Quantitative Video analysis for Qualitative Research: Glossary

Key Points

  • A video file contains a series of images, and each image is a matrix that can be operated on.

  • A framerate of 25 fps means that there are 25 frames (images) per second.

  • The bit rate tells about the variation of each pixel, an 8-bit image stores values from 0-255.

  • A colour image uses 4 planes, Alpha, Red, Green, Blue, while greyscale only uses 1 plane.

Getting around Matlab
  • A semicolon at the end of a line will not display the commands output.

  • Toolboxes need to be added to path before they can be used.

Video visualization
  • Motion videos display the parts of the video that change between frames.

  • A motiongram displays how motion changes over time.

  • Quantity and centroid of motion are quantitative features.

More visualization
  • A motion average image is a kind of open-shutter technique.

  • A motion history video visualizes motion trajectories.

  • All functions can be run on any type of video input.

Preprocessing video
  • A video is trimmed in time, cropped in space, and downsampled in pixels.

  • There are a number of things to set and modify in the code, most of which are documented in the help files.

Advanced techniques
  • Optical flow shows the direction of motion, but is much more CPU-consuming.

  • MGT is the starting point for a lot of more advanced analyses.

Combining video, audio and mocap
  • An MGT struct can be used to organize and manipulate different types of data.