gluUnProject

OpenGL OpenGL: Basic Coding
1

Hi,

I have a couple questions conserning gluUnProject().

  1. What the heck is the winZ coordinate? And how do I get it?
  2. Where should I grap the modelview and projection matrices?
  3. Where can I find tutorials on how to deal with gluUnProject()?

Thanks for your help,

Martin

2

Explaination:

The Z co-ordinate usually comes from a picking render or some kind of hit test. In order to correctly unproject a point, you need to know its distance in the scene.

As for getting model and projection matrices, see below. The method below takes mouse co-ordinates (from windows) and unprojects them. The `vertex’ parameter contains the x, y mouse co-ordinates and the z co-ordinate given by the picking render.

//
// Unproject the given mouse point back into the scene.
//

void CSelectionBuffer :: UnProjectMouse ( CVertex3D& Point, CVertex3D& Out )

{
//
// Finally, we need to reverse transform the values we have back into the world to get correct world x, y, z.
//

GLdouble	Projection	[16], Model [16];
GLint		Viewport	[4];

//
// Now get the matrices and viewport.
//

glGetDoublev ( GL_PROJECTION_MATRIX, Projection );
glGetDoublev ( GL_MODELVIEW_MATRIX, Model );
glGetIntegerv( GL_VIEWPORT, Viewport );

//
// We will need to invert mouse y (because its top left whereas our window is bottom left origin).
//

double Invert_m_y = static_cast<double> ( Viewport [3] ) - static_cast<double> ( Point.v [1] );

HITRECORD h1, h2;

//
// Inverse transform the given point - note its important that the viewing transforms are setup as they were rendered.
//

gluUnProject (	static_cast<double> ( Point.v [0] ),	// Mouse x.
				Invert_m_y,								// Inverted mouse y.
				0.0,									// The z depth result we got from selection.
				Model,									// Model view matrix.
				Projection,								// Projection matrix.
				Viewport,								// Viewport co-ordinates.
				&h1.x,									// Final result x.
				&h1.y,									// Final result y.
				&h1.z );								// Final result z.

//
// Copy results across ( from double to float ).
//

Out.v [0] = static_cast<float> ( h1.x );
Out.v [1] = static_cast<float> ( h1.y );
Out.v [2] = static_cast<float> ( h1.z );

}

[This message has been edited by Robbo (edited 05-24-2002).]

3

Okay,

it works, at least somehow. If I rotate my model so it is rotated 90° the coordinates I get back are everything but not right!

HEEELLLPP!

4

Well, some source-code would be nice - we need to see how you get your z and what your screen x, y are. Also, how you unproject. Don’t forget that your transform (model) matrix must be the same when you unproject as it was when you rendered the scene in the first place. ie:

glPushMatrix ()
…ApplyMyCameraTransform ()
…DrawMyStuff ()
…
glPopMatrix ()

…
… then, before you unproject,

ApplyMyCameraTransform ()
…Do My unprojection
…

etc…

5

Hm, sorry about my panic, but it was my confused mind which made me go berserk and mess up my code.
It works now, thanks for your help!!

Martin