After my initial foray into graphics I wanted to see what was possible with the Atmega328p and the SSD1306 screen. Lots of applications which use this little microcontroller involve pretty mundane, uninspiring things like home automation and I like the idea of giving it a wee chance to shine and do something fun. The constraints of a relatively slow processor, extremely low memory, as well as a tiny and monochrome display were pretty enticing to me, especially after reading some of the early stuff on folklore.org on the hacks Bill Atkinson, Andy Hertzfeld et al employed to get most of out the Macintosh's 68000 processor.
Firstly we need to know the limits of the hardware. I wasn't sure whether the processor, the SPI bus or something inside the display itself would limit the frame rate to something unacceptable and put the kibosh on any sort of fast/responsive visualisations. So I whipped up a quick test which sent a bunch of empty frames to the display and every so often output the average frame rate.
Mercifully it seems that we're able to push as much as 207fps to the SSD1306, which means that we've got a fair bit of room to play with and that if anything the Atmega328p is the bottleneck.
My first attempt was a little clone of breakout. I am apparently hopeless at it - but it's actually the first "game" I'd ever written, so that's fun.
Since the Breakout clone worked smoothly, wanted to see if it was possible to produce simple 3D graphics - eventually settling on creating a spinning cube bouncing around the screen.
Initially I got a little too ambitious and attempted to roll my own general purpose library for 3D graphics - with a set of functions to manipulate a stack of transformation matrices. However I kept on blowing through my stack space and clobbering a load of program state. This makes sense, since if we have a matrix implementation of 4x4 floats, we'd need to create one for the camera, a rotation and translation then that's already 3 matrices of 64 bytes each. Then we have to multiply the matrices and apply this to 8 vertices you can easily see how a naive implementation could at some point go a bit haywire and chew through more stack than it should.
Instead of taking the time to trim this down using the limited debug tools available to me (no stepping, watch, gdb through my simple USB cable) I just manually calculated the transformations I needed for my rotating/bouncing cube ahead of time and implemented them so they could be parameterised. So starting with the following (with Tx, Ty & Tz representing the center position of the cube, θ being angle of rotation and x, y and z being the co-ordinates of each cube)
Which means that each point can be represented by the following vector
Since I just wanted a rotating cube with wireframe lines I didn't need to do much other than use this to calculate the expected x/y positions of each point on screen and then use existing drawLine() function to connect each. Which resulted in this slightly clumsy function:
The final video involved two rotating cubes, one of which is flying around the screen.
It's quite satisfying result as it's the first time I'd produced a from-scratch 3D projection since university, and was actually surprisingly fast - the video looks a little shakey but in person it's as smooth as butter.
... later that day
And after all that hard work, I saw some stuff that left me pretty deflated - two excellent implementations of graphics libraries for AVR microcontrollers. The first one is u8g - a slightly more heavy duty library than the one Adafruit provided. It also provides some nicer font handling ... including a tiny font similar to the one I did. Here's an example of someone using it to draw a rotating cube at a reasonable clip, ~40 fps no less:
The second one is even more impressive and needs to be seen to be believed. Someone managed to create a library to render a 16 bit colour scene in 3D with texturing and lighting.
Be sure to check out the video at the bottom of this link, it's a little humbling after you've just thrown together a couple of simple monochrome visualisations - http://hackaday.com/2016/01/02/better-3d-graphics-on-the-arduino