A small VRML viewer using OpenGL and MFC

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FIG. 1. You can easily display a wrl-based terrain using OpenGL and MFC.
 

This contribution is a small MFC sample to learn how to :

• display a VRML file
• use OpenGL display lists
• superimpose wireframe on a flat or smoothly shaded mesh
• smoothly subdivide a 3D triangular mesh (from Charles Loop)
• implement mouse interaction (rotation and translation)
• build a scene graph from a vrml 2.0 file (hand-made and not lex-based)

DISPLAY LIST

Using display lists is a nice way to accelerate your rendering application. A display list compiles a sequence of gl drawings using standard OpenGL calls, then can be recalled later using a simple list id number. The resulting list is thus resident in the main memory in a precompilated mode, the which greatly accelerates rendering loops. A good command sequence to build a display list may be :

int list = ::glGenLists(1); // ask for a free id number 
::glNewList(list,GL_COMPILE_AND_EXECUTE); 
  ::glBegin(GL_TRIANGLES); 
  // std gl calls here... fill vertices, normals, colors 
  ::glEnd(); 
::glEndList();

A good command sequence to use a display list may be :

if(::glIsList(list) == GL_TRUE)
  ::glCallList(m_ListOpenGL);

The sample builds a scene graph from a vrml 2.0 file (exported via 3D Studio Max only), then uses display lists. Each 3D mesh contains a list number, and use a glCallList command instead of standards glBegin(GL_TRIANGLES) commands when its list is built. A flag m_Modified permits to rebuild the list when the mesh is modified.

//******************************************** 
// The 3D mesh class definition 
//******************************************** 
class CMesh2d : public CObject3d 
{ 
private : 

 // Std datas 
 CArray<CVertex3d> m_ArrayVertex; 
 CArray<CFace3d>   m_ArrayFace; 

 // OpenGL-specific 
 unsigned int m_ListOpenGL; 
 BOOL m_ListDone; 
 BOOL m_Modified; 
 .../... 

public : 
 BOOL glDraw(); 
 .../... 
} 

//******************************************** 
// Mesh drawing 
//******************************************** 
BOOL CMesh2d::glDraw() 
{ 
 // Build list at first 
 if(!m_ListDone || m_Modified) 
  glBuildList(); 

 // Is the list valid ? 
 if(::glIsList(m_ListOpenGL)==GL_TRUE) 
 { 
   ::glCallList(m_ListOpenGL); 
  return TRUE; 
 } 
 return FALSE; 
}

SUPERIMPOSING WIREFRAME

Sometime you would like to view the wireframe superimposing the flat or smooth shaded mesh. A good way to do this is to use the glPolygonOffset command, which creates a z-buffer offset. The following code shows the RenderScene function of the document, if one resumes two rendering passes are necessary, the first render the mesh using lighted flat mode, the second cut off the light, set the line mode, set a z-buffer offset, then draw the mesh again.

//*********************************************** 
// RenderScene 
//*********************************************** 
void CMeshDoc::RenderScene() 
{ 
 // Main drawing 
 m_SceneGraph.glDraw(); 

 // Add wireframe (no light, and line mode) 
 if(m_AddWireframe) 
 { 
  // Set state 
  ::glDisable(GL_LIGHTING); 
  ::glPolygonMode(GL_FRONT_AND_BACK,GL_LINE); 
  ::glEnable(GL_POLYGON_OFFSET_LINE); 
  ::glPolygonOffset(m_PolygonOffset,-1.0f); 

  // Draw again... 
  m_SceneGraph.glDraw(TYPE_MESh2D); 

  // Restore light and mode 
  ::glDisable(GL_POLYGON_OFFSET_LINE); 
  ::glPolygonMode(GL_FRONT_AND_BACK,GL_FILL); 
  ::glEnable(GL_LIGHTING); 
 } 

 ::glFlush(); 
}

FIG. 2. You can superimpose a wireframe on a flat-rendered mesh, using a second rendering loop with the line option.

