Subpixel Rendering For Impossibly Small Terminal Text

When it comes to text, how small is too small? The experts say a six point font is the minimum for readability, but as [James Bowman] shows us, you can get away with half of that. 

The goal is to produce a 40-character display on a 24 mm x 24 mm LCD that has a resolution of 240 x 240 to show a serial terminal (or other data) on the “TermDriver2” USB-to-Serial adapter. With 24 lines, that’s a line per millimeter: very small text. Three points, to be precise, half what the experts say you need. Diving this up into 40 columns gives a character cell of six by nine pixels. Is it enough?

Not by itself, no. That’s where the hack comes in: sub-pixel rendering. After all, a “white” pixel on an LCD is actually three elements: a red, a green, and a blue subpixel, stacked side-by-each. Drive each of those subpixels independently and 240 pixels now becomes 720. That’s plenty for a 40 column terminal.

The article discusses how, in general terms, they pulled off the subpixel rendering and kept the font as legible as possible. We think it’s a good try, though the colored fringe around the characters can be uncomfortable to look at for some people — and then we can’t forget the physical size of the characters being 1 mm tall.

If this trick were being used on a larger display with a 240-wide resolution, we’d say “yes, very legible, good job!”– but at this size? We hope we can find our reading glasses. Still, it’s a neat trick to have in your back pocket for driving low-resolution LCDs.

It may not surprise you that aside from improving legibility, subpixel rendering is also used for pixel (er, sub-pixel) art.