY, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &mouseZ);

to:

glReadPixels(mouseX, realY, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, depth);

then I just changed gluUnProject() from:

gluUnProject((GLdouble)mouseX, (GLdouble)realY, mouseZ, mvmatrix, projmatrix, viewport, &wx, &wy, &wz);

to:

gluUnProject((GLdouble) mouseX, (GLdouble) realY, depth[0], mvmatrix, projmatrix, viewport, &wx, &wy, &wz);

then I could get the Z value I wanted.

Then I just added a small value to wz, for example: wz += 1; to make sure the quad appeared above the place I clicked and it seems to work as I intended now.