I'm Morgan McGuire (@CasualEffects). I've been working on computer graphics and games for 20 years at great places including NVIDIA, Williams College, Brown University, and Activision.

See my home page for a full index of my blog posts, books, research, and projects.

Thursday, May 23, 2013

Upsampling Pixel Art with Alpha on OS X

It is easy to find low-resolution pixel art "sprites" for indie games, for example, by browsing the legally-distributed, Creative-Commons-licensed art at OpenGameArt.org. Such sprites are popular partly for their nostalgia factor for 30-something developers, but I think primarily because such low-fidelity graphics are easier both for indies to produce and to match surrounding image quality for. I use such art extensively in my teaching programming samples and jam games.

Unfortunately, in a world of high-resolution displays, 32x32 pixel art sprites are simply too small. Nearest neighbor upsampling retains the pixel quality but often (to my eyes) plays up the lo-fi nature a bit too much.  I prefer to increase sprite resolution using a pseudo-vectorizing filter such as Maxim Stepin's hq4x. It yields results like this:
BeforeAfter

















There are several open source implementations of hq4x available. Finding one that actually builds without a number of dependencies and supports your operating system (mine is OS X when doing artwork) is tricky, however.

I use Nelson's Windows + Linux implementation (which works fine for OS X if you compile from the Makefile) that simply wraps Maxim's code and doesn't introduce any library dependencies.  To install this on OS X:

  1. Download the source from http://www.somebits.com/weblog/tech/hqnx/
  2. Double-click to unzip the code 
  3. Type "sudo make install" in the directory to compile the programs
The resulting script doesn't work correctly because it uses the Linux-specific tempfile command and because the hqnx code doesn't write correct .bmp files out.  I originally started patching this, but found that it was easier to simply invoke the hqx routines directly on .tga files, which it handles fine.  To upsample artwork simply use:

    hq4x source.tga destination.tga

This process does not preserve alpha (transparency). There are some variants that do support alpha, but again, it was easier for me to solve this problem in Photoshop. I paint backgrounds green (0, 255, 0) and then use the free Photoshop Green Screen filter.  You can also roll your own green (or other) screen removal code, since the algorithms were published by Smith and Blinn in 1996.

Here's the process for a spaceship that I downloaded from http://opengameart.org/content/modular-ships and then modified:



If you like this particular space ship, then you can download the set of 12 that I created by the process described here (as Public Domain content) at http://opengameart.org/content/2d-spaceship-sprites-with-engines.

In closing, I'd like to note that there is still a great need for all-in-one pixel art resampling, and providing this would be a great way for a junior game developer or Ph.D. student to gain great exposure. One could provide a simple, artist-ready tool that wraps hq4x and directly supports input alpha, or perform true vectorization for more significant upsampling, say, in the style of Depixelizing Pixel Art.



Morgan McGuire is a professor of Computer Science at Williams College and a professional game developer. He is the author of The Graphics Codex, an essential reference for computer graphics that runs on iPhone, iPad, and iPod Touch.