The quintessential role of the expert witness in the courtroom is assistive by design. Expert witnesses are only permitted to testify if they possess scientific, technical, or other specialized knowledge that exceeds that of the lay person and only if their knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue. Thus, experts must make an epistemic contribution that specifically helps the trier of fact perform their role more effectively and with greater confidence. The more experts are of assistance to the trier of fact, the more likely opposing counsel will object to their testimony. 

Motion Tracking

The focus of this article is on the admissibility of motion tracking performed by qualified forensic video analysts as a method of assisting the trier of fact in understanding motion under dynamic circumstances. The recent case of Commonwealth of Pennsylvania v. Williams, 2021 WL 1575445 (Superior Court of Pennsylvania), illustrates how motion tracking can be utilized. In this case, two men were shot in the parking lot of a bar at nighttime by a single gunman. The events leading up to the shooting, the shooting itself, and the aftermath were recorded collectively by up to sixteen CCTV cameras. The state retained the services of a qualified forensic video analyst (the “analyst”) to examine the video evidence and present it in a manner that would be helpful for the trier of fact.  

The analyst focused on three individuals depicted in the video images and identified them as “Male #1”, “Male #2”, and “Male #3”. Male #1 was identified as being the shooter based on his actions in the video. Utilizing a superimposed arrow pointing at Male #1, the analyst tracked his movement throughout the videos as well as the movements of Male #2 and Male #3. Male #1 was observed holding a firearm, raising his arm, and firing multiple rounds into a nearby vehicle where both victims were seated. Both victims exited the vehicle and attempted to flee but soon collapsed. Male #1 was observed to stand over one of the victims and shoot him again. Medical evidence presented at trial revealed that this victim received twelve gunshot wounds and was paralyzed as a result. The second victim was shot three times and died. 

The defendant filed a pre-trial motion seeking to preclude the analyst from testifying and argued that the proposed evidence was improper demonstrative evidence, that he should not be permitted to bolster the video evidence with his analysis, and that he should not be allowed to offer an opinion identifying the defendant as the shooter. A hearing was held pursuant to the motion and the analyst testified regarding his analysis. During the motion hearing the analyst testified that it would be very difficult to accurately track the individuals depicted without the analysis that was conducted for the court. Numerous difficulties were articulated, including the following:

• MPEG video does not necessarily accurately reflect movement and some images are predictive in nature, a fact the lay person may not appreciate. 
• The cameras employed charge coupled devices which can lead to clipping and oversaturation and may cause bright objects to appear larger than they are in reality.
• Infrared cameras can make individuals appear to be different when in fact they are the same. 
• Lens distortion may incorrectly suggest movement and change body shape. 
• Differing image capture rates require interpretation.
• Incorrect aspect ratios may show individuals, vehicles, and objects incorrectly proportioned. 

The analyst stated that motion tracking allows the viewer to follow the movement of individuals (including limb movement), vehicles, and objects. Software permits accurate tracking which is capable of validation. The analyst made the following findings in his expert report:

• Male #1 was dressed differently from all other individuals shown in the video images.
• The date and time stamps as between cameras were properly synchronized. 
• Male #1 was reliably tracked throughout all depicted camera views.
• Male #1 is the shooter. 

At no time did the analyst offer an opinion as to the identification of Male #1, only that he was the shooter.

The trial judge found that the technological limitations of the video evidence were more apt to mislead the jury without the expert assistance provided by the analyst. The trial judge accordingly denied the defendant’s motion and ruled the expert evidence admissible. The defendant was convicted and appealed his conviction to the Superior Court. On appeal, he argued that while the compilation and synchronization of the video evidence was proper, the expert exceeded permissible limits by adding the motion tracking arrow pointing at Male #1. By identifying Male #1 as the shooter and identifying him with the arrow, the expert effectively identified the appellant as the shooter, thus intruding into the domain of the trier of fact. 

