 # Introduction

## Problem

We already have motion sensors in play that does this sort of thing easily, but if we want to use computer vision, things get tricky. How to get the speed from just pictures may seem impossible at first but after reading this, hopefully things get better.

## Approach

To do this we need to follow the following steps

• Feed the two images to our application - obviously ðŸ˜‚
• Get the difference between the two images
• Calculate the ðŸš—ðŸ’¨ from the image difference

Sounds pretty easy right? Lemme take you on a ðŸŽ¢

## Feed The Images To Our Application

We are using the default Buffered Image class provided by JAVA to read in the files with their paths specified.

Implementation

``````BufferedImage image1 = ImageIO.read(new File(img1));
``````

## Image Subtraction To Get The Difference

Now that we have our two images, we then perform Image Subtraction. You may be wondering what image subtraction is.
Images are pretty much an array of many pixel values that have been combined together.
Image Subtraction in the simplest of terms is Finding the difference between every pixel in an image and the corresponding pixel in another image. To perform image subtraction, both images need to have the same resolution (since we are subtracting every pixel in one from the other).
When we subtract the image, we set a threshold and set any pixel value higher than the threshold to BLACK, and pixel values lower than the threshold to WHITE ( We are doing this because we want to see the cars in black after subtracting the images )

Implementation

``````public BufferedImage ImageSubtract(BufferedImage img1 , BufferedImage img2){

int imageheight = img1.getHeight();
int imagewidth = img1.getWidth();

WritableRaster image1 = img1.getRaster();
WritableRaster image2 = img2.getRaster();

// pixel values have their, red , green and blue components

int diffred;
int diffblue;
int diffgreen;

// set threshold for subtraction

Color treshold = new Color(30,30,30);

// Create an empty image
BufferedImage DiffImage = new BufferedImage(imagewidth,imageheight,BufferedImage.TYPE_INT_RGB);

for(int y = 0 ; y < imageheight ; y++){
for( int x = 0 ; x < imagewidth ; x++){
diffred = Math.abs( image1.getSample(x, y, 0) - image2.getSample(x,y,0));
diffgreen = Math.abs(image2.getSample(x,y,1) - image1.getSample(x, y, 1));
diffblue = Math.abs(image2.getSample(x,y,2) - image1.getSample(x, y, 2));

// combine individual rgb components to give one color

Color diff = new Color(diffred,diffgreen,diffblue);
DiffImage.setRGB(x, y, diff.getRGB());

// to visualize the differences in the image

if(DiffImage.getRGB(x, y) < treshold.getRGB()){
diff= Color.WHITE;
DiffImage.setRGB(x, y,diff.getRGB());
}
else{
diff = Color.BLACK;
DiffImage.setRGB(x,y,diff.getRGB());
}
}
}
try {
// write the result image to a file
ImageIO.write(DiffImage,"jpg",new File("MotionDetected.jpg"));
} catch (IOException e) { e.printStackTrace(); }
return DiffImage;
}
``````

## Speed Calculation From Image Difference

Now that we have the difference visualized, its time for the "fun" part ðŸ•ºðŸ¾. Getting the speed from the image. To this, we applied some fun logic
Disclaimer : There may be a better way to implement this, but I was gunning for the method most easy to explain

We run through the "difference" image and then pick the first BLACK pixel you find and then save it. Then we run through the "difference" image again and then pick the last BLACK pixel we find.

Now some assumptions will have to be made

• Time passed between the first and second picture say 5s
• Assumed Magnification of the camera used to take the pictures
• Length of the car in millimeters (say 6000mm)

Once these assumptions have been made, we then covert the length of the car to pixels to make life easier and then get the distance moved,
`distance_moved = Math.abs(x2 - x1) - car_length_in_px;`
We subtracted the car length in pixel because the distance moved is from the front of the car.
We then solve for speed
`speed = (double) distance_moved / timepassed;`

Implementation

``````public double SpeedCalc(BufferedImage img){
double speed = 0.0;
int x1 = 0;
int x2 = 0;
WritableRaster newImg = img.getRaster();
for(int y = 0; y< img.getHeight(); y++){
for(int x = 0; x < img.getWidth(); x++){
if( newImg.getSample(x, y, 0)== 0){
x1 = x; // thats the the first x black value
break;
}
else{
continue;
}
}
if(x1 > 0){
break;
}
}
for(int y=0; y< img.getHeight(); y++){
for(int x = 0; x<img.getWidth(); x++){
if( newImg.getSample(x, y, 0)== 0){
x2 = x; // thats the last x black value
break;
}
}
}
// we only really need the x values
int timepassed = 10; // in seconds
double mag = 0.09; // assumed magnification more details in report
int car_length = 6000; // in millimeters
int car_px = (int)(car_length * mag);
System.out.printf("Final Position of Car:%d, Initial Position of Car:%d \n" , x1,x2);
int distance_moved = Math.abs(x2 - x1) - car_px;
speed = (double) distance_moved / timepassed;
return speed;
}
``````

Results

``````\$ java MotionDetection images/car_moving_final_1.PNG images/car_moving_final_2.PNG
The resulting difference of the image is stored in MotionDetected.jpg
Time Passed between 2 images 7 seconds
Final Position of Car:370, Initial Position of Car:1311
Speed of car is 0.63 metres/second
``````

Here's a link to the github repo if you want to see the full source code
https://github.com/oreHGA/Speed-Detector

Thanks for riding with me! ðŸ‘¨ðŸ¾”ðŸ’»

### Discussion   