How do I find Waldo with Mathematica?
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This was bugging me over the weekend: What is a good way to solve those Where's Waldo?  ['Wally' outside of North America] puzzles, using Mathematica (image-processing and other functionality)?

Here is what I have so far, a function which reduces the visual complexity a little bit by dimming
some of the non-red colors:

And an example of a URL where this 'works':

(Waldo is by the cash register):

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Quentin Jerde Created at: 2013-11-13 17:07:18 UTC By Quentin Jerde
Now I need to play a game of "Where is the cash register?"... - Jacinthe Senger
@yoda -- top left, table with lots of shoes, a cash register and Waldo near the corner of the table. - Katharina Becker
Related: keshavsaharia.com/2011/03/16/theres-waldo - Fae Hessel
"Where's Wally." >.< - Dorian Jacobi Sr.
As a Waldo, I approve of this question - Garrison Hyatt
4 Answers
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I agree with @GregoryKlopper that the right way to solve the general problem of finding Waldo (or any object of interest) in an arbitrary image would be to train a supervised machine learning classifier. Using many positive and negative labeled examples, an algorithm such as Support Vector Machine, Boosted Decision Stump or Boltzmann Machine could likely be trained to achieve high accuracy on this problem. Mathematica even includes these algorithms in its Machine Learning Framework.

The two challenges with training a Waldo classifier would be:

Determining the right image feature transform.  This is where @Heike's answer would be useful: a red filter and a stripped pattern detector (e.g., wavelet or DCT decomposition) would be a good way to turn raw pixels into a format that the classification algorithm could learn from. A block-based decomposition that assesses all subsections of the image would also be required ... but this is made easier by the fact that Waldo is a) always roughly the same size and b) always present exactly once in each image.
Obtaining enough training examples. SVMs work best with at least 100 examples of each class. Commercial applications of boosting (e.g., the face-focusing in digital cameras) are trained on millions of positive and negative examples.
A quick Google image search turns up some good data -- I'm going to have a go at collecting some training examples and coding this up right now!

However, even a machine learning approach (or the rule-based approach suggested by @iND) will struggle for an image like the Land of Waldos! 
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I don't know Mathematica . . . too bad.  But I like the answer above, for the most part.  

Still there is a major flaw in relying on the stripes alone to glean the answer (I personally don't have a problem with one manual adjustment).  There is an example (listed by Brett Champion, here) presented which shows that they, at times, break up the shirt pattern.  So then it becomes a more complex pattern.  

I would try an approach of shape id and colors, along with spacial relations.  Much like face recognition, you could look for geometric patterns at certain ratios from each other.  The caveat is that usually one or more of those shapes is occluded.  

Get a white balance on the image, and red a red balance from the image.  I believe Waldo is always the same value/hue, but the image may be from a scan, or a bad copy.  Then always refer to an array of the colors that Waldo actually is: red, white, dark brown, blue, peach, {shoe color}. 

There is a shirt pattern, and also the pants, glasses, hair, face, shoes and hat that define Waldo.  Also, relative to other people in the image, Waldo is on the skinny side.  

So, find random people to obtain an the height of people in this pic.  Measure the average height of a bunch of things at random points in the image (a simple outline will produce quite a few individual people).  If each thing is not within some standard deviation from each other, they are ignored for now.  Compare the average of heights to the image's height.  If the ratio is too great (e.g., 1:2, 1:4, or similarly close), then try again.  Run it 10(?) of times to make sure that the samples are all pretty close together, excluding any average that is outside some standard deviation.  Possible in Mathematica?  

This is your Waldo size.  Walso is skinny, so you are looking for something 5:1 or 6:1 (or whatever) ht:wd.  However, this is not sufficient.  If Waldo is partially hidden, the height could change.  So, you are looking for a block of red-white that ~2:1.  But there has to be more indicators.

