# Physically Based Soap Bubble Synthesis for VR
## 1 Introduction  
With the recent advances in computer graphics technology, the extended reality (XR) market, which encompasses virtual reality (VR) and augmented reality (AR), is rapidly growing, and various content is being produced. In recent years, even artificial intelligence (AI) has been applied to VR applications and has been used in various fields such as tourism, leisure, education, games, and health care fields [1]. Orchestra performances and music videos by singers are provided in VR, and the types of application that are viewed are continually growing. However, to watch an immersive soap bubble show, we need real materials such as soap bubbles and bubble tools along with a special concert hall, and it must be accompanied by experienced performers who produce it. Therefore, in this work, we propose a VR bubble show framework that has no space-time constraints and can be directed without special bubble art materials and equipment. In addition, at a time when outdoor activities are restricted due to COVID-19 Pandemic, soap bubble can be considered a timely technology to experience performance indoors as well.  
To synthesize a soap bubble show in VR, we need to analyze and model the actual physical shape and movement of soap bubbles. However, modeling soap bubbles in a VR environment requires too much computation and there are hardware limitations. Nevertheless, the realistic expression of soap bubbles is very important and it must satisfy users' high expectations. To realize a soap bubble show in VR, we first need to understand the basic properties of soap bubbles. Soap bubbles are very light in the form of a thin layer of soap that surrounds the air, with large changes caused even with a small force [2]. The soap bubbles used in a bubble show are more stable than typical soap bubbles because they use a liquid containing surfactant ingredients that stops the bubble from bursting easily [3]. In addition, many physical laws apply to a single soap bubble and determine its movement patterns. In computer graphics, soap bubbles were realistically simulated as clusters of many soap bubbles using vortex sheets [4]. As hardware capable of real-time rendering was developed, motion recognition was used to simulate the generation and bursting of soap bubbles [5]. A study even examined color changes in bubbles due to lightcancellation and interference [6]. To the best of our knowledge, no study has used soap bubble modeling to determine the size of a soap bubble based on actual parameters or applied the laws of physics to bubbles that are to be used in VR content, nor has a study produced a rendering model to which environmental variables can be applied. In this study, we propose a physically based soap bubble synthesis that can be experienced in a virtual reality environment. To this end, we analyzed measured soap bubble data and optimized the motion vector texture to express the velocity of fluid flow. In addition, for optimized modeling, the shape and motion of soap bubbles were measured and parameters describing the physical characteristic were extracted. We measured the movement path, internal and external pressure difference, damping force, volume, mass, cross-sectional area, air resistance acceleration, and fall acceleration of soap bubbles. Finally, we implemented a soap bubble experience in VR by applying the physics-based rendering technology.  
The contributions of this study are summarized as follows: We created a soap bubble model by measuring several physical variables of soap bubbles; we developed a soap bubble modeling method optimized for VR by applying the physical variables required for soap bubble modeling and rendering. Finally, we provided tools that can be experienced in VR environments and tools for creating customized hand gestures such as clap, grab, fanning, bouncing and piercing.