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A small VRML viewer using OpenGL and MFC

, 30 Nov 1999
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<h1>
A small VRML viewer&nbsp;</h1></center>

<center>
<h1>
using OpenGL and MFC</h1></center>

<center>
<h4>
<a href="mailto:pierre.alliez@cnet.francetelecom.fr">Pierre Alliez</a></h4></center>

<center>
<h4>
&nbsp;</h4></center>
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<tr>
<td WIDTH="50%"><b>Environment</b>: VC6 SP2, NT4 SP5</td>

<td WIDTH="50%"><b>Keywords</b>: OpenGL, WRL, GUI</td>
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<p><img SRC="mnt.jpg" height=482 width=600>
<p><i><font size=-1>FIG. 1. You can easily display a wrl-based terrain
using OpenGL and MFC.</font></i>
<br>&nbsp;
<p>This contribution is a small MFC sample to learn how to :
<ul>
<li>
display a VRML file</li>

<li>
use OpenGL display lists</li>

<li>
superimpose wireframe on a flat or smoothly shaded mesh</li>

<li>
smoothly subdivide a 3D triangular mesh (from Charles Loop)</li>

<li>
implement mouse interaction (rotation and translation)</li>

<li>
build a scene graph from a vrml 2.0 file (hand-made and not lex-based)</li>
</ul>

<h3>
DISPLAY LIST</h3>
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 :
<pre>int list = ::glGenLists(1); <font color="#009900">// ask for a free id number</font>&nbsp;
::glNewList(list,GL_COMPILE_AND_EXECUTE);&nbsp;
&nbsp; ::glBegin(GL_TRIANGLES);&nbsp;
&nbsp; <font color="#009900">// std gl calls here... fill vertices, normals, colors</font>&nbsp;
&nbsp; ::glEnd();&nbsp;
::glEndList();</pre>
A good command sequence to use a display list may be :
<pre><font face="Courier New,Courier">if(::glIsList(list) == GL_TRUE)
&nbsp; ::glCallList(m_ListOpenGL);</font></pre>
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.
<pre><font face="Courier New,Courier"><font color="#009900">//********************************************&nbsp;
// The 3D mesh class definition&nbsp;
//********************************************&nbsp;
</font>class CMesh3d : public CObject3d&nbsp;
{&nbsp;
private :&nbsp;

&nbsp;// Std datas&nbsp;
&nbsp;CArray&lt;CVertex3d> m_ArrayVertex;&nbsp;
&nbsp;CArray&lt;CFace3d>&nbsp;&nbsp; m_ArrayFace;&nbsp;

&nbsp;// OpenGL-specific&nbsp;
&nbsp;unsigned int <b>m</b>_<b>ListOpenGL</b>;&nbsp;
&nbsp;BOOL m_ListDone;&nbsp;
&nbsp;BOOL m_Modified;&nbsp;
&nbsp;.../...&nbsp;

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

<font color="#009900">//********************************************&nbsp;
// Mesh drawing&nbsp;
//********************************************&nbsp;
</font>BOOL CMesh3d::glDraw()&nbsp;
{&nbsp;
&nbsp;// Build list at first&nbsp;
&nbsp;if(!m_ListDone || m_Modified)&nbsp;
&nbsp; glBuildList();&nbsp;

&nbsp;// Is the list valid ?&nbsp;
&nbsp;if(::glIsList(m_ListOpenGL)==GL_TRUE)&nbsp;
&nbsp;{&nbsp;
&nbsp;&nbsp; ::<b>glCallList</b>(m_ListOpenGL);&nbsp;
&nbsp; return TRUE;&nbsp;
&nbsp;}&nbsp;
&nbsp;return FALSE;&nbsp;
}</font></pre>

<h3>
SUPERIMPOSING WIREFRAME</h3>
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.
<pre><font color="#009900">//***********************************************&nbsp;
// RenderScene&nbsp;
//***********************************************</font>&nbsp;
void CMeshDoc::RenderScene()&nbsp;
{&nbsp;
&nbsp;// Main drawing&nbsp;
&nbsp;m_SceneGraph.glDraw();&nbsp;

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

&nbsp; // Draw again...&nbsp;
&nbsp; m_SceneGraph.glDraw(TYPE_MESH3D);&nbsp;

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

&nbsp;::glFlush();&nbsp;
}</pre>

<h3>
<img SRC="superimp.jpg" height=454 width=600></h3>
<i><font size=-1>FIG. 2. You can superimpose a wireframe on a flat-rendered
mesh, using a second rendering loop with the line option.</font></i>
<h3>
</h3>

<h3>
SMOOTH SUBDIVISION</h3>
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.
<pre><img SRC="subdivision.jpg" height=191 width=600></pre>
<i><font size=-1>FIG.3. The one-to-four triangle subdivision scheme used
by method.</font></i>
<pre><font color="#006600">//***********************************************&nbsp;
// Smooth subdivision&nbsp;
</font><font color="#009900">//***********************************************</font>&nbsp;
void CMeshDoc::OnMeshLoop()&nbsp;
{&nbsp;
&nbsp;BeginWaitCursor();&nbsp;
&nbsp;int NbObject = m_SceneGraph.NbObject();&nbsp;
&nbsp;for(int i=0;i&lt;NbObject;i++)&nbsp;
&nbsp;{&nbsp;
&nbsp;&nbsp; CObject3d *pObject3d = m_SceneGraph[i];&nbsp;
&nbsp;&nbsp; if(pObject3d->GetType() == TYPE_MESH3D)&nbsp;
&nbsp; {&nbsp;
&nbsp;&nbsp; CMesh3d *pMesh&nbsp; = (CMesh3d *)pObject3d;&nbsp;
&nbsp;&nbsp; pMesh-><b>SubdivisionLoop</b>();&nbsp;
&nbsp; }&nbsp;
&nbsp;}&nbsp;
&nbsp;UpdateAllViews(NULL);&nbsp;
&nbsp;EndWaitCursor();&nbsp;
}</pre>

