./backlog: Charlie's blog

meanderings through tidbits of mathsy computery stuff

Making a Mosaic from Random Images

You may have noticed that this blog has been quiet for a while. That's because I've been writing my undergraduate dissertation (the subject of another soon-to-be blog post, and hopefully a published paper)! This post is to make up for my lack of activity on here lately -- so let's get started.

If you've seen any recent pop art (or in fact, kept abreast of the arts section in most newspapers), you'll probably have seen things like this (found here);

A mosaic of Marilyn Monroe made from

Or this;

A mosaic of Barack Obama made out of

Yup, that really is a mosaic of Barack Obama's face made out of toast (of varying degrees of toasted-ness).

Lately, I was thinking about making something similar as a side project, using a set of random images as mosaic tiles. The idea was to make something -- maybe to print out a bunch of pictures and arrange them into a poster, for example. However, the process of finding a large enough and diverse enough set of images is time-consuming, and figuring out which images should go where is really labour-intensive. I'm FAR too lazy to do all of that by hand.

Fortunately, this is actually pretty easy to do if you know a bit of Python... which I do. So then, let's get started. There are two steps to this mini-project: acquiring and pre-processing a set of images, and then arranging them into a mosaic.

Acquiring and pre-processing image tiles

I wasn't sure where to get images from originally -- maybe Wikipedia Commons? Maybe Flickr? Maybe some public-domain images? Eventually (because of laziness), I simply settled on using Flickr images. I wrote a short Python script to search Flickr for photos with a particular set of tags (and of course, filtered the images by Creative Commons licences), and download them.

Then, I wrote another script to centre, crop, and resize these images into 32x32 pixel tiles. Now that the images are preprocessed into tiles of the same dimensions, they are ready to be used in the mosaic.

For the sake of prototyping, I went ahead and downloaded some 500-odd Creative Commons images from Flickr and pre-processed them into tiles. It's a really small set of images, so I didn't expect it to work very well -- but as you'll see later, I was pleasantly surprised.

Arranging into a mosaic

This is actually blindingly straightforward. A simple (plain English) algorithm for arranging these pre-processed images goes something like this:

  • Read in the target image (the image to be made into a mosaic)
  • Read in all of the image tiles
  • For each image tile
    • Calculate the tile's average colour
    • Store that in a data structure somewhere (we will use this later)
  • Create a blank image, with scaled up dimensions - to accommodate the tiles
  • For each pixel in the target image
    • Get the pixel's colour in RGB
    • Find the image tile with the closest-matching colour
    • Paste the corresponding image tile into the blank image we created earlier
  • Save the output image

Finding the best-matching tile

This is dead simple, too. Because it was the easiest thing to do, I simply averaged each colour channel across the entire image, and made the assumption that the resulting colour was the canonical colour of the entire image.

Finding the best match for an individual pixel in a set of image tiles is actually quite straightforward, too -- just take some distance measure (I used Manhattan distance) between each colour channel between the pixel and the average colour of each tile, and then sort the image tiles by distance. The tile at the top of the list will be the best match. Simples.

The results

I did a little experimentation to figure out if it worked. Initially, it worked surprisingly well, but (as I'd guessed would probably happen), a lot of images got re-used and made the overall effect a bit rubbish. I'll demonstrate this using a rather fetching picture of President Obama (in keeping with the toasty tribute earlier).

For reference, here's what the target image looks like:

President Obama's face

My initial attempt worked, but looked a bit like this:

Crappy first attempt

Great! It works! ... but so many images are used several times, it sort-of ruins the effect. OK, so what if... instead of choosing the closest matching tile, I randomly choose one from the top 5 closest matches?

Much better second

Much better! That's definitely more interesting. And for the sake of completeness, let's try another face... what about Michelle Obama?

Michelle Obama's face as a

Awesome. What abouuuuuuut (crap, I've run out of Obamas, what next?)... um... what about my own face? (sorry)

Here's what I look like normally:


And here's what I look like as a mosaic:


Success! Not bad for a couple of afternoons' work. If you're interested, the code is available on my GitHub account (it's very sloppily put together, so don't judge the code quality too harshly ;) ).