Tutorial 7: Collision

We will describe 2 methods: Automatic collision detection for moving through 3d worlds with stair climbing and sliding, and manual scene node and triangle picking using a ray. In this case, we will use a ray coming out from the camera, but you can use any ray.

To start, we take the program from tutorial 2, which loads and displays a quake 3 level. We will use the level to walk in it and to pick triangles from. In addition we'll place 3 animated models into it for triangle picking. The following code starts up the engine and loads the level, as per tutorial 2.

#include <irrlicht.h>
#include "driverChoice.h"

using namespace irr;

#ifdef _MSC_VER
#pragma comment(lib, "Irrlicht.lib")
#endif

enum
{
    // I use this ISceneNode ID to indicate a scene node that is
    // not pickable by getSceneNodeAndCollisionPointFromRay()
    ID_IsNotPickable = 0,

    // I use this flag in ISceneNode IDs to indicate that the
    // scene node can be picked by ray selection.
    IDFlag_IsPickable = 1 << 0,

    // I use this flag in ISceneNode IDs to indicate that the
    // scene node can be highlighted.  In this example, the
    // homonids can be highlighted, but the level mesh can't.
    IDFlag_IsHighlightable = 1 << 1
};

int main()
{
    // ask user for driver
    video::E_DRIVER_TYPE driverType=driverChoiceConsole();
    if (driverType==video::EDT_COUNT)
        return 1;

    // create device

    IrrlichtDevice *device =
        createDevice(driverType, core::dimension2d<u32>(640, 480), 16, false);

    if (device == 0)
        return 1; // could not create selected driver.

    video::IVideoDriver* driver = device->getVideoDriver();
    scene::ISceneManager* smgr = device->getSceneManager();

    device->getFileSystem()->addFileArchive("../../media/map-20kdm2.pk3");

    scene::IAnimatedMesh* q3levelmesh = smgr->getMesh("20kdm2.bsp");
    scene::IMeshSceneNode* q3node = 0;

    // The Quake mesh is pickable, but doesn't get highlighted.
    if (q3levelmesh)
        q3node = smgr->addOctreeSceneNode(q3levelmesh->getMesh(0), 0, IDFlag_IsPickable);

So far so good, we've loaded the quake 3 level like in tutorial 2. Now, here comes something different: We create a triangle selector. A triangle selector is a class which can fetch the triangles from scene nodes for doing different things with them, for example collision detection. There are different triangle selectors, and all can be created with the ISceneManager. In this example, we create an OctreeTriangleSelector, which optimizes the triangle output a little bit by reducing it like an octree. This is very useful for huge meshes like quake 3 levels. After we created the triangle selector, we attach it to the q3node. This is not necessary, but in this way, we do not need to care for the selector, for example dropping it after we do not need it anymore.

scene::ITriangleSelector* selector = 0;

if (q3node)
{
    q3node->setPosition(core::vector3df(-1350,-130,-1400));

    selector = smgr->createOctreeTriangleSelector(
            q3node->getMesh(), q3node, 128);
    q3node->setTriangleSelector(selector);
    // We're not done with this selector yet, so don't drop it.
}

We add a first person shooter camera to the scene so that we can see and move in the quake 3 level like in tutorial 2. But this, time, we add a special animator to the camera: A Collision Response animator. This animator modifies the scene node to which it is attached to in order to prevent it moving through walls, and to add gravity to it. The only thing we have to tell the animator is how the world looks like, how big the scene node is, how much gravity to apply and so on. After the collision response animator is attached to the camera, we do not have to do anything more for collision detection, anything is done automatically. The rest of the collision detection code below is for picking. And please note another cool feature: The collision response animator can be attached also to all other scene nodes, not only to cameras. And it can be mixed with other scene node animators. In this way, collision detection and response in the Irrlicht engine is really easy.

Now we'll take a closer look on the parameters of createCollisionResponseAnimator(). The first parameter is the TriangleSelector, which specifies how the world, against collision detection is done looks like. The second parameter is the scene node, which is the object, which is affected by collision detection, in our case it is the camera. The third defines how big the object is, it is the radius of an ellipsoid. Try it out and change the radius to smaller values, the camera will be able to move closer to walls after this. The next parameter is the direction and speed of gravity. We'll set it to (0, -10, 0), which approximates to realistic gravity, assuming that our units are metres. You could set it to (0,0,0) to disable gravity. And the last value is just a translation: Without this, the ellipsoid with which collision detection is done would be around the camera, and the camera would be in the middle of the ellipsoid. But as human beings, we are used to have our eyes on top of the body, with which we collide with our world, not in the middle of it. So we place the scene node 50 units over the center of the ellipsoid with this parameter. And that's it, collision detection works now.

