Getting started

Shall we start practicing some spells? For that, we need a Wizard!

from expelliarmus import Wizard

Let’s cast a spell called read() and read this RAW file to a structured NumPy array!

wizard = Wizard(encoding="evt3", fpath="./pedestrians.raw")
arr = wizard.read()
print(arr.shape) # Number of events encoded to the NumPy array.

(39297796,)

The array is a collection of (timestamp, x_address, y_address, polarity) tuples.

print(arr.dtype)

[('t', '<i8'), ('x', '<i2'), ('y', '<i2'), ('p', 'u1')]

A typical sample looks like this:

print(arr[0])

(5840504, 707, 297, 0)

If we would like to reduce the EVT3 file size, we can use the cut(fpath_out, new_duration) spell to limit the recording time duration to 12ms, for instance:

nevents = wizard.cut(fpath_out="./pedestrians_cut.raw", new_duration=12)
print(f"Number of events embedded in the cut file: {nevents}.") # The number of events embedded in the output file.

Number of events embedded in the cut file: 540.

This can be verified by reading the new file in an array.

cut_arr = wizard.read(fpath="./pedestrians_cut.raw")
print(f"Length of array extracted from the cut recording: {len(cut_arr)}.")

Length of array extracted from the cut recording: 540.

The files are consistent:

print(f"First original sample: {arr[0]} | First cut sample: {cut_arr[0]}.")
print(f"{nevents}th original sample: {arr[nevents-1]} | Last cut sample: {cut_arr[-1]}.")
print((arr[:nevents]==cut_arr[:]).all())

First original sample: (5840504, 707, 297, 0) | First cut sample: (5840504, 707, 297, 0).
540th original sample: (5853218, 1208, 253, 0) | Last cut sample: (5853218, 1208, 253, 0).
True

The time duration is, more or less, the desired one (the events are discrete, hence we have not a fine control over them).

print(f"New recording duration: {((cut_arr['t'][-1] - cut_arr['t'][0])/1000):.2f} ms") 

New recording duration: 12.71 ms

What if your wand is not powerful enough to read the full recording to memory? Well, we can try to read the files one chunk of events at time…

wizard.set_chunk_size(chunk_size=512)
print(f"Length of the chunk: {len(next(wizard.read_chunk()))}.")

Length of the chunk: 512.

Let’s read less events, so that we are able to visualize them

wizard.set_chunk_size(chunk_size=16)
print(next(muggle.read_chunk()))

[(5848837,  610, 296, 1) (5848843,  834, 302, 1) (5848846,  593, 254, 1)
 (5848846, 1003, 298, 1) (5848859,  610, 299, 1) (5848887,  709, 306, 0)
 (5848888,  756, 292, 0) (5848895,  704, 300, 0) (5848903,  744, 169, 1)
 (5848904, 1209, 252, 0) (5848905,  709, 307, 0) (5848911,  139, 315, 0)
 (5848918,  603, 301, 1) (5848918,  708, 299, 1) (5848924,  778, 295, 1)
 (5848967,  140, 315, 0)]