  This paper introduces a novel, markerless, step-by-step, in-situ 3D Augmented
Reality (AR) instruction method and its application - BRICKxAR (Multi 3D
Models/M3D) - for small parts assembly. BRICKxAR (M3D) realistically visualizes
rendered 3D assembly parts at the assembly location of the physical assembly
model (Figure 1). The user controls the assembly process through a user
interface. BRICKxAR (M3D) utilizes deep learning-trained 3D model-based
registration. Object recognition and tracking become challenging as the
assembly model updates at each step. Additionally, not every part in a 3D
assembly may be visible to the camera during the assembly. BRICKxAR (M3D)
combines multiple assembly phases with a step count to address these
challenges. Thus, using fewer phases simplifies the complex assembly process
while step count facilitates accurate object recognition and precise
visualization of each step. A testing and heuristic evaluation of the BRICKxAR
(M3D) prototype and qualitative analysis were conducted with users and experts
in visualization and human-computer interaction. Providing robust 3D AR
instructions and allowing the handling of the assembly model, BRICKxAR (M3D)
has the potential to be used at different scales ranging from manufacturing
assembly to construction.
