Validation of a videogrammetry technique for analysing American football helmet kinematics

ABSTRACT

Professional American football games are recorded in digital video with multiple cameras, often at high resolution and high frame rates. The purpose of this study was to evaluate the accuracy of a videogrammetry technique to calculate translational and rotational helmet velocity before, during and after a helmet impact. In total, 10 football impacts were staged in a National Football League (NFL) stadium by propelling helmeted 50th percentile male crash test dummies into each other or the ground at speeds and orientations representative of concussive impacts for NFL players. The tests were recorded by experienced sports film crews to obtain video coverage and quality typically available for NFL games. A videogrammetry procedure was used to track the position and rotation of the helmet throughout the relevant time interval of the head impact. Compared with rigidly mounted retroreflective marker three dimensional (3-D) motion tracking that was concurrently collected in the experiments, videogrammetry accurately calculated changes in translational and rotational velocity of the helmet using high frame rate (two cameras at 240 Hz) video (7% and 15% error, respectively). Low frame rate (2 cameras at 60 Hz) video was adequate for calculating pre-impact translational velocity but not for calculating the translational or rotational velocity change of the helmet during impact.     

Is academic tracking related to gains in learning competence? Using propensity score matching and differential item change functioning analysis for better understanding of tracking implications

This study analyzes gains in cognitive components of learning competence with respect to cohorts based on ability tracking in a Czech longitudinal study. Propensity score matching is used to form parallelized samples of academic and non-academic track students and to eliminate the effect of selective school intake. We applied regression models on the total scores to test for the overall track effect. Furthermore, we analyze scores and gains on the subscores and check for differential item functioning in Grade 6 and in change to Grade 9. While after 3 years, no significant difference between the two tracks was apparent in the total learning competence score, we did, however, find significant differences in some subscores and in the functioning of some items. We argue that item-level analysis is important for deeper understanding of the tracking implications and may provide the basis for more precise evidence-based decisions regarding the tracking policy.     

A compressed self-index using a Ziv–Lempel dictionary

A compressed full-text self-index for a text T, of size u, is a data structure used to search for patterns P, of size m, in T, that requires reduced space, i.e. space that depends on the empirical entropy (H _k or H ₀) of T, and is, furthermore, able to reproduce any substring of T. In this paper we present a new compressed self-index able to locate the occurrences of P in O((m + occ)log u) time, where occ is the number of occurrences. The fundamental improvement over previous LZ78 based indexes is the reduction of the search time dependency on m from O(m ²) to O(m). To achieve this result we point out the main obstacle to linear time algorithms based on LZ78 data compression and expose and explore the nature of a recurrent structure in LZ-indexes, the suffix tree. We show that our method is very competitive in practice by comparing it against other state of the art compressed indexes.

从图式理论看汉语带"鸡"字成语的翻译

英汉两种语言中都有大量的动物成语。由于英汉两个民族文化背景不同，相应的民族背景下的读者对同一动物形象所形成的图式知识不尽相同，这给动物成语的翻译带来了一定困难。为了尽可能准确完整地传递汉语带“鸡”字成语所承载的完整涵义，翻译中应灵活采取图式对应、图式不对应、图式扩展和图式整合等不同的方法。     

加权的布尔型文本匹配算法研究

本文分析了传统的布尔型文本匹配算法的弊端和缺陷,结合图书文本信息的快速、高效的检索匹配要求,提出了加权的布尔型文本匹配算法。该算法对传统的文本匹配算法中漏检、误检和模糊查询等方面的问题进行了加权处理和结果校正,既保证查询算法的高效与全面性,同时又有效地控制了匹配的准确度,为图书信息查询系统的建立提供一种更好的解决方法。     

基于Fourier-Mellin变换的全景图像拼接与曲线匹配

基于Fourier—Mellin变换的图像配准方法,本文对于图像拼接与曲线匹配进行新探讨。该方法在不需要准确控制相机运动,不需要知道相机的焦距等内部参数及检测图像特征,在配准精度要求不是很高的情况下,直接生成的全景图像可以满足很多实际需求;先将图像曲线转化为二值图像,然后应用Fourier—Mellin变换对这些二值图像进行配准,从而达到两条曲线的匹配。     

图式与语境相结合的立体化听力理解模式

通过对图式听力理解模式和语境化听力理解模式的分析、探讨,提出心理学与语用学相结合的立体化听力理解模式的构建设想,探究了立体化听力理解模式对指导听力教学实践,有效提高听力理解能力的启示。     

图式理论在英文报刊阅读中的应用

英文报刊阅读越来越受到重视,但读者往往只注重词汇、语法等语言知识的提高,而忽视图式的积累。本文探讨了图式理论在英文报刊阅读中的应用,证实了图式理论能够指导读者进行有效的英文报刊阅读。     

图式理论与英语阅读

图式理论是英语阅读理论中非常重要的理论,它一直受到英语阅读领域的关注。正确认识并应用图式理论,有利于英语阅读,并有效促进英语阅读能力的快速提高。     

不及物动词带宾语的典型性研究

认知语言学的兴起对传统语法的范畴分类提出了挑战。运用认知语言学的原型范畴化理论和意象图式理论对汉语不及物动词带宾语的典型性进行了研究,揭示了不及物动词带宾语语法结构中同一类别(从最典型成员到最不典型的成员)所表现出的一定程度上的等级性和此种语法结构是如何通过隐喻意象图式扩展范畴。此外,还揭示了就汉语不及物动词带宾语而言,英语不及物动词带宾语的独特的语言特征。