<h3>
<img SRC="subd_mesh.jpg" height=414 width=600></h3>
<i><font size=-1>FIG.4. Two successives iterations of one-to-four subdivision
scheme.</font></i><i><font size=-1></font></i>
<p><img SRC="subd_smooth.jpg" height=414 width=600><i><font size=-1></font></i>
<p><i><font size=-1>FIG.5. See the visual enhancement obtained by a smooth
subdivsion scheme..</font></i>
<br><i><font size=-1></font></i>&nbsp;
<h3>
MOUSE INTERACTION</h3>
A few variables and commands inserted in the view permit mouse interaction.
<pre><font color="#009900">//***********************************************&nbsp;
// Left button -> x/y translation&nbsp;
//***********************************************&nbsp;
</font>void CMeshView::OnLButtonDown(UINT nFlags, CPoint point)&nbsp;
{&nbsp;
&nbsp;m_LeftButtonDown = TRUE;&nbsp;
&nbsp;m_LeftDownPos = point;&nbsp;
&nbsp;SetCapture();&nbsp;
&nbsp;CView::OnLButtonDown(nFlags, point);&nbsp;
}&nbsp;
void CMeshView::OnLButtonUp(UINT nFlags, CPoint point)&nbsp;
{&nbsp;
&nbsp;m_RightButtonDown = FALSE;&nbsp;
&nbsp;m_LeftButtonDown = FALSE;&nbsp;
&nbsp;ReleaseCapture();&nbsp;
&nbsp;CView::OnLButtonUp(nFlags, point);&nbsp;
}&nbsp;

<font color="#009900">//***********************************************&nbsp;
// Right button : z translation&nbsp;
//***********************************************&nbsp;
</font>void CMeshView::OnRButtonDown(UINT nFlags, CPoint point)&nbsp;
{&nbsp;
&nbsp;m_RightButtonDown = TRUE;&nbsp;
&nbsp;m_RightDownPos = point;&nbsp;
&nbsp;SetCapture();&nbsp;
&nbsp;CView::OnRButtonDown(nFlags, point);&nbsp;
}&nbsp;
void CMeshView::OnRButtonUp(UINT nFlags, CPoint point)&nbsp;
{&nbsp;
&nbsp;m_RightButtonDown = FALSE;&nbsp;
&nbsp;m_LeftButtonDown = FALSE;&nbsp;
&nbsp;ReleaseCapture();&nbsp;
&nbsp;CView::OnRButtonUp(nFlags, point);&nbsp;
}&nbsp;

<font color="#009900">//***********************************************&nbsp;
// Mouse move&nbsp;
// Both : rotation&nbsp;
// Left : x / y translation&nbsp;
// Right : z translation&nbsp;
//***********************************************&nbsp;
</font>void CMeshView::OnMouseMove(UINT nFlags, CPoint point)&nbsp;
{&nbsp;
&nbsp;// Both : rotation&nbsp;
&nbsp;if(m_LeftButtonDown &amp;&amp; m_RightButtonDown)&nbsp;
&nbsp;{&nbsp;
&nbsp; if(m_xyRotation)&nbsp;
&nbsp; {&nbsp;
&nbsp;&nbsp; m_yRotation -= (float)(m_LeftDownPos.x - point.x) * m_SpeedRotation;&nbsp;
&nbsp;&nbsp; m_xRotation -= (float)(m_LeftDownPos.y - point.y) * m_SpeedRotation;&nbsp;
&nbsp; }&nbsp;
&nbsp; else&nbsp;
&nbsp; {&nbsp;
&nbsp;&nbsp; m_zRotation -= (float)(m_LeftDownPos.x - point.x) * m_SpeedRotation;&nbsp;
&nbsp;&nbsp; m_xRotation -= (float)(m_LeftDownPos.y - point.y) * m_SpeedRotation;&nbsp;
&nbsp; }&nbsp;
&nbsp; m_LeftDownPos = point;&nbsp;
&nbsp; m_RightDownPos = point;&nbsp;
&nbsp; InvalidateRect(NULL,FALSE);&nbsp;
&nbsp;}&nbsp;

&nbsp;else&nbsp;

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

&nbsp;else&nbsp;

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

&nbsp;CView::OnMouseMove(nFlags, point);&nbsp;
}</pre>

<h3>
Downloads</h3>
<a href="wrl_viewer.zip">Download demo project - 1000 Kb</a>
<br>&nbsp;
<h3>
History</h3>
Date Posted: November 15, 1999
<br>Last Updated: November 15, 1999
<br>&nbsp;
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Pierre Alliez

France France
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