In continuation of my earlier post about Instagram’s Engineering Challenge on an Image Unshredder, I took some time out yesterday and completed the automatic strip width detection piece. The approach was easy, the euclidean distance on the strip edge will be too high than the normal values.

For example the values may look like,

3, 4, 3, 5, 3, 2, 6, <b>22</b>, 4, 3, 2, 3, 4, 3, 2, <b>27</b>, 3, ...

One just needs to take the average of above values, then find the average of values greater than this average. Once you have the maxAverage, one can run a loop and find the value where the ratio of (value / minimum) is close to the ratio of (maxAverage / minimum). The first index where this happens is the strip width. To be doubly sure that we have chosen the right value, make sure the strip width divides the image width in whole.

I tested my code with many a free images from Flickr and it all worked. Though I did find a couple of issues with images that had a dark background and no front object - like a huge clear sky running in.

As the challenge was to unshred the shredded Tokyo image, it stands completed. The remaining improvements, I will leave for some other day.

I have also added an image shredder and a test suite that takes in images from a given folder, shreds and then unshreds them.

For those interested the code is posted on my GitHub repository at

Hope this helps!