Received: 20 March 2018 Accepted: 21 March 2018 DOI: 10.1002/cav.1839 SPECIAL ISSUE PAPER Time-varying light motion in single convergence Xingyu Lin 1 Mingxuan Chai 1 Sheng Li 1,2 Guoping Wang 1,2 1 Graphics and Interaction Laboratory, Peking University, Beijing, China 2 Beijing Engineering Technology Research Center of Virtual Simulation and Visualization, Peking University, Beijing, China Correspondence Sheng Li, Graphics and Interaction Laboratory, Peking University, Beijing, China; or Beijing Engineering Technology Research Center of Virtual Simulation and Visualization, Peking University, Beijing, China. Email: lisheng@pku.edu.cn Funding information National Key Research and Development Program of China, Grant/Award Number: 2017YFB0203002 and 2017YFB1002705; The National Key Technology Research and Development Program of China, Grant/Award Number: 2015BAK01B06; National Natural Science Foundation (NSFC) of China, Grant/Award Number: 61421062 and 61472010 Abstract As light travels fast in the physical world, it is generally hard to capture the propagation of light with a real camera. However, with the development of pho- ton mapping, it is possible to simulate and visualize such fantastic effect. In this paper, we propose a new algorithm based on progressive photon mapping to render the time-varying light motion. By incorporating our algorithm with participating media, we synthesis the animation of time-varying light beams expanding in space with slow motion. Interesting phenomena can be observed in our experiments through designating either a constant light source or a bul- let light that only emits instantaneously. Our algorithm is efficient in the sense that all frames of the animation can be rendered with only one convergence of the progressive photon mapping, showing the advantage of revealing the light propagation process by our algorithm. The experiments demonstrate the effectiveness and efficiency of our approach. KEYWORDS motion, participating media, progressive photon mapping, time-varying 1 INTRODUCTION Slow motion creates visual art. It is often used for artistic effect, to create a romantic or suspenseful aura or to stress a moment in life. To create the slow motion of lighting would help us see a phenomenon that usually cannot be captured by human eyes and can also help us visualize and analyze the propagation of light. The slow motion of a phenomenon is usually achieved by capturing the object's motion in a much higher rate and replaying it in normal speed. However, to capture light in slow motion in the physical world is not easy, as light travels at a very high speed (3 × 10 8 m/s) and may require a trillion frame per second (fps) camera to accomplish this task. Only until very recently, scientists are able to capture light with a half trillion fps camera and image synthesis analysis. 1 However, the field of computer graphics has long been rendering photorealistic images with the powerful framework of ray tracing. 2 Previously, other authors 3–5 proposed methods for rendering light shaft in a flash or of real-time rendering. With the recent development of photon mapping algorithms, 6,7 it becomes possible to simulate light traveling and illumination propagation in slow motion. In this paper, we focus on simulating the magical slow motion lighting phenomenon based on progressive photon map- ping (PPM). PPM is an iterative multipass algorithm, with a ray tracing pass to record all hit points for the rays shooting from camera, and multiple photon tracing passes to accumulate the photon energy onto the nearby hit points. To ren- der the light in slow motion, a time tag is well designed when tracing each photon. During the gathering process, the energy of these photons will be accumulated to different frames of the animation based on their time tags associated. Comput Anim Virtual Worlds. 2018;29:e1839. wileyonlinelibrary.com/journal/cav Copyright © 2018 John Wiley & Sons, Ltd. 1 of 11 https://doi.org/10.1002/cav.1839