The realistic interaction of real and synthetic content is an important part of creating a believable augmented reality. This augmentation may be as simple as rendering markers as if they are actually attached to physical objects, or as complex as adding 3D special effects to live video, but in either case the synthetic geometry must appear fixed to the same frame as the objects around it in order to be believable. The key to rendering geometry in a fixed frame is to maintain an accurate estimate of the camera position relative to the environment. Purely IMU-based solutions suffer from inaccuracy, and particularly lag, in the estimated position which results in synthetic geometry which appears to float over the real geometry. More fundamentally, however, the fact that no analysis is carried out on the captured image stream means that the system cannot know what is actually within the field of view of the camera. Thus, the system cannot take into account line of sight and must render synthetic content based only on viewpoint direction, rather than on the basis of the immediately visible environment. ACVT has undertaken a large body of fundamental research in this field and has several active projects looking at many different applications of the technology.