Improving Mid-Air Sketching in Room-Scale Virtual Reality with Dynamic Color-to-Depth and Opacity Cues

Immersive 3D mid-air sketching systems liberate users from the confines of traditional 2D sketching canvases. However, complications from perceptual challenges in Virtual Reality (VR), combined with the ergonomic and cognitive challenges of sketching in all three dimensions in mid-air lower the accuracy and aesthetic quality of 3D sketches. This paper explores how color-to-depth and opacity cues support users to create and perceive freehand, 3D strokes in room-scale sketching, unlocking a full 360° of freedom for creation. We implemented three graphic depth shader cues modifying the (1) alpha, (2) hue, and (3) value levels of a single color to dynamically adjust the color and transparency of meshes relative to their depth from the user. We investigated how these depth cues influence sketch efficiency, sketch quality, and total sketch experience with 24 participants in a comparative, counterbalanced, 4 x 1 within-subjects user study. First, with our graphic depth shader cues we could successfully transfer results of prior research in seated sketching tasks to room-scale scenarios. Our color-to-depth cues improved the similarity of sketches to target models. This highlights the usefulness of the color-to-depth approach even for the increased range of motion and depth in room-scale sketching. Second, our shaders assisted participants to complete tasks faster, spend a greater percentage of task time sketching, reduced the feeling of mental tiredness and improved the feeling of sketch efficiency in room-scale sketching. We discuss these findings and share our insights and conclusions to advance the research on improving spatial cognition in immersive sketching systems.