// Set a jump speed of 3 units per second, which gives a fairly realistic jump
// when used with the gravity of (0, -10, 0) in the collision response animator.
scene::ICameraSceneNode* camera =
    smgr->addCameraSceneNodeFPS(0, 100.0f, .3f, ID_IsNotPickable, 0, 0, true, 3.f);
camera->setPosition(core::vector3df(50,50,-60));
camera->setTarget(core::vector3df(-70,30,-60));

if (selector)
{
    scene::ISceneNodeAnimator* anim = smgr->createCollisionResponseAnimator(
        selector, camera, core::vector3df(30,50,30),
        core::vector3df(0,-10,0), core::vector3df(0,30,0));
    selector->drop(); // As soon as we're done with the selector, drop it.
    camera->addAnimator(anim);
    anim->drop();  // And likewise, drop the animator when we're done referring to it.
}

// Now I create three animated characters which we can pick, a dynamic light for
// lighting them, and a billboard for drawing where we found an intersection.

// First, let's get rid of the mouse cursor.  We'll use a billboard to show
// what we're looking at.
device->getCursorControl()->setVisible(false);

// Add the billboard.
scene::IBillboardSceneNode * bill = smgr->addBillboardSceneNode();
bill->setMaterialType(video::EMT_TRANSPARENT_ADD_COLOR );
bill->setMaterialTexture(0, driver->getTexture("../../media/particle.bmp"));
bill->setMaterialFlag(video::EMF_LIGHTING, false);
bill->setMaterialFlag(video::EMF_ZBUFFER, false);
bill->setSize(core::dimension2d<f32>(20.0f, 20.0f));
bill->setID(ID_IsNotPickable); // This ensures that we don't accidentally ray-pick it

Add 3 animated hominids, which we can pick using a ray-triangle intersection. They all animate quite slowly, to make it easier to see that accurate triangle selection is being performed.

    scene::IAnimatedMeshSceneNode* node = 0;

    video::SMaterial material;

    // Add an MD2 node, which uses vertex-based animation.
    node = smgr->addAnimatedMeshSceneNode(smgr->getMesh("../../media/faerie.md2"),
                        0, IDFlag_IsPickable | IDFlag_IsHighlightable);
    node->setPosition(core::vector3df(-90,-15,-140)); // Put its feet on the floor.
    node->setScale(core::vector3df(1.6f)); // Make it appear realistically scaled
    node->setMD2Animation(scene::EMAT_POINT);
    node->setAnimationSpeed(20.f);
    material.setTexture(0, driver->getTexture("../../media/faerie2.bmp"));
    material.Lighting = true;
    material.NormalizeNormals = true;
    node->getMaterial(0) = material;

    // Now create a triangle selector for it.  The selector will know that it
    // is associated with an animated node, and will update itself as necessary.
    selector = smgr->createTriangleSelector(node);
    node->setTriangleSelector(selector);
    selector->drop(); // We're done with this selector, so drop it now.

    // And this B3D file uses skinned skeletal animation.
    node = smgr->addAnimatedMeshSceneNode(smgr->getMesh("../../media/ninja.b3d"),
                        0, IDFlag_IsPickable | IDFlag_IsHighlightable);
    node->setScale(core::vector3df(10));
    node->setPosition(core::vector3df(-75,-66,-80));
    node->setRotation(core::vector3df(0,90,0));
    node->setAnimationSpeed(8.f);
    node->getMaterial(0).NormalizeNormals = true;
    node->getMaterial(0).Lighting = true;
    // Just do the same as we did above.
    selector = smgr->createTriangleSelector(node);
    node->setTriangleSelector(selector);
    selector->drop();

    // This X files uses skeletal animation, but without skinning.
    node = smgr->addAnimatedMeshSceneNode(smgr->getMesh("../../media/dwarf.x"),
                        0, IDFlag_IsPickable | IDFlag_IsHighlightable);
    node->setPosition(core::vector3df(-70,-66,-30)); // Put its feet on the floor.
    node->setRotation(core::vector3df(0,-90,0)); // And turn it towards the camera.
    node->setAnimationSpeed(20.f);
    node->getMaterial(0).Lighting = true;
    selector = smgr->createTriangleSelector(node);
    node->setTriangleSelector(selector);
    selector->drop();


