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3. Playing with Args

Learning Goal: Understand the role of the Args class.

Before you get started, it might not be a bad idea to check out the fmdt.args module to read up on the Args class and it's composition. Nevertheless, we are going to begin with a simple example.

In previous tutorials, we've been passing our arguments directly to fmdt.detect like so:

fmdt.detect(vid_in_path="demo.mp4", trk_path="trk.txt")

Creating Args

However, we can explicitly create an Args object first, using either fmdt.detect_args or fmdt.Args.new.

args = fmdt.detect_args(vid_in_path="demo.mp4", trk_path="trk.txt")
args = fmdt.Args.new(vid_in_path="demo.mp4", trk_path="trk.txt")

These two calls are semantically equivalent; the difference between Args.new() and detect_args() is that Args.new includes extra options for fmdt-log-parser & fmdt-visu, such as vid_out_path, that a call to fmdt-detect does not use.

For example, the following code is valid:

args = fmdt.Args.new(vid_in_path="demo.mp4", vid_out_path="demo_visu.mp4")

whereas 

args = fmdt.detect_args(vid_in_path="demo.mp4", vid_out_path="demo_visu.mp4") #!!! ERROR !!!#

is not.

Danger

TODO: The latest does not fail (while it should) because of the **args parameter in the fmdt.detect_args function.

Calling fmdt-detect from an Args instance

We can actually use our new args to call fmdt-detect with the detect function. Since the parameters are already stored in args, we don't have to pass anything:

args.detect()

The benefits of this paradigm shift are subtle but important to reflect on. The main advantage is that this allows us to take a single core set of parameters and apply them across a list of videos.

Consider the case where we have a list of videeo filenames stored in vid_list:

vid_list = ["Draconids-6mm1-04.avi", ..., "Draconids-6mm2-18.avi"]

We can then define a set of parameters:

args = fmdt.detect_args(ccl_hyst_lo=150, ccl_hyst_hi=160)

And call fmdt-detect on all of the videos with args:

for v in vid_list:
    args.detect_args.vid_in_path = v # Update the --vid-in-path parameter
    args.detect()                    # Call fmdt-detect

Next Steps

Learn how to access the statistics stored in fmdt-detect's log files by following along here.