Question

I am trying to pick objects in the bullet physics world but all I seem to be able to pick is the floor/ground plane!!! I am using the Vuforia SDK and have altered the ImageTargets demo code. I have used the following code to project my touched screen points to the 3d world:

void projectTouchPointsForBullet(QCAR::Vec2F point, QCAR::Vec3F &lineStart, QCAR::Vec3F &lineEnd, QCAR::Matrix44F &modelViewMatrix)
{

QCAR::Vec4F normalisedVector((2 * point.data[0] / screenWidth - 1),
        (2 * (screenHeight-point.data[1]) / screenHeight - 1),
        -1,
        1);
QCAR::Matrix44F modelViewProjection;
SampleUtils::multiplyMatrix(&projectionMatrix.data[0],  &modelViewMatrix.data[0] , &modelViewProjection.data[0]);
QCAR::Matrix44F inversedMatrix = SampleMath::Matrix44FInverse(modelViewProjection);

QCAR::Vec4F near_point = SampleMath::Vec4FTransform( normalisedVector,inversedMatrix);

near_point.data[3] = 1.0/near_point.data[3];
near_point = QCAR::Vec4F(near_point.data[0]*near_point.data[3], near_point.data[1]*near_point.data[3], near_point.data[2]*near_point.data[3], 1);

normalisedVector.data[2] = 1.0;//z coordinate now 1
QCAR::Vec4F far_point = SampleMath::Vec4FTransform( normalisedVector, inversedMatrix);

far_point.data[3] = 1.0/far_point.data[3];
far_point = QCAR::Vec4F(far_point.data[0]*far_point.data[3], far_point.data[1]*far_point.data[3], far_point.data[2]*far_point.data[3], 1);

lineStart = QCAR::Vec3F(near_point.data[0],near_point.data[1],near_point.data[2]);
lineEnd = QCAR::Vec3F(far_point.data[0],far_point.data[1],far_point.data[2]);
} 

when I try a ray test in my physics world I only seem to be hitting the ground plane! Here is the code for the ray test call:

   QCAR::Vec3F intersection, lineStart;
   projectTouchPointsForBullet(QCAR::Vec2F(touch1.tapX, touch1.tapY), lineStart, lineEnd,inverseProjMatrix, modelViewMatrix);
    btVector3 btRayFrom = btVector3(lineEnd.data[0], lineEnd.data[1], lineEnd.data[2]);
    btVector3 btRayTo = btVector3(lineStart.data[0], lineStart.data[1], lineStart.data[2]);

    btCollisionWorld::ClosestRayResultCallback rayCallback(btRayFrom,btRayTo);
    dynamicsWorld->rayTest(btRayFrom, btRayTo, rayCallback);
    if(rayCallback.hasHit())
    {
        char* pPhysicsData = reinterpret_cast<char*>(rayCallback.m_collisionObject->getUserPointer());//my bodies have char* messages attached to them to determine what has been touched
        btRigidBody* pBody = btRigidBody::upcast(rayCallback.m_collisionObject);
        if (pBody && pPhysicsData)
        {
            LOG("handleTouches:: notifyOnTouchEvent from physics world!!!");
            notifyOnTouchEvent(env, obj,0,0, pPhysicsData);
        }

    }

I know I am predominantly looking top-down so I am bound to hit the ground plane, I at least know my touch is being correctly projected into the world, but I have objects lying on the ground plane and I can’t seem to be able to touch them! Any pointers would be greatly appreciated 🙂

Answer 1

I found out why I wasn’t able to touch the objects – I am scaling the objects up when they are drawn, so I had to scale the view matrix by the same value before I projected my touch point into the 3d world (EDIT I also had the btRayFrom and btRayTo input cooordinates reversed, it is now fixed):

//top of code
int kObjectScale = 100.0f
....
...
//inside touch handler method
SampleUtils::scalePoseMatrix(kObjectScale, kObjectScale, kObjectScale,&modelViewMatrix.data[0]);
    projectTouchPointsForBullet(QCAR::Vec2F(touch1.tapX, touch1.tapY), lineStart, lineEnd,inverseProjMatrix, modelViewMatrix);
    btVector3 btRayFrom = btVector3(lineStart.data[0], lineStart.data[1], lineStart.data[2]);
    btVector3 btRayTo = btVector3(lineEnd.data[0], lineEnd.data[1], lineEnd.data[2]);

My touches are projected correctly now 🙂