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<h1>The select example</h1>
<center>
<img src="../images/select.jpg" width="300" height="200" alt="select"/>
</center>
<p>
Selection of objects of the scene using <code>select()</code> and an OpenGL GL_SELECT render mode.
</p>
<p>
Use the <code>select()</code> callback function to implement your object selection function. This
examples is based on a generic GL_SELECT implementation that can easily be cut and pasted in your
applications.
</p>
<p>
Analytic intersection computations are also possible once the screen coordinates have be converted
to a half line using <code>convertClickToLine()</code>. Make a selection and then move the camera
to see a representation of the intersection line.
</p>
<h2>select.h</h2>
<pre>
<span class="dir">#include <QGLViewer/qglviewer.h>
</span>
<span class="key">class </span>Viewer : <span class="key">public </span>QGLViewer
{
<span class="key">protected </span>:
<span class="key">virtual </span><span class="typ">void </span>draw();
<span class="key">virtual </span><span class="typ">void </span>select(<span class="typ">const </span>QMouseEvent*);
<span class="key">virtual </span><span class="typ">void </span>init();
<span class="key">virtual </span>QString helpString() <span class="typ">const</span>;
<span class="key">private </span>:
qglviewer::Vec orig, dir, selectedPoint;
};</pre>
<h2>select.cpp</h2>
<pre>
<span class="dir">#include </span><span class="dstr">"select.h"</span><span class="dir">
</span><span class="dir">#include <math.h>
</span><span class="dir">#include <limits.h> </span><span class="com">// UINT_MAX
</span><span class="dir"></span>
<span class="key">using namespace </span>std;
<span class="com">// The id of the selected object. Should be encapsulated.
</span><span class="com">// -1 means no object is selected.
</span><span class="typ">static int </span>selected;
<span class="typ">static void </span>drawSpiral(<span class="typ">const bool </span>specialColor = <span class="key">false</span>)
{
<span class="typ">const float </span>nbSteps = <span class="num">100.0</span>;
glBegin(GL_QUAD_STRIP);
<span class="key">for </span>(<span class="typ">float </span>i=<span class="num">0</span>; i<nbSteps; ++i)
{
<span class="key">if </span>(specialColor)
glColor3f((nbSteps-i)/nbSteps, .<span class="num">8 </span>, i/nbSteps/<span class="num">2.0</span>);
<span class="key">else
</span>glColor3f((nbSteps-i)/nbSteps, .<span class="num">2 </span>, i/nbSteps);
<span class="typ">float </span>angle = i/<span class="num">4.0</span>;
<span class="typ">float </span>c = cos(angle);
<span class="typ">float </span>s = sin(angle);
<span class="typ">float </span>r1 = <span class="num">0.5 </span>- i/(<span class="num">3.</span>f*nbSteps);
<span class="typ">float </span>r2 = <span class="num">0.3 </span>- i/(<span class="num">3.</span>f*nbSteps);
<span class="typ">float </span>alt = i/nbSteps <span class="num">- 0.5</span>;
<span class="typ">const float </span>nor = .<span class="num">5</span>;
<span class="typ">const float </span>up = sqrt(<span class="num">1.0</span>-nor*nor);
glNormal3f(nor*c, nor*s, up);
glVertex3f(r1*c, r1*s, alt);
glVertex3f(r2*c, r2*s, alt+<span class="num">0.05</span>);
}
glEnd();
}
<span class="typ">static void </span>drawScene(<span class="typ">bool </span>pushId = <span class="key">false</span>)
{
<span class="com">// Draw the scene, with a possible pushName() for selection
</span>
<span class="com">// Consider using several stack levels for different objects, or to separate
</span> <span class="com">// the triangles, edges and vertices of the same object. Example :
</span> <span class="com">// glPushName(0)
</span> <span class="com">// for all triangles i, glPushName(i), draw triangle, glPopName()
</span> <span class="com">// glPopName()
</span> <span class="com">//
</span> <span class="com">// glPushName(1)
</span> <span class="com">// for all edges i, glPushName(i), draw edge, glPopName()
</span> <span class="com">// glPopName()
</span> <span class="com">//
</span> <span class="com">// glPushName(2)
</span> <span class="com">// for all vertex i, glPushName(i), raster vertex, glPopName()
</span> <span class="com">// glPopName()
</span> <span class="com">// As a result, you have a two level stack, with a type id (0, 1 or 2 here)
</span> <span class="com">// which indicates the type of the primitive, and then the id of the primitive.
