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Propagation of Visual Entity Properties Under Bandwidth Constraints

May 24, 2007 Article Start Page 1 of 7 Next
 

1. Introduction

The Saga of Ryzom is a persistent massively-multiplayer online game (MMORPG) released in September 2004 throughout Europe and North America, localised in 3 languages so far. It has been developed by Nevrax since 2000, and was taken over by Gameforge in late 2006.

The Nevrax team built Ryzom from scratch, starting with the Nevrax Library (NeL), made available as free software under the General Public License (GPL). On top of NeL, a server technology was created to handle an immersive virtual world. This article focuses on the network techniques developed to display a smooth scene of moving entities, the dynamic properties of which are propagated to all connected end-user clients having an avatar in the same virtual area.

I would like to thank Daniel Miller, CTO of Nevrax and Gameforge France, for his enlightening contributions and reviews of this article.

Purpose of the Works

Our client software had to display an animated 3D scene figuring moving and animated entities. Players would have direct control over their avatars, as point & click control (the alternative usually found in 2D or ¾ perspective games, even sometimes in 3D games) was considered not immersive enough. While the role-playing genre usually does not require fast input from the players, like in first person shooters (where the first player to shoot, will be the one who survives) or in sport games (where synchronization is very important for collective actions), movements had to be consistent & smooth.

We wanted visible micro-teleportations, or position jolts, to be less frequent than in other MMOGs, in which, at least at the time when the Ryzom project started, entities often seemed to go back in time or jump forward. Another important feature was to allow the characters to move in a seamless environment, without loading time between predetermined small geographical zones, during which the player would be idle.

In the present article, which explains the techniques we tried and which ones were eventually chosen, a visual property refers to any dynamic state or attribute of a player or non-player entity that must be known by client software to render it, such as position & heading, 3D model identifier, clothes set, current animation, etc. We assume the client software has local access to some static data such as 3D models, textures and animations. In most of the document we will focus on the propagation of position, because the continuous nature of position paths makes greatly noticeable when smooth propagation is not achieved.

Challenges

In an online game, especially a massively multiplayer one, the bandwidth constraints prevents from trivially sending every position change to all players. At the planned time of Ryzom launch, 56k modems still were widespread among players, ADSL was still in its infancy. Our MMORPG had to work nominally on a 56k or even 14k connection.

Besides, in an online game, cheating utterly destroys the game experience of the victims (the players put at a disadvantage) and must be prevented. From the ever-popular adage “Don’t trust the client”, even more present as Nevrax’s founders’ intention was to release the Ryzom client under a free software license, it was quickly clear that the client would be a display and input device, while all game logic would have to be on the server.

Handling dynamic game information on the server side would prevent the appearance of player-made radars, for instance. Then, after excluding a peer-to-peer network solution, the bandwidth constraints did not only originate from the low bandwidth available in consumers’ homes, but also in the server farms that we would have to rent.

MMORPG playing experience was known to come with lag, an unpleasant noticeable delay between an action and its viewing, often visible as a stopped animation or animation jerk. This lag phenomenon had to be minimized by design. It usually originated either from an inadequate information propagation system under tight bandwidth constraints and latency-prone networks such as the Internet, or from delay in CPU-processing by the server.

Our work then had to focus on studying the known techniques of visual property propagation, and possibly creating better ones. The CPU performance of the software was also critical, to avoid time-consuming peaks.


Article Start Page 1 of 7 Next

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