Perceived Weight of Mediated Reality Sticks

TL;DR

The paper investigates how mediated reality visual alterations to a stick—specifically length extensions/shortenings and middle cutouts—alter perceived weight and center of gravity during dynamic touch. Using two within-subject experiments (stretched and cutout conditions) with $N=10$ participants, the authors measure perceived weight and reported COG, linking findings to an inertia-based model that contrasts real inertia $I_r$ with visually inferred inertia $I_v$. They show that longer visual lengths lead to heavier-to-lighter shifts in perceived weight and that cutouts do not change weight but shift COG reporting, with the reported COG providing a better predictor of weight than the physical COG. These results advance understanding of dynamic touch in mediated reality and offer design guidance for MR systems that modulate perceived heaviness without high-end haptic hardware.

Abstract

Mediated reality, where augmented reality (AR) and diminished reality (DR) meet, enables visual modifications to real-world objects. A physical object with a mediated reality visual change retains its original physical properties. However, it is perceived differently from the original when interacted with. We present such a mediated reality object, a stick with different lengths or a stick with a missing portion in the middle, to investigate how users perceive its weight and center of gravity. We conducted two user studies (N=10), each of which consisted of two substudies. We found that the length of mediated reality sticks influences the perceived weight. A longer stick is perceived as lighter, and vice versa. The stick with a missing portion tends to be recognized as one continuous stick. Thus, its weight and center of gravity (COG) remain the same. We formulated the relationship between inertia based on the reported COG and perceived weight in the context of dynamic touch.

Figures (9)

• Figure 1: Mediated reality sticks with weight--COG inconsistencies. All illustrated sticks have the same physical weights and COGs but different mediated realities. We investigate the characteristics of mediated reality sticks in different lengths in Experiment 1. Mediated reality sticks with cutout parts are analyzed in Experiment 2. Note that the cutout stick could have different COGs depending on whether the user recognizes the object as a whole or two separate objects. To grasp this understanding, besides measuring the perceived weight, we collected the perceived COG as an alternative length-related measure, since the length was apparent by the appearance.
• Figure 2: Setup of our study. (a) Participants held a $60$ cm stick and observed it through a VST-HMD. Both the HMD and the stick were tracked using Polhemus magnetic tracking sensors. (b) The participants uniformly waved the stick to a metronome sound. They verbally reported the perceived weight in magnitude in experiments 1-1 and 1-2. (c) The participants reported the COG by shifting the red dot and pressing a keyboard button to indicate the location in experiments 1-2 and 2-2.
• Figure 3: Image processing. We prepared a 3D-scanned replica of our study's setup and a 3D model of the stick in advance and registered them in the mediated reality space. We extracted the participant’s hand in the screen space and overlaid it onto the rendered scene and the stick. The alpha composition of all the rendered content resulted in a mediated reality space from a user's perspective. Because the background layer covered the real stick, we were able to render an arbitrarily sized stick.
• Figure 4: Visual stimuli. The real stick was $60$ cm. Therefore, the conditions of $l = 20$ and $40$ showed virtually shortened sticks while $l = 80$ and $100$ showed virtually extended sticks. In experiments 2-1 and 2-2, we introduced $l_\mathrm{dim} = 10$, $20$, $30$, and $40$ cm, representing sticks that are partially missing their middle portions. Note that the conditions $l=20$ and $l_\mathrm{dim}=40$ are the same. The conditions $l=60$ and $l_\mathrm{dim}=0$ are the same as the real stick. The COG of $l_{\mathrm{dim}}=30$, $20$, and $10$ cm sticks depends on whether users recognize them as a whole or as independent parts.
• Figure 5: The results of Experiment 1-1. The vertical axis of the graph shows the length of the virtual stick, $l \in \{20, 40, 80, 100\}$, and the horizontal axis shows the magnitude estimate $w_l$. The shorter mediated reality sticks are on the "heavier" side ($w_l > 100$), and the longer ones are on the "lighter" side ($w_l < 100$). The differences are observed between one of the shorter sticks and one of the longer sticks.