Graph Reading for Plot Velocities

[Maya Srinivasan]

Hi there, 

My colleagues and I were wondering how to read and understand the plot velocities graph format that Ctrax produces after tracking a video. I will insert one of our output graphs below, but we couldn't really find what the zero line and the negative values mean. 

Thank you!

[John Bender]

Hi Maya,

Zero velocity means the fly isn't moving. Negative velocity should mean the fly is moving backwards. Judging by this graph, your tracking isn't particularly good. The spikes mean the fly is moving 100 pixels per video frame, where generally even a fast-walking fly would only move a few pixels per frame. (You could do some measurements to convert pixels into millimeters and video frames into milliseconds.) So I suspect that Ctrax isn't accurately finding your flies in the video, so it thinks the flies are jumping large distances forward and backward, leading to spikes in the velocity graph.

JB

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