Evaluating digital video transcoding for forensic derivative results

Video data received for analysis often come in a variety of file formats and compression schemes. These data are often transcoded to a consistent file format for forensic examination and/or ingesting into a video analytic system. The file format often requested is an MP4 file format. The MP4 file format is a very common and a universally accepted file format. The practical application of this transcoding process, across the analytical community, has generated differences in video quality. This study sought to explore possible origins of the differences and assist the practitioner by defining minimum recommendations to ensure that quality of the video data is maintained through the transcoding process. This study sought to generate real world data by asking participants to transcode provided video files to an MP4 file format using programs they would typically utilize to perform this task. The transcoded results were evaluated based on measurable metrics of quality. As the results were analyzed, determining why these differences might have occurred became less about a particular software application and more about the settings employed by the practitioner or of the capabilities of the program. This study supports the need for any video examiner who is transcoding video data to be cognizant of the settings utilized by the programs employed for transcoding video data, as loss of video quality can affect analytics as well as further analysis.     

Agonal Sequences in Four Filmed Hangings: Analysis of Respiratory and Movement Responses to Asphyxia by Hanging*

Abstract: The human pathophysiology of asphyxia by hanging is still poorly understood, despite great advances in forensic science. In that context, filmed hangings may hold the key to answer questions regarding the sequence of events leading to death in human asphyxia. Four filmed hangings were analyzed. Rapid loss of consciousness was observed between 13 sec and 18 sec after onset of hanging, closely followed by convulsions (at 14–19 sec). A complex pattern of decerebration rigidity (19–21 sec in most cases), followed by a quick phase of decortication rigidity (1 min 00 sec–1 min 08 sec in most cases), an extended phase of decortication rigidity (1 min 04 sec–1 min 32 sec) and loss of muscle tone (1 min 38 sec–2 min 47 sec) was revealed. Very deep respiratory attempts started between 20 and 22 sec, the last respiratory attempt being detected between 2 min 00 sec and 2 min 04 sec. Despite differences in the types of hanging, this unique study reveals similarities that are further discussed.     

Body Height Measurements in Images

Abstract:  For any measurement of a person's height in an image, a difference exists between the actual height of the person and the image measurement. In order to gain knowledge about statistical behavior of differences between actual and measured heights it is necessary to make reference recordings, e.g., of test persons under the same recording conditions. To test whether the differences are dependent on camera and further circumstances, an experiment was set up which involved the measurement of 22 test persons using three cameras of varying quality. Reproducibility of measurements per image appears to be strongly dependent on the camera (quality), whereas systematic bias differs with the view point of the camera. Operator dependency of the measurement process is found, so its repetition by different operators is recommended.     

案事件调查中视频证据的内容解析

案事件的调查中,常常需要专业人员出具书面的报告,对视频记载的事件过程进行描述和解释。实际案件中,视频内容的分析报告多采用时间顺序,以语言描述为主。本文提出视频内容解析的概念,综合运用图像处理、图像测量、现场实验等技术方法,对视频中人、车、物等状态及其相互关系进行定性和定量的分析。解析结果以文字与图表相结合的形式呈现,可以更清晰完整地表述视频中反映的事件过程,已在多起重特大案事件的现场视频分析中应用。对于持续时间较长、关键目标动作较复杂的案事件形成视频解析报告,有助于对事件过程的还原,提高了视频证据的证明能力。     

论视频监控中的问题及其法律规制

在现代警务中,视频监控系统对于社会秩序之维护、警察任务之完成有着积极的作用。但由于缺乏基本的法律授权和法律规范,视频监控系统对民众的宪法基本权有侵害之虞。故此,遵循警察补充性、正当法律程序、合目的性、安全保护等基本原则,并从设置主体、设置地点、设置实质标准、监控系统选择等方面强化视频监控的法律规制,才能真正发挥视频监控的应有作用。     

文检仪显现同种同色掩盖字迹条件的初探

目的研究运用文检仪显现同种同色掩盖字迹的显出条件。方法以黑色签字笔为例,依据不同型号笔的油墨在不同光源下的荧光性质和红外吸收能力的差异,利用文检仪进行分析检验。结果只有在一定条件下被掩盖字迹才能被显出。结论在LUMI=IR红外荧光（兰光）下,只能显现激发荧光的油墨掩盖不能激发荧光的油墨的字迹和激发荧光能力弱的油墨掩盖激发荧光能力强的油墨的字迹;在红外吸收光和365nm长波紫外表面光下,只能显现颜色浅的油墨掩盖颜色深的油墨的字迹。     

基于视频的车速鉴定方法

视频监控设备的广泛应用为各类案件的侦破及取证提供了重要线索。利用高质量视频图像进行近景摄影测量、运动物体识别及跟踪的方法和技术已经较为成熟,但利用广角、低像素的视频监控录像定量描述车辆行驶速度的技术研究尚未开展。针对交通事故技术鉴定的实际需求,根据速度的一般计算方法和计算机视觉的基本原理,提出可以应用目前广泛使用的公共视频监控设备所拍摄的录像进行车辆速度计算的方法。     

基于数码相机黑白模式的分色照相探讨

利用数码相机进行分色照相,是数码照相机用于现场照相后所带来的新问题。能否直接在拍照中进行分色,而不是拍照后用图像处理软件进行分色,对于保持照片效果的原始性具有一定的意义。     

一种水下金属物证探测器的设计

目的 设计一种水下金属物证探测器,用于公安人员在池塘、沟渠、水井等水域中搜寻金属类物证.方法 采用金属探测技术、水下视频采集技术和无线传输技术相结合,实现探测器的多功能特性.结果 在深度不超过4m的水域中,借助探测器可以准确搜寻到金属物证.结论 该探测器具有金属探测以及视频图像采集、无线发送和保存的功能,能满足一定范围内搜寻水下金属物证的工作需求.     

Determination of Vehicle Speed from Recorded Video Using Reverse Projection Photogrammetry and File Metadata

The prevalence of security and in‐car video has increased the number of motor vehicle accidents captured on digital video. However, inconsistencies in how to accurately determine time and distance for vehicle speed has led to examinations with varying results. A potential solution for calculating time intervals is to use frame timing contained within many digital video file's metadata, recorded with 0.000001 sec precision. This paper examines a fatal motor vehicle accident where frame timing information was used with distance measurements from reverse projection photogrammetry to calculate vehicle speed. A margin of error was then calculated based on the accuracy in performing reverse projection photogrammetry distance measurements. The resulting speed calculation was then compared to event data recorder data and found to be within an average of ±1.43538 MPH. Using specific time intervals may lead investigators to more accurate speed calculations, specifically those involving variable frame rate video.