JeVois Tutorials  1.16
JeVois Smart Embedded Machine Vision Tutorials
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Adding frames/s and CPU load information to Python modules

The core JeVois software provides a number of helper classes to achieve things that are often useful in machine vision modules. One of them is a Timer class that computes average frames/s of processing, and adds some information about CPU usage, temperature, and frequency.

JeVois Timer class

The jevois::Timer class is designed to be very simple to use. It has two functions, start() and stop(). From the Timer documentation:

This class reports the time spent between start() and stop(), at specified intervals. Because JeVois modules typically work at video rates, this class only reports the average time after some number of iterations through start() and stop(). Thus, even if the time of an operation between start() and stop() is only a few microseconds, by reporting it only every 100 frames one will not slow down the overall framerate too much. See Profiler for a class that provides additional checkpoints between start() and stop().

Setting up

Here, we will start from where we ended in ProgrammerInvHello: a simple ArUco tag detector written in Python + OpenCV using JeVois Inventor.

If you have not yet done so, create a new module called Hello now, as follows:

  • Select New Python Module... from the pull-down menu of JeVois Inventor (or press CTRL-N).
  • Fill in the details as shown below:

See Hello JeVois using JeVois Inventor for more details.

Trying it out

We add a timer to our ArUco detector as follows:

  • In the constructor of our class (the __init__() function), instantiate a Timer
  • After we have received our input frame, start a measuring period
  • Process the input frame, here to detect ArUco tags
  • After all processing is done, stop the measuring period
  • The stop() function returns a pre-formatted string with frames/s, CPU, etc info, so we can just write that into our output image using cv2.putText()
import libjevois as jevois
import cv2
import numpy as np
class Hello:
def __init__(self):
self.dict = cv2.aruco.Dictionary_get(cv2.aruco.DICT_4X4_50)
self.params = cv2.aruco.DetectorParameters_create()
self.timer = jevois.Timer('ArUco detection', 50, jevois.LOG_DEBUG) # new code
def process(self, inframe, outframe):
img = inframe.getCvBGR()
self.timer.start() # new code
grayimg = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
corners, ids, rej = cv2.aruco.detectMarkers(grayimg, self.dict, parameters = self.params)
img = cv2.aruco.drawDetectedMarkers(img, corners, ids)
fps = self.timer.stop() # new code
cv2.putText(img, fps, (3,img.shape[0]-7), cv2.FONT_HERSHEY_SIMPLEX,
0.4, (255,255,255), 1, cv2.LINE_AA) # new code

A few notes:

  • In python, the time measured by our timer here does not include conversion of the input image from the camera, from the native pixel format of the camera to BGR. It also does not include conversion of the input from BGR to the output pixel format that will be sent over USB.
  • In C++ modules, in which we often access camera and USB buffers at a lower level, we usually make these conversions explicitly and then also include their time in the measurements reported by the timer.
  • Hence, do not get too excited if you create a module in Python that seems to operate faster than its C++ counterpart, maybe the difference is just due to the fact that the timer in Python is not including the image format conversion times.

Going further

  • JeVois also provides a Profiler class that allows you to measure timing for different checkpoints between start() and stop(). Launch DarknetSingle and enable log outputs (in JeVois Inventor, switch to the Console tab and toggle log messages to USB) to see how it can be used, for example, to measure time spent computing each layer of a multi-layer neural network. Every few seconds, you will get a report of time spent computing each layer.