SMOOTH SUBDIVISION

From a given 3D mesh, how can we improve the geometric appearence on smooth models ? The Charles Loop smooth subdivision comes here to help us. Each triangle is divided in four triangles (see figure 3), and a filtering function permits the mesh to be smoothed. The command is easy-to-use in the document, and I let you discover the details in the mesh's code.

FIG.3. The one-to-four triangle subdivision scheme used by method.

//*********************************************** 
// Smooth subdivision 
//*********************************************** 
void CMeshDoc::OnMeshLoop() 
{ 
 BeginWaitCursor(); 
 int NbObject = m_SceneGraph.NbObject(); 
 for(int i=0;i<NbObject;i++) 
 { 
   CObject3d *pObject3d = m_SceneGraph[i]; 
   if(pObject3d->GetType() == TYPE_MESh2D) 
  { 
   CMesh2d *pMesh  = (CMesh2d *)pObject3d; 
   pMesh->SubdivisionLoop(); 
  } 
 } 
 UpdateAllViews(NULL); 
 EndWaitCursor(); 
}

FIG.4. Two successives iterations of one-to-four subdivision scheme.

FIG.5. See the visual enhancement obtained by a smooth subdivsion scheme..
 

MOUSE INTERACTION

A few variables and commands inserted in the view permit mouse interaction.

//*********************************************** 
// Left button -> x/y translation 
//*********************************************** 
void CMeshView::OnLButtonDown(UINT nFlags, CPoint point) 
{ 
 m_LeftButtonDown = TRUE; 
 m_LeftDownPos = point; 
 SetCapture(); 
 CView::OnLButtonDown(nFlags, point); 
} 
void CMeshView::OnLButtonUp(UINT nFlags, CPoint point) 
{ 
 m_RightButtonDown = FALSE; 
 m_LeftButtonDown = FALSE; 
 ReleaseCapture(); 
 CView::OnLButtonUp(nFlags, point); 
} 

//*********************************************** 
// Right button : z translation 
//*********************************************** 
void CMeshView::OnRButtonDown(UINT nFlags, CPoint point) 
{ 
 m_RightButtonDown = TRUE; 
 m_RightDownPos = point; 
 SetCapture(); 
 CView::OnRButtonDown(nFlags, point); 
} 
void CMeshView::OnRButtonUp(UINT nFlags, CPoint point) 
{ 
 m_RightButtonDown = FALSE; 
 m_LeftButtonDown = FALSE; 
 ReleaseCapture(); 
 CView::OnRButtonUp(nFlags, point); 
} 

//*********************************************** 
// Mouse move 
// Both : rotation 
// Left : x / y translation 
// Right : z translation 
//*********************************************** 
void CMeshView::OnMouseMove(UINT nFlags, CPoint point) 
{ 
 // Both : rotation 
 if(m_LeftButtonDown && m_RightButtonDown) 
 { 
  if(m_xyRotation) 
  { 
   m_yRotation -= (float)(m_LeftDownPos.x - point.x) * m_SpeedRotation; 
   m_xRotation -= (float)(m_LeftDownPos.y - point.y) * m_SpeedRotation; 
  } 
  else 
  { 
   m_zRotation -= (float)(m_LeftDownPos.x - point.x) * m_SpeedRotation; 
   m_xRotation -= (float)(m_LeftDownPos.y - point.y) * m_SpeedRotation; 
  } 
  m_LeftDownPos = point; 
  m_RightDownPos = point; 
  InvalidateRect(NULL,FALSE); 
 } 

 else 

 // Left : x / y translation 
 if(m_LeftButtonDown) 
 { 
  m_xTranslation -= (float)(m_LeftDownPos.x - point.x) * m_SpeedTranslation; 
  m_yTranslation += (float)(m_LeftDownPos.y - point.y) * m_SpeedTranslation; 
  m_LeftDownPos = point; 
  InvalidateRect(NULL,FALSE); 
 } 

 else 

 // Right : z translation 
 if(m_RightButtonDown) 
 { 
  m_zTranslation += (float)(m_RightDownPos.y - point.y) * m_SpeedTranslation; 
  m_RightDownPos = point; 
  InvalidateRect(NULL,FALSE); 
 } 

 CView::OnMouseMove(nFlags, point); 
}