Question

I’m importing an object file, let’s say it’s a plane for example. I want to be able to draw on the plane and I just can’t seem to figure out how it’s done. It’s fairly trivial to know the x and y coordinates of the mouse click but what about the z coordinate? I need to be able to do some sort of ray tracing that shoots out a ray and tells me as soon as it reaches the object. What I’m trying to know is the z coordinate of the click which is on the object. There are no other objects in my space is a good assumption.

Answer 1

Googling a bit gave me these nice advices:

• one

• two

• three

• four

Some time ago i used them (almost all of them) to create my application. For some reasons i have not got this application now but i can show you my code getting 3D-vector of pick point in a 2 .. 3 hrs =)

If you do want to implement “drawing on an object” i suggest you need to find the UV (texture) coordinates of the pick point and then perform some drawing operations on the object texture.

UPADTE: my picking function implementation

sf::Vector3f ScreenToSpace(int x, int y)
{
    GLint viewport[4];
    GLdouble modelview[16];
    GLdouble projection[16];
    GLfloat winX, winY, winZ;
    GLdouble posX, posY, posZ;

    glGetDoublev(GL_MODELVIEW_MATRIX, modelview);
    glGetDoublev(GL_PROJECTION_MATRIX, projection);
    glGetIntegerv(GL_VIEWPORT, viewport);

    winX = (float) x;
    winY = (float) viewport[3] - (float) y;
    glReadPixels(x, int(winY), 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &winZ);

    gluUnProject(winX, winY, winZ, modelview, projection, viewport, &posX, &posY, &posZ);

    return sf::Vector3f((float) posX, (float) posY, (float) posZ);
}

Here, sf::Vector3f is simply a SFML Vector class.