Python方式记录while循环内的更改

Question

I have a Python script that triggers the computer webcam and is able to detect movement in the video being captured using opencv.

The way it works is Python reads the very first frame of the video and stores it as a numpy array in a variable. Then, there is a while loop running in the script that basically compares the first video frame with each current frame of the video. There are around 30 frames every second being iterated in the loop. When the current frame is different than the first frame, I update a variable inside the while loop by assigning the value 1 to it. So, as the loop goes on, you may have 0, 0, 0 and then 1, 1, 1, 1 depending on whether there is motion or not. My purpose is to record the time when motion starts. In other words, the time when my variable changes from 0 to 1.

Here is my pseudo code:

start webcam
times_list=[]
motion_list=[]
while True:
    my_variable=0
    frame_difference=current_frame - first_frame
    if frame_difference > 0:
        continue
    my_variable=1
    motion_list.append(my_variable)
    #The motion_list will get big, so let's keep only the last two items to avoid memory problems
    #The last two items is all we need.
    #Check if there was a change from non-motion to motion
    if motion_list[-1]==1 and motion_list[-2]==0:
        times_list.append(datetime.datetime.now())

So, at the end I have a time_list with all times when motion started.

Is this a good solution or am I missing something here?

Answer 1

Update:

First suggestion:

There are few improvements you could do, especially if, as it is often in practice (but not necessarily), you intend to run your application on Raspberry Pi .

In total, there are 4 cases to consider about and that your program must cover them all:

1. No motion: ⇢ ⓪ ⇢ ⓪ ⇢ ⓪ ⇢

2. Continuous motion: ⇢ ❶ ⇢ ❶ ⇢ ❶ ⇢

3. From no motion to motion: ⓪ ⇢ ❶

4. From motion to no motion: ❶ ⇢ ⓪

The only time to register is the one described by the case 3; while during the motion (case 2), we won't care about the time.

To resolve this, I suppose , previous to the while loop:

• You saved first_frame somewhere
• You set my_variable to 0

Here is the pseudocode:

times_list=[]
my_variable = 0
while True:
    frame_difference = current_frame - first_frame
    # Case 1:
    if difference_frame == 0 and my_variable == 0:
        continue
    # Case 2:
    if difference_frame != 0 and my_variable == 1:
        continue
    # Case 3:
    if difference_frame != 0 and my_variable == 0:
        my_variable = 1
        # Your comments say this is the only instant you are interested in
        times_list.append(datetime.datetime.now())
    # Case 4:
    if difference_frame == 0 and my_variable == 1:
        my_variable = 0

Notes:

1. For the sake of performance, it is better to test cases 1 and 2 before the rest as, in practice, they occur more frequently than the two other cases. So by running continue you avoid wasting time in checking cases 3 and 4 which are less frequent.
2. We got rid of motion_list and not need to allocate a memory for this list anymore because my_variable plays the role of a flag and it satisfies your need when by re-setting its value to 0 or 1 in the last two cases.

Second suggestion:

If you are not doing this application as a university project but for more practical/commercial reasons, you may think of this improvement: as reading frames from camera is an I/O bound task, you may be interested in increasing webcam FPS (and more probably in increasing Raspberry Pi FPS )