Okay I am starting to understand the idea of neural networks but I still haven't been able to understand the use/benefits/implementation of convolutional neural networks especially for image processing.

What I don't understand is how to detect an object of different size.

Say we have a weights matrix of 50*50 and a face in the training set which is of 50*50 size . Now when you take the weighted sum of the matrix it will return a particular value say "X" . So now I understand the idea is to run the weights matrix over the entire image to get a lock over a region which gives a weighted sum of ~"X" now you have detected a face.

If the above mentioned understanding is true Then how is that an image of size 25*25 going to churn out a value anywhere clone to "X" . So the basic question is how to deal with relative sizes of the image .

If possible please suggest some good tutorials for the same .


First of all, CNNs are typically used with identical sized inputs. So, you resize your input to a constant size, no matter what.

Next, detecting an object and recognizing an object over many samples are different tasks. For object detection, you could be better of with R-CNNs. Its sources are also available and there is a Caffe tutorial for R-CNNs. Of course there are many other works on the same topic. For example, this one.

  • $\begingroup$ As in say like we have an image of 50*50 , with in the image there is a face in the background which is made using 20*20. So how would the weights matrix return the same value for a face . I read some where that we can resize the same image so that there might come a time when the face is enlarged to fit a 50*50 region . But i dont find this a very efficient way of doing the same. $\endgroup$ – Chris Thaliyath Aug 13 '15 at 6:05
  • $\begingroup$ No it's not efficient. Your network should be trained with scaled samples of faces to do that. You should also learn it. $\endgroup$ – Tolga Birdal Aug 13 '15 at 6:18
  • $\begingroup$ Okay so say an object say face would be having a range of values which is applicable $\endgroup$ – Chris Thaliyath Aug 13 '15 at 6:24
  • $\begingroup$ Thanks buddy , now I have another question about back propagation. $\endgroup$ – Chris Thaliyath Aug 13 '15 at 6:27
  • $\begingroup$ Like how do I go about it using gradient descent method and why exactly do we need a sigmoid function $\endgroup$ – Chris Thaliyath Aug 13 '15 at 6:28

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