Waldo has glasses.  Search for two circles 0.5:1 above the red-white.  
Blue pants.  Any amount of blue at the same width within any distance between the end of the red-white and the distance to his feet.  Note that he wears his shirt short, so the feet are not too close.
The hat.  Red-white any distance up to twice the top of his head.  Note that it must have dark hair below, and probably glasses.
Long sleeves.  red-white at some angle from the main red-white.
Dark hair.  
Shoe color.  I don't know the color.
Any of those could apply.  These are also negative checks against similar people in the pic -- e.g., #2 negates wearing a red-white apron (too close to shoes), #5 eliminates light colored hair.  Also, shape is only one indicator for each of these tests . . . color alone within the specified distance can give good results.

This will narrow down the areas to process.

Storing these results will produce a set of areas that should have Waldo in it.  Exclude all other areas (e.g., for each area, select a circle twice as big as the average person size), and then run the process that @Heike laid out with removing all but red, and so on.  

Any thoughts on how to code this?

Edit:

Thoughts on how to code this . . . exclude all areas but Waldo red, skeletonize the red areas, and prune them down to a single point.  Do the same for Waldo hair brown, Waldo pants blue, Waldo shoe color.  For Waldo skin color, exclude, then find the outline.  

Next, exclude non-red, dilate (a lot) all the red areas, then skeletonize and prune.  This part will give a list of possible Waldo center points.  This will be the marker to compare all other Waldo color sections to.

From here, using the skeletonized red areas (not the dilated ones), count the lines in each area.  If there is the correct number (four, right?), this is certainly a possible area.  If not, I guess just exclude it (as being a Waldo center . . . it may still be his hat).

Then check if there is a face shape above, a hair point above, pants point below, shoe points below, and so on.

No code yet -- still reading the docs.
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My guess at a "bulletproof way to do this" (think CIA finding Waldo in any satellite image any time, not just a single image without competing elements, like striped shirts)... I would train a Boltzmann machine on many images of Waldo - all variations of him sitting, standing, occluded etc; shirt, hat, camera, and all the works. You don't need a large corpus of Waldos (maybe 3-5 will be enough), but the more the better.

This will assign clouds of probabilities to various elements occurring in whatever the correct arrangement, and then establish (via segmentation) what an average object size is, fragment the source image into cells of objects which most resemble individual people (considering possible occlusions and pose changes), but since Waldo pictures usually include a LOT of people at about the same scale, this should be a very easy task, then feed these segments of the pre-trained Boltzmann machine. It will give you probability of each one being Waldo. Take one with the highest probability.

This is how OCR, ZIP code readers, and strokeless handwriting recognition work today. Basically you know the answer is there, you know more or less what it should look like, and everything else may have common elements, but is definitely "not it", so you don't bother with the "not it"s, you just look of the likelihood of "it" among all possible "it"s you've seen before" (in ZIP codes for example, you'd train BM for just 1s, just 2s, just 3s, etc, then feed each digit to each machine, and pick one that has most confidence). This works a lot better than a single neural network learning features of all numbers.
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I've found Waldo!



How I've done it

First, I'm filtering out all colours that aren't red

waldo = Import["http://www.findwaldo.com/fankit/graphics/IntlManOfLiterature/Scenes/DepartmentStore.jpg"];
red = Fold[ImageSubtract, #[[1]], Rest[#]] &@ColorSeparate[waldo];


Next, I'm calculating the correlation of this image with a simple black and white pattern to find the red and white transitions in the shirt. 

corr = ImageCorrelate[red, 
   Image@Join[ConstantArray[1, {2, 4}], ConstantArray[0, {2, 4}]], 
   NormalizedSquaredEuclideanDistance];


I use Binarize to pick out the pixels in the image with a sufficiently high correlation and draw white circle around them to emphasize them using Dilation

pos = Dilation[ColorNegate[Binarize[corr, .12]], DiskMatrix[30]];


I had to play around a little with the level. If the level is too high, too many false positives are picked out. 

Finally I'm combining this result with the original image to get the result above

found = ImageMultiply[waldo, ImageAdd[ColorConvert[pos, "GrayLevel"], .5]]

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