Screen to World Coordinates

[atcBrambo]

Hi.
I have found code like this also under many C# C++ (and C-- ahh ahh)
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function GetOpenGLPosFromMouse(X, Y: Integer): TGLVectord3;
var
    viewport:   TGLVectori4;
    modelview:  TGLMatrixd4;
    projection: TGLMatrixd4;
    winZ,winY:  Single;
begin
    glGetDoublev(GL_MODELVIEW_MATRIX, @modelview );   // Modelview
    glGetDoublev(GL_PROJECTION_MATRIX, @projection );    // Projection
    glGetIntegerv(GL_VIEWPORT, @viewport );                      // Viewport

  winY := viewport[3] - y;                                                 //Change from Win32 to OpenGL coordinate system

    glReadPixels(X, Round(winY), 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, @winZ );
    gluUnProject(X, winY, winZ,
                           modelview, projection, viewport,                             
               @Result[0], @Result[1], @Result[2]);  
end;

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But it doesn't work; in fact I tested with this
---------------------------------------------

OpenGLMousePosition:=GetOpenGLPosFromMouse(DelphiMousePosition.X,DelphiMousePosition.Y);

    glDisable(GL_DEPTH_TEST);
             
    Main_Form.Caption:= 'Mouse Position is '+
                        'x..:'+IntToStr(DelphiMousePosition.X)+'  '+
                        'y..:'+IntToStr(DelphiMousePosition.y)+'  '+
                        '   OpenGl Mouse '+
                        'x..:'+FormatFloat('#,##0.0;-#,##0.0;0.0',OpenGLMousePosition[0])+'  '+
                        'y..:'+FormatFloat('#,##0.0;-#,##0.0;0.0',OpenGLMousePosition[1])+'  '+
                        'z..:'+FormatFloat('#,##0.0;-#,##0.0;0.0',OpenGLMousePosition[2])+'  ';
                        
    glColor3d(1.0,1.0,0.0);
    glLineWidth(3);

    glBegin(GL_LINES);
      glVertex3d( 0, 0, 0);
      glVertex3d( OpenGLMousePosition[0],OpenGLMousePosition[1],OpenGLMousePosition[2]);
    glEnd();

------------------------------------------------------------------------------------
while moving the mouse the line goes somewhere else. Occasionally near (maybe about 40 pixels away) the mouse.

Did someone already faced the headache ?

JPG sample cannot be upload .... too large.
I will be happy to send it.


Giovanni.
Italy.