The Sportvision PITCHf/x system is composed of three basic parts: the pitchtracking cameras, the trajectory reconstruction software, and the image rendering software. In this brief note, I will discuss only the first two of these. Sportvision公司的PITCHf/x系统是由三个基本部分所构成: 投球追踪摄影机，轨迹重建 软体，与影像建构软体。在这篇简短的文章中，我将只讨论前面两个部分。 To track a pitch in three dimensions requires two cameras with their axes ideally oriented perpendicular to each other. The PITCHf/x system utilizes three cameras, but only two are used for the actual tracking. A nontracking third camera is placed in centerfield and is only used for setting the height of the strike zone. In a typical setup (e.g., Fenway Park), the tracking cameras are placed high above home plate looking down ("high home") and high above the first-base line looking down ("high first"). In other venues, such as Busch Stadium, there is a "high third" instead of a "high home" camera. Both of the tracking cameras are permanently afixed to the stadium structure in an location where they are not easily bumped. The zoom on the lens is adjusted so that the field of view runs from approximately a few feet in front of the pitching rubber to the point of home plate. The cameras operate at 60 frames/s, meaning that a typical pitch of transit time 0.45 sec will provide up to 27 images for each of the two tracking cameras. 要以三度空间的形式追踪投手投出的球需要两部摄影机，且摄影机的中心线方向必须形成 直角。PITCHf/x系统则使用了三部摄影机，但是只有两部是用来追踪轨迹。第三台不用来 追踪轨迹的摄影机架设在中外野，唯一的目的是设定好球带的高度。以一般的设置(例如 : 芬威球场)而言，追踪摄影机架设在本垒板上方朝下方拍摄("home high")与一垒边线上 方朝下拍摄("hign first")。在其他的地点，如布希球场，是以"high third"取代"high home"摄影机。两部追踪摄影机都被固定在球场结构中不易遭受碰撞的位置。镜头的变焦 经过调整後使得视角位在投手丘前面几尺到本垒板。摄影机以每秒60 个画面拍摄，这代 表两台追踪摄影机在一般球的飞行时间0.45秒内各可以提供27张影像。 To do the trajectory reconstruction, very clever proprietary software is used to do pattern recognition. For each frame, each camera provides a two-dimensional image of the baseball in pixels. To convert pixels into x,y,z coordinates, the cameras need to be calibrated, which is accomplished initally by placing markers at precisely known locations in the field of view of the cameras. Using this information, a prescription (more precisely, a matrix transformaton) is established for transforming the 2-dimensional pixel information from the two cameras into x,y,z coordinates on a frame-by-frame basis. The resulting trajectory x(t),y(t),z(t) (t is the time) is then "smoothed" by fitting it to a function corresponding to constant acceleration (click here for more details). Other quantities, such as the pitch speed, the location of the pitch as it crosses the plate, the "break" of the pitch, etc., are derived from the smoothed trajectory. 要达成轨迹重建必须使用非常聪明的专属软体来做图形识别。每个画面，每部摄影机提供 了以像素为单位的二维棒球影像。要将像素转换为x,y,z座标，摄影机必须经过教点，起 初先在摄影机视野内的精确的已知位置做记号，使用这些资料可以建立一套规则(更精确 的说法是矩阵转换)，将从两台摄影机得到以逐格动画为依据的二维像素资讯，转换成 x,y,z座标。产生的轨迹x(t)，y(t)，z(t)(t代表时间)透过一个对应於固定加速度的函数 (点此了解更多细节)做"平滑处理"。其他的数据资料，如球速，球经过本垒版的位置，球 的变化量...等等，也都能由经过平滑处理的轨迹取得。 For a more detailed and technical description of an early version of the K-Zone tracking technology, see the article "Tracking Pitches for Broadcast Television", by Dr. Andre Guiziec. This system only tracked the pitch over a small range close to home plate. The current PITCHf/x system tracks over most of the distance between pitcher and home plate. For a report on a different technique for tracking a pitched baseball, see Tracking High-Speed Motion with Multi-Exposure Images. Although not directly related to the MLB Gameday system and although the techinque is not applicable outside of a laboratory setting, it is a very nice piece of experimental work that demonstrates how smart people can do excellent studies with inexpensive equpiment. 想要得到更多较早版本K-Zone追踪技术的细节与技术说明，请参阅由Dr Andre Guiziec所 撰写的"Tracking Pitches for Broadcast Television"这篇文章。这个系统只追踪球接 近本垒板时的小范围。目前的PITCHf/x系统则追踪从投手到本垒板大部分的距离。另一种 追踪投出的球的技术报导请参阅Tracking High-Speed Motion with Multi-Exposure Images。虽然这个技术与大联盟的Gameday系统没有直接的关系，并且尚未使用在实验室 以外的环境，但是这是一个非常棒的专题研究，它显示聪明的家伙如何运用便宜的设备进 行杰出的研究。 http://webusers.npl.uiuc.edu/~a-nathan/pob/technology.htm -- Gary says... http://www.gary711.net http://gary711.idv.st --

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