</span> <span class="com">// See the man page of glSelectBuffer() for details.
</span>
<span class="typ">const int </span>nb = <span class="num">10</span>;
<span class="key">for </span>(<span class="typ">int </span>i=<span class="num">0</span>; i<nb; ++i)
{
glPushMatrix();
glTranslatef(cos(<span class="num">2.0</span>*i*M_PI/nb), sin(<span class="num">2.0</span>*i*M_PI/nb), <span class="num">0.</span>);
<span class="key">if </span>(pushId)
{
glPushName(i);
drawSpiral();
glPopName();
}
<span class="key">else
</span>drawSpiral(i==selected);
glPopMatrix();
}
}
<span class="typ">void </span>Viewer::select(<span class="typ">const </span>QMouseEvent* e)
{
<span class="com">// You should cut-paste this rather tricky function as your own "select()" function.
</span> <span class="com">// You need a drawing function that pushes and pops names to tag objects' id (see drawSpiral() above).
</span> <span class="com">// You have to change the call to the drawWithPush (here a simple drawScene()).
</span> <span class="com">// "selected" is set to the closest object that was picked (or -1 if no object was selected)
</span>
<span class="com">// Make openGL context current
</span> makeCurrent();
<span class="typ">const int </span>SENSITIVITY = <span class="num">4</span>;
<span class="typ">const int </span>NB_HITS_MAX = <span class="num">64</span>;
<span class="com">// Prepare the selection mode
</span> <span class="typ">static </span>GLuint hits[NB_HITS_MAX];
glSelectBuffer(NB_HITS_MAX, hits);
glRenderMode(GL_SELECT);
glInitNames();
<span class="com">// Loads the matrices
</span> glMatrixMode(GL_PROJECTION);
glLoadIdentity();
GLint viewport[<span class="num">4</span>];
camera()->getViewport(viewport);
gluPickMatrix(<span class="key">static_cast</span><GLdouble>(e->x()), <span class="key">static_cast</span><GLdouble>(e->y()), SENSITIVITY, SENSITIVITY, viewport);
<span class="com">// loadProjectionMatrix() first resets the GL_PROJECTION matrix with a glLoadIdentity.
</span> <span class="com">// Give false as a parameter in order to prevent this and to combine the matrices.
</span> camera()->loadProjectionMatrix(<span class="key">false</span>);
camera()->loadModelViewMatrix();
<span class="com">// Render scene with objects ids.
</span> <span class="com">// Change here to call your own "drawWithPush()" function.
</span> drawScene(<span class="key">true</span>);
glFlush();
<span class="com">// Get the number of objects were seen through the pick matrix frustum. Reset GL_RENDER mode.
</span> GLint nb_hits = glRenderMode(GL_RENDER);
<span class="com">// Compute orig and dir, used to draw a representation of the intersecting line
</span> camera()->convertClickToLine(e->x(), e->y(), orig, dir);
<span class="com">// Display of selection results
</span> cout << <span class="str">"("</span> << e->x() << <span class="str">","</span> << e->y() << <span class="str">") : "</span>;
<span class="key">if </span>(nb_hits <= <span class="num">0</span>)
{
selected = <span class="num">-1</span>;
cout << <span class="str">"No object selected"</span> << endl;
<span class="key">return</span>;
}
cout << nb_hits << <span class="str">" spiral"</span> << ((nb_hits><span class="num">1</span>)?<span class="str">"s"</span>:<span class="str">""</span>) << <span class="str">" under the cursor"</span> << endl;
<span class="com">// Interpret results : each object created 4 values in the "hits" array :
</span> <span class="com">// hits[4*i+1] is the object minimum depth value, while hits[4*i+3] is the id pushed on the stack.
