227 VGA donut
227 : VGA donut
- Author: Andy Sloane
- Description: Renders a 3D torus on a VGA display
- GitHub repository
- Clock: 48000000 Hz
VGA Donut
How it works
Renders a faceted donut to a VGA monitor.
Like my other demo on tt08, this runs in a weird VGA resolution: 1220x480, but still 4:3 aspect ratio like 640x480.
Interestingly, it is not actually rendering any polygons; this is sphere traced (AKA raymarched), using a CORDIC unit to calculate the distance between a point and the surface of the torus. But, because we don't have much time (we're racing the VGA beam!), we do just two or three CORDIC iterations, which causes the donut surface to actually become polyhedral. This trick was accidentally discovered by Bruno Levy while playing with a C version of my original donut code and I had to try it out in Verilog -- so here we are.
The reason it has such low horizontal resolution is because it's doing 16 ray marching steps per "pixel", with five CORDIC iterations unrolled into one clock cycle (three iterations for the major axis, and two for the minor axis).
In order to fit this into 2x2 TinyTapeout tiles, a lot of sacrifices were made; for one, it doesn't have a multiplier so the ray steps are by approximate orders of magnitude. New donut "pixels" are rendered every 16 clock cycles, so the demo makes heavy use of dithering in both space and time -- the video looks much better than the screenshot above.
How to test
Connect VGA Pmod to output, set clock to 48MHz, and give it a reset pulse.
External hardware
TinyVGA Pmod for video on o[7:0].
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | R1 | ||
| 1 | G1 | ||
| 2 | B1 | ||
| 3 | VSync | ||
| 4 | R0 | ||
| 5 | G0 | ||
| 6 | B0 | ||
| 7 | HSync |