  Augmented Reality (AR) embeds digital information into objects of the
physical world. Data can be shown in-situ, thereby enabling real-time visual
comparisons and object search in real-life user tasks, such as comparing
products and looking up scores in a sports game. While there have been studies
on designing AR interfaces for situated information retrieval, there has only
been limited research on AR object labeling for visual search tasks in the
spatial environment. In this paper, we identify and categorize different design
aspects in AR label design and report on a formal user study on labels for
out-of-view objects to support visual search tasks in AR. We design three
visualization techniques for out-of-view object labeling in AR, which
respectively encode the relative physical position (height-encoded), the
rotational direction (angle-encoded), and the label values (value-encoded) of
the objects. We further implement two traditional in-view object labeling
techniques, where labels are placed either next to the respective objects
(situated) or at the edge of the AR FoV (boundary). We evaluate these five
different label conditions in three visual search tasks for static objects. Our
study shows that out-of-view object labels are beneficial when searching for
objects outside the FoV, spatial orientation, and when comparing multiple
spatially sparse objects. Angle-encoded labels with directional cues of the
surrounding objects have the overall best performance with the highest user
satisfaction. We discuss the implications of our findings for future immersive
AR interface design.
