A few months ago I saw a cool mosaic effect in a Wired ad for CA Technologies. Here’s what part of the ad looked like:

I liked the ad, so I wondered how they did it. Can you see out how to create a similar effect? Take a minute to figure it out as an exercise.

Here’s what I came up with: divide the image into tiles. For each tile, compute an average overall color for that tile. Then go back and blend every pixel in that tile with the average color. So if a tile is partly dark and partly blue, the average color is a dark blue, so the blue in that tile becomes even darker. I like that the effect is pretty simple once you figure out how to do it.

Of course, once I had an idea of how to do it, I wanted to write some code and see whether I could recreate the effect. Go has good libraries for handling images and I’ve been meaning to try Go. I ended up with about 70 lines of moderately-ghastly Go code that did the job.

For this Creative Commons image (thanks Fuelrefuel/Wikimedia Commons!)

I ended up with a photomosaic like this:

As far as I can tell, that’s pretty much the same filter that ran in the ad. Here’s another example. First, a picture of me:

and here’s the resulting mosaic’ed image:

That’s all the interesting stuff. You can stop reading now.

This part is boring. Really. No need to keep reading. The code I came up with is really ugly, but the pseudo-code is pretty simple:


- Read the picture into a go image
- Number of horizontal tiles = image_width / desired_tile_width
- Number of vertical tiles = image_height / desired_tile_height
- Loop through tiles with nested vertical and horizontal for loops
- For each tile, loop over the tile's pixels to compute average RGB values
- Loop over the tile's pixels again & set new_color = (avg_color+curr_color)/2
- Write the image out as a new picture


That’s it! I wanted a quick and dirty test, so I didn’t worry about things like the leftover pixels if the tiles didn’t evenly divide the image.

Let’s see, what else. Things I liked about Go:
– It’s super-easy to read and write images, so I could concentrate on the fun stuff.
– I like that documentation like this gives a clear, easy way to set up your environment. The golang tour is great too. And installing Go on Ubuntu is easy: “sudo apt-get install golang” and you’re done.
– The language makes a lot of sense to me, in a C kind of way.

Some things didn’t make as much sense to me, or at least I need to do more reading:
– My initial program just read a JPEG and wrote it back out, and the output image was considerably dimmer. I was just using default encoding values, so maybe some gamma values got left out, but it was a little weird. I was expecting read->decode->encode->write to be a no-op.
– When I read the JPEG into an image and tried to write directly to that image, Go gave me an error. That was a little strange. I ended up copying the JPEG to a new image and then I could write.
– In the spirit of just doing stuff without reading the documentation, it seemed like Go images stored their At() component colors with 16 bits of range (from 0..65536). But when I wanted to write colors with Set() it seemed like Go wanted 8 bits in the example I found. So for a while I was casting stuff with (uint8) and getting totally random bits written into the image. That also generated a fun image:

but it took me a few minutes to figure out what was going on. I’m sure some reading would clear things up, but.. who cares? I was also doing some weird float arithmetic to compute color averages. This was just quick/dirty code, and I can read more about the nitty gritty later. As soon as I got the effect I wanted, I rapidly lost interest. I even hard-coded image filenames because I couldn’t be bothered to search for go command-line flag info. All in good fun.
– Arrays and slices are cool, but allocating 2D arrays and slices seems a little verbose.
– I like that Go’s designers have opinions and enforce them, at least 99% of the time. When you’re hacking ugly code, it was annoying to get the “you didn’t use this variable” errors. But I understand the rationale and it’s probably a good idea for writing Real Code that’s not intended to be thrown away.
– I was all set to grouse about go fmt’s enforced indentations/spacing, but it actually looks pretty reasonable. Basically, each indent is a tab. Then if you’re a 3 or 4 space indent kind of guy, you can configure your editor like vim or emacs to change how the tab width is displayed.

Historically, Python is my language of choice to knock out a quick script thing–I love Python dictionaries. But with Go’s speed, support for dictionaries/maps, and capability to do HTTP servers very easily, I might end up switching to Go. I think I’ll use Go for my next little fun project.

Added: Thanks to Tom Madams who whipped a prototype of this filter in video using Shadertoy!