Motion History Images

The motion history image (MHI) is a static image template helps in understanding the motion location and path as it progresses. In MHI, the temporal motion information is collapsed into a single image template where intensity is a function of recency of motion. Thus, the MHI pixel intensity is a function of the motion history at that location, where brighter values correspond to a more recent motion. Using MHI, moving parts of a video sequence can be engraved with a single image, from where one can predict the motion flow as well as the moving parts of the video action.

Some important features of the MHI representation are:

• It represents motion sequence in a compact manner. In this case, the silhouette sequence is condensed into a grayscale image, where dominant motion information is preserved.
• MHI can be created and implemented in low illumination conditions where the structure cannot be easily detected otherwise.
• The MHI representation is not so sensitive to silhouette noises, holes, shadows, and missing parts.
• The gray-scale MHI is sensitive to the direction of motion because it can demonstrate the flow direction of the motion.
• It keeps a history of temporal changes at each pixel location, which then decays over time.
• The MHI expresses the motion flow or sequence by using the intensity of every pixel in a temporal manner.

General Algorithm

for each time t
  Bt := absolute_difference(It, It-1) > threshold
end for

for each time t
  for each pixel (x,y)
    if Bt(x,y) = 1
      MHIt(x,y) := τ
    else if MHIt-1 ≠ 0
      MHIt(x,y) := MHIt-1(x,y) - 1
    else
      MHIt(x,y) := 0
    end if
end for