Training your own Object Detector

Hello people! Hope you all are enjoying the journey of learning computer vision with me. Remember, the OpenCV code we wrote for face detection. We had used the pre-built classifier 'haarcascade_frontalface_alt.xml’. Did you guys think on what this the xml file is? How was it generated? How can you have your xml file which will help you have a model capable of detecting objects of your interest? 

Here, we will try to answer all of your above questions and at the end you will be in a position to have your own model. 

Training your model:

The xml file is cascade trained for object detection as you may have correctly predicted. Now to train a cascade, you will need loads of data i.e images of objects you want your model to be able to recognise. You will also need images which do not contain the object of your interest. The images with the object in them are referred to as ‘Positive images’ and images without the object are called ‘Negative images’. Here, I will be using the database of cars freely available at this lhttp://cogcomp.cs.illinois.edu/Data/Car/. 

It has 550 positive , 500 negative and few test images to check the cascade we just trained ourselves. Now that we have the dataset of cars,we will have a model trained which is capable of detecting cars in unknown images. We would now want all the image details be listed with the correct names so that reading those images from folder isn’t a problem. One way is to type down all the names manually in text file and drain your energy doing nothing good. Other option is using a ubuntu inbuilt command. I and all the smart people ( which you are since you are reading this blog :P) will go for second option.

Open the terminal on your system. Go to folder where the car images are present. For convenience I have the positive and negative image folder saved on desktop. So I will do the following:  

This will create a info.txt file in the folder with all the image files listed. We would now give it the absolute path so that we can use the details from the desktop directory too.  Same is repeated for negative images.

Now I have the list of positive and negative images ready. The list of positive images should have one more detail with its name i.e the location where the object of our interest is present. In this case we have the isolated cars in images and all have the same dimension. This simplifies the work for us. You should now have the info.txt as shown below: 

I will now move the info.txt and neg.txt to Desktop.

The training of cascade requires the data of object to be present in a ‘vec file’. So we will now have the vec file generated. The command should do the work for you. 

$opencv_createsamples -info info.txt -num 500 -w 48 -h 24 -vec car.vec 

The width and height are set to that ratio since the car has greater width and lesser height. Also, the number of samples we use is generally less than the number of actual images we have and so we take 500 in this case. Now create a folder ‘data’ which will contain all the information of training stages and also have the final trained cascade.

Now run the command 

$opencv_traincascade -data data -vec car.vec bg neg.txt -numPos 400 -numNeg 500 -numStages 13 -w 48 -h 24 -featureType LBP -maxFalseAlarmRate 0.4 -minHitRate 0.99 -precalcValBufSize 20488 -precalcIdxBufSize 2048

This command has started the training process for cascade. You will see something like this:

Depending upon the number of stages we want the cascade to train itself, it will take sometime and the process will complete. This should take good amount of time depending your system configuration. Also, the number of images we took here is quiet less if we want the cascade to be very accurate. And increasing the number of images will definitely add to the time consumption.  You can see something like this:

Now, you can see the cascade.xml in the data folder. It also has various stage.xml. The stage.xml is the result obtained after it has completed that many stages of training. It may not really seem useful since we already have obtained the final cascade within hardly some significant time. That may not be case always, especially when the dimensions of the object is big and large number of images are used. Now, imagine that the training stops due to some unexpected interruption like power cut or something that sort. How frustrating it would be start the training all over again and wasting the time. This is where the stages.xml come to rescue. The training will resume only from the stage where it last stopped and not from stage 0. 

Thus, you have now trained the cascade for car detection. You can definitely go ahead with training your object detector! Now its time to check how does the cascade work. So pick up any image from the test data set or whichever image of car you have. The only thing to make sure is that car has the shape similar to the images which were used for training.

Copy the code given below and keep your fingers crossed.

Yeah!! The car detector worked. Now start collecting the images of object you would like your model to recognise and start training the cascade. Explaining every steps in details was not possible right now. Do write to me, if you get struck somewhere or have any particular doubts. Subscribe to regularly get updates in your mail box.

CheERs!!