ISSN 2464-4617(print) ISSN 2464-4625(CD) CSRN 2701 Computer Science Research Notes http://www.WSCG.eu Increasing diversity and usability of crowd animation systems Ciprian Paduraru University of Bucharest and Electronic Arts Bucharest, Romania [email protected] ABSTRACT Crowd systems are a vital part in virtual environment applications that are used in entertainment, education, training or different simulation systems such as evacuation planning. Because performance and scalability are key factors, the implementation of crowds poses many challenges in many of its aspects: behaviour simulation, animation, and rendering. This paper is focusing on a different model of animating crowd characters that support dynamically streaming of animation data between CPU and GPU. There are three main aspects that motivate this work. First, we want to provide a greater diversity of animations for crowd agents than was possible before, by not storing any predefined animation data. Another aspect that stems from the first improvement is that this new model allows the crowd simulation side to communicate more efficiently with the animation side by sending events back and forth at runtime, fulfilling this way use-cases that different crowd systems have. Finally, a novel technique implementation that blends between crowd agents’ animations is presented. The results section shows that these improvements are added with negligible cost. Keywords animation, crowd, skeleton, GPU, blending, memory, compute shader, vertex shader 1 INTRODUCTION of the crowd’s agents. The focus of this paper is on the render side, and especially on how to deal with Crowd simulations are becoming more and more the agents’ animations. The main motivation to study common in many computer graphics applications. It is animation systems for crowds is that realistic motion a critical component nowadays in video games industry and diversity of animations are important aspects of (games like Fifa 2017®, Ubisoft’s Assassin’s Creed®, user perception. There are three contributions that RockStar’s Grand Theft Auto® series), evacuation this paper adds to the field of animations for crowd planning software (e.g. Thunderhead’s Pathfinder systems: software®) or phobia treatments applications, where it is being used to create realistic environments. However, crowd simulation and rendering is a costly • We do not require to store all animations pose data operation and several techniques were developed to on the GPU side as similar solutions does. In- optimize CPU, GPU or bandwidth usage to have as stead, we use a streaming model of poses which many agents as possible. In our paper, by agent, we has insignificant performance overhead; This allows mean an individual visible entity in the crowd. Entities crowd systems to have a greater diversity of anima- can be humans, cars or every other instance the client tion data than before ([Rud05], [Ngu07], [Sho08], desires to configure and use. We split a crowd system [Bea14]). implementation into two logical parts: simulation and render side. The simulation usually deals with driving • Fulfill the requirements of modern crowd systems, behaviours, actions, and events for crowd agents, while where the simulation side often needs to request the render side deals with animation and rendering complex blending between the motion of groups or individual agents, by feeding dynamic input states or events. The inverse communication is also possi- ble: animations can trigger events to the simulation Permission to make digital or hard copies of all or part of side (more details on Section 3). this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and • At the moment of this paper, the first documented the full citation on the first page. To copy otherwise, or re- method to perform blending between animations of publish, to post on servers or to redistribute to lists, requires agents that share the same animation data streams prior specific permission and/or a fee. with different time offsets. Full Papers Proceedings 1 ISBN 978-80-86943-49-7 ISSN 2464-4617(print) ISSN 2464-4625(CD) CSRN 2701 Computer Science Research Notes http://www.WSCG.eu Overall, we consider that by using our approach, ani- instancing. In the shader program, the current view- mation systems for crowds can get more usability and ing frustum and character heading can be obtained to variation with minimal performance loss. compute the texture coordinates that are most similar The rest of the paper is organized as follows. Related to the current view and animation pose. In the same work in the field is presented in Section 2. A deeper in- category of optimizations, but targeted for simplifying troduction into current techniques and some other tech- the work of content creators (animators), is the cage- nical requests that are driving our work are presented based method mentioned in [Kim14]. This method can in Section 3. Section 4 describes the usability of our be used for lower level of details in parallel with our framework and strategies that we use to implement it. technique, to increase the number of animations avail- Comparative results are shown in Section 5. Finally, able for lower LODs. conclusions are given in the last section. In our approach, we consider that blending between motions of individual or larger groups of agents can 2 RELATED WORK be requested by the simulation side (e.g an agent ex- iting from a boids behavior because an emergent event The ability to adjust the skin mesh of a character with- started). This means that a decision mechanism at simu- out having to redefine the animation is highly desir- lation layer (e.g. a finite state machine, a behaviour tree able, and this is possible using a technique called vertex or even a neural network) feeds our animation blend- skinning [Ryd05], [Bea16]. By using vertex shaders, a ing mechanism with concrete parameters. Several pa- lot of the computation that was previously done on the pers presented below provided an inspirational point for CPU side is now moved on GPU side, which proves to our work and can be used in conjunction with our tech- be faster for this kind of computations [Sho08]. The niques as plugins. main technique applied for rendering animated charac- ters using vertex skinning in crowd systems is called Motion warping [Wit95] is one of the core techniques skinned instancing, which significantly reduce the num- used to create whole families of realistic motions which ber of draw calls needed to render agents. The way can be derived from a small subset of captured motion skinned instancing work in [Rud05], [Ngu07], [Sho08], sequences and by modifying a few keyframes. [Kov02] [Bea14] is by writing all the animations data on a GPU presents a method that automatically blends motions in texture at once, then have the vertex shader perform the form of a motion graph. This method can be used by skinning operations by extracting the correct pose from the locomotion system of a crowd to choose the anima- that texture for each individual instance. In our ap- tions needed at runtime from an initial database of as- proach, the difference is that the GPU texture doesn’t sets. The main strategy is to identify a portion between have to be filled at initialization time with all anima- two motion frames that are sufficiently similar such that tions data. Instead, with some bandwidth cost, we send blending is almost certain to produce a nice looking the poses of some template animations from CPU to transition. The identification process compares the dis- the GPU, facilitating this way client driven blending be- tances between the pairs of frames of the two motions tween motions. to identify the best points for transition. The concept of registration curve was introduced by [Kov03]. Con- While the skinned instancing and skeletal animation are sidering the problem of blending two motions, the reg- highly desirable for rendering high-quality agents for istration curve creates an alignment curve which aligns crowds, mainly because of the current GPUs capabil- the frames for each point on the timewarp curve. A sta- ities nowadays, there are other techniques that can be tistical model that can do unsupervised learning over used to limit the number of polygons rendered with- a motion captured database and generate new motions out having users notice artifacts. These methods are or resynthesize data from it, is introduced by [Bra00] described below and are used in our framework to de- under the name of Style Machines. Finally, an interest- cide how to render characters at a lower level of details ing survey of the animation techniques used by several (LOD). Even if the focus of our paper is to use skele- games engines is presented by [Greg14]. This proved tal animations using instancing for crowds, these tech- to be a motivation for this paper, due to the complex niques can reduce substantially the resources needed requirements of different game engines. to render and animate agents at lower LODs. Image- based approaches have been used to simplify render- Compression techniques have also been studied in sev- ing of large crowds in an urban simulation [Tec00]. A eral papers. One that can be adapted to the same use discrete set of possible views for a character is pre- case as our paper, is [Ari06] where the author is mainly rendered and stored in memory. A single quad is then using PCA and clustering (along other tricks, such as used when rendering the character by using the clos- virtual markers per bone instead of angle) to compress est view from the set. Also, as mentioned in [Mil07], clips of a motion database that is to be streamed at impostors present great advantages in current graph- runtime.