# Rendering Strategy to Counter Mutual Masking Effect in Multiple Tactile Feedback
## 1 Introduction  
With the advancement of mobile devices and virtual reality systems, the demand for haptic technology is rapidly growing as it can drastically improve the user's experience. Specifically, methods and devices that simultaneously utilize two or more tactile feedback types have been recently proposed for more immersive interaction with virtual objects. There are four types of highly sensitive mechanoreceptive units in the human hand. These units actively respond to different tactile feedback types, including pressure, shear stretch, and low- and high-frequency vibrations [1; 2; 3]. In this regard, researchers have proposed to simultaneously use two tactile feedback types, and the feasibility of presenting multiple tactile information, such as shape (geometry) and texture, has been demonstrated [4; 5; 6]. Furthermore, devices that simultaneously provide three or four tactile feedback types have also been proposed [7; 8]. Nevertheless, so far, relatively little consideration was given to the possible masking effect when multiple tactile feedback types are mixed. By definition, a masking effect is when the perception of one stimulus is affected by another stimulus. The maskingeffect appears in various sensory modalities [9; 10; 11; 12; 13]. It can be examined by investigating changes in the absolute threshold (absolute limen (AL)) and the difference threshold (difference limen (DL)) of the target stimulus, in association with the masking stimulus (masker) intensity. The AL is particularly known to significantly change according to the masker's intensity [12; 13; 14]. Thus, the masked AL should be investigated before designing multiple tactile feedback, as the users perceive the stimulus intensity at the sensation level (SL); SL refers to the number of decibels above the AL. If the masking effect exists, the perceived intensity should be considered based on the masked AL rather than the unmasked AL. Indeed, the masking effect has been a well-established concept in haptics since the 1980s. However, the interplay of two tactile stimuli, presented to the same site, has been scarcely studied.  
In this paper, we show that a significant interplay appears when simultaneously applying two tactile stimuli, and suggest a method to overcome this mutual masking effect. In our previous report [14], we investigated the masking effect caused by mechanical vibration (MV) on the perception of electrovibration (EV). We found that MV significantly shifts the AL of EV. The main goals of the current study were to investigate the _mutual_ masking effect further and propose a new rendering algorithm to overcome it. First, we derived a generalized form of masking function through a series of psychophysical experiments. Then, based on the masking functions, we proposed a rendering algorithm that eliminated the mutual masking effect. We proved the proposed concept mathematically, and then validated through a user test where the geometry and texture of the virtual object were presented jointly. Our primary contributions can be summarized as follows.  
* We investigated the mutual masking effect when two tactile feedback types were presented simultaneously.
* We propose and have validated a novel rendering algorithm that compensates for the mutual masking effect.