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

Question

I am using SDL with OpenGL and so far I have been doing everything in 2d (using glOrtho ())

Now I want to try to draw things in 3D, but, of course, since now I have chosen a 3-dimensional dimension in the mixture, things get complicated.

What I want to do, take the coordinates of the cursor screen, translate them into (?) World coordinates (the coordinates that I can use in OpenGL, since I now have a perspective, etc.), and draw a square in this position (cursor the mouse is in the center of the quad)

I have read several gluUnProject () tutorials and I understand what I have to do, but as I study it myself, it is very easy to get confused and misunderstand what I am doing.

As such, I basically copied from the examples I found.

Below is my code (I took out error checking, etc., trying to shorten it a bit)

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

I tried using glReadPixel () to get mouseZ, but I'm not sure if I am doing this correctly.

The problem is that when I click, the square is not drawn in the correct position (I think, partly because I don’t know how to get mouseZ correctly, so I just replaced it with gluUnProject () 1,0?)

(wx, wy, wz) glTranslatef(), , , mouseZ. , , . wz -499, , , , , ( )

- , , - , .

#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

, :

GLfloat depth[2];

glReadPixels() :

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

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

gluUnProject() :

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

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

Z, .

wz, : wz += 1;, , , , , , , .

Answer 2

Z, z:

, , ( , ) 3D- . Z- .

( ), 2D-, ( , 2D ), z.

Z - , .

Z, .

2D 3D openGL , OpenGl Z = 1. [-1,1] .