28-01-2011, 10:09 AM
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INTRODUCTION
Digital processing is done with a digital computer or some special purpose digital hardware.
The word Digital implies that the information in the computer is represented and sent by variables that taken limited number of discrete values.
Increasing demand for road traffic data of all sorts.
Variation of parameters in real-world traffic.
Aimed to measure queue parameters accurately.
Queue detection algorithm has two operations :
vehicle detection
motion detection.
Motion detection is based on applying a differencing technique on the profiles of the images along the road.
The vehicle detection is based on applying edge detection on these profiles.
Image analysis system structure
Stages of image analysis:
Image sensors used:
Improved video cameras: automatic gain control, low SNR
ADC Conversion:
Analog video signal received from video camera is converted to digital/binary form for processing
Pre-processing:
High SNR of the camera output reduces the quantity of processing enormous data flow.
Two jobs to be done:
Green light on:
Determine no. of vehicles moving along particular lanes and their classification by shape and size.
Red light on:
Determine the backup length along with the possibility to track its dynamics and classify vehicles in backup.
Queue Detection Algorithm
Approach described here is a spatial-domain technique to detect queue - implemented in real-time using low-cost system.
For this purpose two different algorithms have been used,
Motion detection operation
Vehicle detection operation
Motion detection is first – as in this case vehicle detection mostly gives positive result, while in reality, there may not be any queue at all
Motion detection operation:
Differencing two consecutive frames.
Histogram of the key region parts of the frames is analyzed by comparing with the threshold value.
Key region should be at least 3-pixel-wide profile of the image along the road.
A median filtering operation is firstly applied to the key region (profile) of each frame and one-pixel-wide profile is extracted.
Difference of two profiles is compared to detect for motion.
When there is motion, the differences of the profiles are larger than the case when there is no motion. The motion can be detected by selecting a threshold value.
Vehicle detection algorithm:
Following the application of the motion detection operation, a vehicle detection operation is applied on the profile of the unprocessed image.
To implement the algorithm in real time, two strategies are often applied:
Key region processing
Simple algorithms.
Most of the vehicle detection algorithms developed so far are based on a background differencing technique, which is sensitive to variations of ambient lighting.
The method used here is based on applying edge detector operators to a profile of the image .
Edges are less sensitive to the variation of ambient lighting and are used in full frame applications (detection).
Methods of vehicle detection:
Background frame differencing:
Grey-value intensity reference image
Inter-frame differencing:
Incoming frame itself becomes the background for the following frame
Segmentation and classification:
Sub division of an image into its constituent parts depending on the context
Edge detectors consisting of separable medium filtering and morphological operators, SMED (Separable Morphological Edge Detector) are applied to the key regions of the image. (The SMED approach is applied (f) to each sub-profile of the image and the histogram of each sub-profile is processed by selecting Dynamic left-limit value and a threshold value to detect vehicles.
SMED has lower computational requirement while having comparable performance to other morphological operators
SMED can detect edges at different angles, while other morphological operators are unable to detect all kinds of edges.
Canny edge detection operator:
Canny is the most famous edge detection operator.
It acts as either 1st order or 2nd order gradient operator.
Canny operator mainly consists of two operations.
Non-maximal Suppression.
Hysteresis Thresholding
In the Non-maximal Suppression, the non-maximal pixel values in the image will suppress to zero and maximal pixel values in the image will tend to a maximum value.
In the Hystersis thresholding, the detection of the threshold will take place.
This program selects a grey value from the histogram of the window, where there are approx. zero edge points above this grey value.
When the window contains an object, the left-limit of the histogram shifts towards the maximum grey value, otherwise it shifts towards the origin.
This process is repeated for a large no. of frames(100),and the minimum of the left-limit of these frames is selected as the left-limit for the next frame.
Threshold selection program:-
The no. of edge points greater than the left limit grey value of each window is extracted for a large no. of frames (200) to get enough parameters below and above a proper threshold value.
These nos. are used to create a histogram where its horizontal and vertical axes correspond to the no. of edge points greater than left limit and the frequency of repetition of these numbers for a period of operation of the algorithm (200 frames).
This histogram is smoothed using a median filter and we expect to get two peaks in the resulted diagram, one peak related to the frames passing a vehicle and the other related to the frames without vehicles for that window.
However, as it can be seen in the figure given below, there are other number of edge points (32-40) between peaks 20 & 60, which are related to those vehicles in each profile. We use statistical approach based on selecting a point on the horizontal axis, where the sum of the entropy of the points above and below this point is maximum. This point is selected as the threshold value for the next period.
Results and Discussions:
The main queue parameters we were interested in identifying were the length of the queue, the period of occurrence and the slope of the occurrence of the queue behind the traffic lights.
To implement the algorithm in real-time, it was decided that the vehicle detection operation should only be used in a sub-profile where we expect the queue will be extended.