    // And this mdl file uses skinned skeletal animation.
    node = smgr->addAnimatedMeshSceneNode(smgr->getMesh("../../media/yodan.mdl"),
                        0, IDFlag_IsPickable | IDFlag_IsHighlightable);
    node->setPosition(core::vector3df(-90,-25,20));
    node->setScale(core::vector3df(0.8f));
    node->getMaterial(0).Lighting = true;
    node->setAnimationSpeed(20.f);

    // Just do the same as we did above.
    selector = smgr->createTriangleSelector(node);
    node->setTriangleSelector(selector);
    selector->drop();

    material.setTexture(0, 0);
    material.Lighting = false;

    // Add a light, so that the unselected nodes aren't completely dark.
    scene::ILightSceneNode * light = smgr->addLightSceneNode(0, core::vector3df(-60,100,400),
        video::SColorf(1.0f,1.0f,1.0f,1.0f), 600.0f);
    light->setID(ID_IsNotPickable); // Make it an invalid target for selection.

    // Remember which scene node is highlighted
    scene::ISceneNode* highlightedSceneNode = 0;
    scene::ISceneCollisionManager* collMan = smgr->getSceneCollisionManager();
    int lastFPS = -1;

    // draw the selection triangle only as wireframe
    material.Wireframe=true;

    while(device->run())
    if (device->isWindowActive())
    {
        driver->beginScene(true, true, 0);
        smgr->drawAll();

        // Unlight any currently highlighted scene node
        if (highlightedSceneNode)
        {
            highlightedSceneNode->setMaterialFlag(video::EMF_LIGHTING, true);
            highlightedSceneNode = 0;
        }

        // All intersections in this example are done with a ray cast out from the camera to
        // a distance of 1000.  You can easily modify this to check (e.g.) a bullet
        // trajectory or a sword's position, or create a ray from a mouse click position using
        // ISceneCollisionManager::getRayFromScreenCoordinates()
        core::line3d<f32> ray;
        ray.start = camera->getPosition();
        ray.end = ray.start + (camera->getTarget() - ray.start).normalize() * 1000.0f;

        // Tracks the current intersection point with the level or a mesh
        core::vector3df intersection;
        // Used to show with triangle has been hit
        core::triangle3df hitTriangle;

        // This call is all you need to perform ray/triangle collision on every scene node
        // that has a triangle selector, including the Quake level mesh.  It finds the nearest
        // collision point/triangle, and returns the scene node containing that point.
        // Irrlicht provides other types of selection, including ray/triangle selector,
        // ray/box and ellipse/triangle selector, plus associated helpers.
        // See the methods of ISceneCollisionManager
        scene::ISceneNode * selectedSceneNode =
            collMan->getSceneNodeAndCollisionPointFromRay(
                    ray,
                    intersection, // This will be the position of the collision
                    hitTriangle, // This will be the triangle hit in the collision
                    IDFlag_IsPickable, // This ensures that only nodes that we have
                            // set up to be pickable are considered
                    0); // Check the entire scene (this is actually the implicit default)

        // If the ray hit anything, move the billboard to the collision position
        // and draw the triangle that was hit.
        if(selectedSceneNode)
        {
            bill->setPosition(intersection);

            // We need to reset the transform before doing our own rendering.
            driver->setTransform(video::ETS_WORLD, core::matrix4());
            driver->setMaterial(material);
            driver->draw3DTriangle(hitTriangle, video::SColor(0,255,0,0));

            // We can check the flags for the scene node that was hit to see if it should be
            // highlighted. The animated nodes can be highlighted, but not the Quake level mesh
            if((selectedSceneNode->getID() & IDFlag_IsHighlightable) == IDFlag_IsHighlightable)
            {
                highlightedSceneNode = selectedSceneNode;

                // Highlighting in this case means turning lighting OFF for this node,
                // which means that it will be drawn with full brightness.
                highlightedSceneNode->setMaterialFlag(video::EMF_LIGHTING, false);
            }
        }

        // We're all done drawing, so end the scene.
        driver->endScene();

        int fps = driver->getFPS();

        if (lastFPS != fps)
        {
            core::stringw str = L"Collision detection example - Irrlicht Engine [";
            str += driver->getName();
            str += "] FPS:";
            str += fps;

            device->setWindowCaption(str.c_str());
            lastFPS = fps;
        }
    }

    device->drop();

    return 0;
}