Real-time streaming mode vs post-process analysis mode: which is better for mocap?

When considering which motion capture system to purchase, it’s of vital importance that you know exactly what data you’re going to be capturing and when you need to view it. 

For example, if you are planning to track robots or drones to confirm they are moving as their algorithm intended,or you’re wanting to provide an athlete in rehabilitation with instant feedback, you’ll need to be recording and viewing your data in real-time mode. The term “real-time” indicates that the motion capture system is translating the motion it’s capturing, as it’s happening, directly into the system for you to see. 

But if you’re picking out events for time-normalizing over recurring cycles to compare to other subjects; or exporting in a selection of formats to analyze in other software packages, you’ll be able to record data and look at it afterward using post-processing mode.

Let’s have a look at some of the features offered by each mode of analysis, and some of the factors you should consider when deciding what mode you’ll need to evaluate your data:

Features of real-time streaming

1. Real-time 2D and 3D viewing
2. Real-time labeled marker data
3. Real-time segment data (both kinematic and kinetic)
4. Streaming to third-party software

Features of post-process analysis

1. Auto-labeled marker data for rapid analysis
2. Synchronized force plate analog data and reference video
3. Powerful data clean up and segmental modeling tools
4. Export data to third-party software in various formats

Factors to consider when deciding between post-process analysis and real-time streaming: 

• The frame rate. Closed-loop feedback for a fast-moving robot may need high speed, and humans can’t visualize or process information this fast, so you can save on camera and computing costs.
• Latency. This is the delay between the action taken and the system’s response to that action. For a motion capture system, this is typically measured from triggering the recording of an image frame in all the synchronized cameras, to data from that frame being presented to an ethernet network interface card (NIC), ready to be passed on. It includes the time to get the image from the sensors, calculating 2D centroids, reconstructing 3D coordinates, labeling the markers and optionally calculating 6DoF of one or more segments. 
• Calculation and graphics rendering. When considering the required latency and frame rate for your application, it is worth considering the modeling and display capacity. Nowadays, vector algebra for segmental modeling requires very little computing power and therefore will only be significant if there are multiple subjects. But, if the feedback method requires highly detailed animation, then that step is likely to demand the greatest processor power. Ask the vendor about distributed computing – whether modeling, analysis and rendering tasks can be easily sent to different computers so they can be undertaken in parallel.
• Data cleanup. In post-process analysis there are many methods for filling gaps, either by interpolation or rigid-body subset back calculation. Ask your vendor which of these methods can be used in real-time and how easy they are to instigate.
• Time normalization/cyclical display. For example, you may be doing treadmill running analysis and want to train your subject to attain a certain maximum or minimum joint ankle at a specific event. Many of the tools used in post-process should be able to also be applied in real time. Ask your vendor if all the built in graphing functions are available in Post-Process and Live Mode.
• Data Streaming Formats: 

Questions to ask include:

• What data streaming formats are supplied?
• Is there a software developer’s kit and plug-ins?
• Is it restricted to certain operating systems? 

There’s a whole lot more where that came from.

We’ve developed an entire Buyers Guide for Motion Capture where we talk about the various types of systems and how they differ; the questions you should be asking vendors; and the most important features you should consider when buying a new mocap system. 

Download our Ultimate Buyer’s Guide for Purchasing Motion Capture Software here.