Car tracking with cascades

Car Tracking with OpenCV
Car Tracking with OpenCV

In this tutorial we will look at vehicle tracking using haar features. We have a haar cascade file trained on cars.

The program will detect regions of interest, classify them as cars and show rectangles around them.

Related courses:

Detecting with cascades

Lets start with the basic cascade detection program:

#! /usr/bin/python
import cv2
face_cascade = cv2.CascadeClassifier('cars.xml')
vc = cv2.VideoCapture('road.avi')
if vc.isOpened():
rval , frame =
rval = False
while rval:
rval, frame =
# car detection.
cars = face_cascade.detectMultiScale(frame, 1.1, 2)
ncars = 0
for (x,y,w,h) in cars:
ncars = ncars + 1
# show result

This will detect cars in the screen but also noise and the screen will be jittering sometimes. To avoid all of these, we have to improve our car tracking algorithm.  We decided to come up with a simple solution.

Car tracking algorithm

For every frame:

  • Detect potential regions of interest
  • Filter detected regions based on vertical,horizontal similarity
  • If its a new region, add to the collection
  • Clear collection every 30 frames

Removing false positives
The mean square error function is used to remove false positives. We compare vertical and horizontal sides of the images. If the difference is to large or to small it cannot be a car.

ROI detection
A car may not be detected in every frame. If a new car is detected, its added to the collection.
We keep this collection for 30 frames, then clear it.

#! /usr/bin/python
import cv2
import numpy as np
def diffUpDown(img):
# compare top and bottom size of the image
# 1. cut image in two
# 2. flip the top side
# 3. resize to same size
# 4. compare difference
height, width, depth = img.shape
half = height/2
top = img[0:half, 0:width]
bottom = img[half:half+half, 0:width]
top = cv2.flip(top,1)
bottom = cv2.resize(bottom, (32, 64))
top = cv2.resize(top, (32, 64))
return ( mse(top,bottom) )
def diffLeftRight(img):
# compare left and right size of the image
# 1. cut image in two
# 2. flip the right side
# 3. resize to same size
# 4. compare difference
height, width, depth = img.shape
half = width/2
left = img[0:height, 0:half]
right = img[0:height, half:half + half-1]
right = cv2.flip(right,1)
left = cv2.resize(left, (32, 64))
right = cv2.resize(right, (32, 64))
return ( mse(left,right) )
def mse(imageA, imageB):
err = np.sum((imageA.astype("float") - imageB.astype("float")) ** 2)
err /= float(imageA.shape[0] * imageA.shape[1])
return err
def isNewRoi(rx,ry,rw,rh,rectangles):
for r in rectangles:
if abs(r[0] - rx) < 40 and abs(r[1] - ry) < 40:
return False
return True
def detectRegionsOfInterest(frame, cascade):
scaleDown = 2
frameHeight, frameWidth, fdepth = frame.shape
# Resize
frame = cv2.resize(frame, (frameWidth/scaleDown, frameHeight/scaleDown))
frameHeight, frameWidth, fdepth = frame.shape
# haar detection.
cars = cascade.detectMultiScale(frame, 1.2, 1)
newRegions = []
minY = int(frameHeight*0.3)
# iterate regions of interest
for (x,y,w,h) in cars:
roi = [x,y,w,h]
roiImage = frame[y:y+h, x:x+w]
carWidth = roiImage.shape[0]
if y > minY:
diffX = diffLeftRight(roiImage)
diffY = round(diffUpDown(roiImage))
if diffX > 1600 and diffX < 3000 and diffY > 12000:
rx,ry,rw,rh = roi
newRegions.append( [rx*scaleDown,ry*scaleDown,rw*scaleDown,rh*scaleDown] )
return newRegions
def detectCars(filename):
rectangles = []
cascade = cv2.CascadeClassifier('cars.xml')
vc = cv2.VideoCapture(filename)
if vc.isOpened():
rval , frame =
rval = False
roi = [0,0,0,0]
frameCount = 0
while rval:
rval, frame =
frameHeight, frameWidth, fdepth = frame.shape
newRegions = detectRegionsOfInterest(frame, cascade)
for region in newRegions:
if isNewRoi(region[0],region[1],region[2],region[3],rectangles):
for r in rectangles:
frameCount = frameCount + 1
if frameCount > 30:
frameCount = 0
rectangles = []
# show result

Final notes
The cascades are not  rotation invariant, scale and translation invariant. In addition, Detecting vehicles with haar cascades may work reasonably well, but there is gain with other algorithms (salient points).

You may like:

Download Code + Video + Cascade file


40 thoughts on “Car tracking with cascades

  1. Frank - May 31, 2017

    try with both Python 2 and Python 3. Can you display the data beforehand?

  2. Sébastien Turrel - May 28, 2017

    Frank did you create the xml yourself ?
    my openCv training on my Debian serv always stop after stage 0 🙁
    btw, when i run your last script with Roi the error is :

        frame = cv2.resize(frame, (frameWidth / scaleDown, frameHeight / scaleDown))
    TypeError: integer argument expected, got float

    can you help me with that my friend ?

    1. Frank - May 29, 2017

      I don’t recall, has been a while. Do you have video capture? Try to show the verify these variables contain the right values using debugger or print function. It mentions a type error, cast frameWidth/scaleDown and the other one to integers using int().

      1. Sébastien Turrel - May 29, 2017

        this code is very interresting my friend.
        when i do :
        frame = cv2.resize(frame, (int(frameWidth / scaleDown), int(frameHeight / scaleDown)))
        frameHeight, frameWidth, fdepth = frame.shape
        it s say :
        left = img[0:height, 0:half]
        TypeError: slice indices must be integers or None or have an __index__ method
        is it possible to you to write it again or something ? i am just discovering haar cascade and it’s very fascinating.

      2. Sébastien Turrel - June 6, 2017

        Still here my friend ? 🙂

        1. Frank - June 9, 2017

          Hi, has been a few busy days. I’ve no problem running the code from the zip file.
          I’m using Python 2.7.13 on Manjaro Linux.
          Did you download the zip file? It has an attached video and haar file.

          1. Sébastien Turrel - June 11, 2017

            yeah i did it, but i am running Python 3.5

  3. Saeed Iqbal - May 15, 2017

    Dear Sir,

    I am facing the below error:
    Unable to stop the stream: Inappropriate ioctl for device

    1. Frank - May 15, 2017

      Hi Saeed, are you using a webcam or video file?

    2. Sébastien Turrel - June 9, 2017

      i did it my friend 😉 maybe it’s just because i am using Python 3.5 😉

  4. Volodymyr Rykhva - March 19, 2017

    hi:) Thanks for posting this tut.It helps me to understand a lot. But I got a problem with improved version of your car detection script. The basic script works well (emm…I meant it shows red rectangle when a car is in focus) and your solution doesn’t show this red frame. What could be the issue here? Thanks

    1. Frank - March 20, 2017

      Is it the same video? The array rectangles is probably empty, inside the loop of detectCars(), check using

      1. Volodymyr Rykhva - March 26, 2017

        yeap, it’s actually empty for some reasons:(

        1. Frank - March 27, 2017

          try changing the method isNewRoi, that decides if its added to to the list. The values are set to 40, which is just set by a test. Try changing the values

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