  3D printers enable end-users to design and fabricate unique physical
artifacts but maintain an increased entry barrier and friction. End users must
design tangible artifacts through intangible media away from the main problem
space (ex-situ) and transfer spatial requirements to an abstract software
environment. To allow users to evaluate dimensions, balance, or fit early and
in-situ, we developed BrickStARt, a design tool using tangible construction
blocks paired with a mixed-reality headset. Users assemble a physical block
model at the envisioned location of the fabricated artifact. Designs can be
tested tangibly, refined, and digitally post-processed, remaining continuously
in-situ. We implemented BrickStARt using a Magic Leap headset and present
walkthroughs, highlighting novel interactions for 3D design. In a user study
(n=16), first-time 3D modelers succeeded more often using BrickStARt than
Tinkercad. Our results suggest that BrickStARt provides an accessible and
explorative process while facilitating quick, tangible design iterations that
allow users to detect physics-related issues (e.g., clearance) early on.
