01 // Uses the mouse position to calculate
02 // a 3D dot in the scene
03 void projection(SoQtRenderArea *renderArea,
04    int mouseX, int mouseY, SbVec3f &interface)
05 {
06    // Position of mouse in rendering area:
07    SbVec2s size = renderArea->getSize(
08    float x = float(mouseX) / size[0];
09    float y = float(size[1] - mouseY) / size[1];
10
11    // Pointer to camera
12    SoGroup *root = (SoGroup *) renderArea->getSceneGraph();
13    SoCamera *camera = (SoCamera *) root->getChild(0);
14
15    // Volume visible to camera
16    SbViewVolume cameraVolume;
17    cameraVolume = camera->getViewVolume();
18
19    // Mouse line vertical to scene
20    // The other endpoint is created by mirroring on the plane
21    SbVec3f p0, p1;
22    cameraVolume.projectDotToLine(SbVec2f(x,y), p0, p1);
23
24    // The center of the line is the position we are looking for
25    // on the surface, which goes through the origin
26    interface = (p0 + p1) / 2.0;
27 }