The Court undertook a Rule 702 analysis and found that the trial court did not abuse its discretion by admitting the expert testimony, noting the testimony of the analyst that an untrained person would have difficulty tracking pixels without assistance. Contrary to the assertion of the appellant, the analyst did not identify him as the shooter and therefore the analyst’s evidence did not enter the fact-finding realm. The expert analysis served to aid the jury in their understanding of the surveillance video evidence so that they could assess the identity the shooter when evaluating all of the evidence presented in the case. The probative value of the motion tracking outweighed any potential inflammatory effect. The Court stated that “…[the analyst’s] use of an arrow to signify Male #1, did not cause unfair prejudice, rather was properly admitted by the trial court in order to assist the jury in interpreting, and understanding, Double D’s Bar surveillance videos.”

Analysis

The roles of the expert witness and the trier of fact are fundamentally different. The role of the expert witness is to assist the trier of fact in their understanding of the evidence so that an informed decision can be made by them about what the facts are and what they mean. The expert owes a duty to the court to provide fair, objective, and non-partisan assistance. The expert does not need to solve the case for the trier of fact. In the Williams case, it was readily apparent that without expert assistance, the jury would have had difficulty understanding the video evidence and what inferences could fairly be drawn from it. The analyst provided the necessary assistance to the jury so that they in turn could do their job fairly and factually informed. This allowed the prosecutor to argue the case bearing in mind all of the evidence presented and to argue that Williams was the shooter. The appellant argued that by identifying Male #1 as the shooter, the analyst had articulated an adamantine link to the defendant, but the analyst did not such thing. If expert evidence allows counsel to argue that such a link exists, that is entirely proper and within the permitted scope of advocacy. Experts present facts while counsel argues what they mean in the context of the case. 

Motion tracking is a useful analytical tool for evaluating dynamic scenes. Forensic examination conducted by a qualified expert allows the viewer to focus on the movement of individuals without getting sidetracked by technical issues. The Williams case is a classic example of an expert providing needed assistance without any attempt to do the prosecution’s job. Williams is not the only example of motion tracking being used to assist the trier of fact. An earlier article on this website entitled California Court of Appeal Rules Upon the Propriety of Forensic Video Analysis Techniques Utilized by Expert discusses the 2020 California Court of Appeal case of People v. Tran, which featured motion tracking. 

Motion tracking has also found favorable reception in Canadian courts, notably in R. v. Pasqua, 2008 ABQB 124 (Alberta Court of Queen’s Bench); appeal allowed 2009 ABCA 247 (Alberta Court of Appeal) for reasons not relating to video evidence, and R. v. Chanthabouala, 2010 BCSC 808 (British Columbia Supreme Court). In Pasqua, motion tracking was helpful in assessing the movement of many individuals on a light rail transit platform and helped to identify which person among them pushed the victim into the path of an oncoming train. In Chanthabouala, the forensic video analyst examined surveillance images from a nightclub’s CCTV system. Color-coded dots were placed above each individual so that their movements could be followed throughout the video. These “tracking photos” were challenged by the defence who argued that the jury should draw their own conclusions from the video images. They argued that the analyst’s evidence might cause the jury to simply adopt the analyst’s conclusions as their own without making their own assessment. The defence argued that if the tracking evidence was to be admitted, each of the tracked individuals should be independently identified before the “tracking photos” were shown to the jury. The Court ruled that the proposed evidence was admissible provided each individual was independently identified prior to the “tracking photos” being shown to the jury. The Court noted “I find that the evidence of the tracking photos was logically probative, relevant and material to issues in the trial and that the photos illustrated evidence that was not easily given by oral testimony.” This was a different approach from that taken in Williams, which did not require prior proof of identification. Indeed, prior identification is not a legal requirement applicable to all cases but rather merely the preference of the judge in Chanthabouala to address a specific defence challenge. 

Utilized correctly by a qualified forensic video analyst, motion tracking is a propitious method of image analysis that will provide meaningful assistance to the trier of fact when assessing scenes involving movement of individuals, vehicles, and other objects. Counsel leading such evidence must be prepared to argue why it is necessary and to emphasize that motion tracking does not change what occurs in the images, but rather draws the attention of the viewer to critical aspects of the images while leaving the full image readily available for examination.