You can grab position values from the modelview matrix and get the distance from the magnitude. The modelview matrix represents the orientation of the trackable with the camera at the origin. The inverse of the modelview matrix represents the orientation of the camera with the trackable at the origin. Since we're just doing distance, we won't bother taking the inverse.

Here's a code snippet:

#include <math.h>

QCAR::Matrix44F modelViewMatrix =
    QCAR::Tool::convertPose2GLMatrix(trackable->getPose());        

QCAR::Vec3F position(modelViewMatrix.data[12], modelViewMatrix.data[13], modelViewMatrix.data[14]);
float distance = sqrt(position.data[0] * position.data[0] +
                      position.data[1] * position.data[1] +
                      position.data[2] * position.data[2]);
LOG("distance: %f", distance);

- Kim