Motion History Images

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The motion history image (MHI) is a static image template helps in understanding the motion location and path as it progresses.[1] 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:[2]

* 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

## References

1. ↑ "Research: Motion History Images". http://web.cse.ohio-state.edu/CVL/Research/MHI/mhi.html. Retrieved 13 November 2014.
2. ↑ Ahad, Md Atiqur Rahman. Motion history images for action recognition and understanding. Springer Science & Business Media, 2012.

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