</span> <span class="com">// Of all the objects that were projected in the pick region, we select the closest one (zMin comparaison).
</span> <span class="com">// This code needs to be modified if you use several stack levels. See glSelectBuffer() man page.
</span> <span class="typ">unsigned int </span>zMin = hits[<span class="num">1</span>];
selected = hits[<span class="num">3</span>];
<span class="key">for </span>(<span class="typ">int </span>i=<span class="num">1</span>; i<nb_hits; ++i)
<span class="key">if </span>(hits[i*<span class="num">4</span>+<span class="num">1</span>] < zMin)
{
zMin = hits[i*<span class="num">4</span>+<span class="num">1</span>];
selected = hits[i*<span class="num">4</span>+<span class="num">3</span>];
}
<span class="com">// Find the selectedPoint coordinates, using camera()->pointUnderPixel().
</span> <span class="typ">bool </span>found;
selectedPoint = camera()->pointUnderPixel(e->x(), e->y(), found);
selectedPoint -= <span class="num">0.01</span>*dir; <span class="com">// Small offset to make point clearly visible.
</span> <span class="com">// Note that "found" is different from (selected>=0) because of SENSITIVITY.
</span>}
<span class="typ">void </span>Viewer::init()
{
restoreFromFile();
<span class="com">// Means no object is selected.
</span> selected = <span class="num">-1</span>;
glLineWidth(<span class="num">3.0</span>);
glPointSize(<span class="num">10.0</span>);
help();
}
<span class="typ">void </span>Viewer::draw()
{
drawScene();
<span class="com">// Draw the previous intersection line
</span> glBegin(GL_LINES);
glVertex3fv(orig.address());
glVertex3fv((orig + <span class="num">100.0</span>*dir).address());
glEnd();
<span class="key">if </span>(selected >= <span class="num">0</span>)
{
glColor3f(.<span class="num">9</span>, .<span class="num">2</span>, .<span class="num">1</span>);
glBegin(GL_POINTS);
glVertex3fv(selectedPoint.address());
glEnd();
}
}
QString Viewer::helpString() <span class="typ">const
</span>{
QString text(<span class="str">"<h2>S e l e c t</h2>"</span>);
text += <span class="str">"Left click while pressing the <b>Shift</b> key to select an object of the scene.<br>"</span>;
text += <span class="str">"Selection is performed using the OpenGL <i>GL_SELECT</i> render mode. "</span>;
text += <span class="str">"A line is drawn between the selected point and the camera selection position. "</span>;
text += <span class="str">"using <i>convertClickToLine()</i>, a useful function for analytical intersections.<br>"</span>;
text += <span class="str">"Feel free to cut and paste this implementation in your own applications."</span>;
<span class="key">return </span>text;
}</pre>
<h2>main.cpp</h2>
<pre>
<span class="dir">#include </span><span class="dstr">"select.h"</span><span class="dir">
</span><span class="dir">#include <qapplication.h>
</span>
<span class="typ">int </span>main(<span class="typ">int </span>argc, <span class="typ">char</span>** argv)
{
<span class="com">// Read command lines arguments.
</span> QApplication application(argc,argv);
<span class="com">// Instantiate the viewer.
</span> Viewer v;
<span class="com">// Make the viewer window visible on screen.
</span> v.show();
<span class="com">// Set the viewer as the application main widget.
</span> application.setMainWidget(&v);
<span class="com">// Run main loop.
</span> <span class="key">return </span>application.exec();
}</pre>
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<i>Last modified on Thursday, February 5, 2004.</i>
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