An ASCII Ray Marcher

All of the code can be found in this github repo.

Introduction

I’ve always found computer graphics intriguing, especially the subfields of ASCII art/animation.

One of the first ‘cool’ things about the command line I discovered was that you could type in

telnet towel.blinkenlights.nl

and watch an entire Star Wars movie through ASCII drawings.

A screenshot from the original website

Another animation that fascinated me was donut.c, which displayed a spinning donut using only ASCII characters (and with donut-shaped C code).

A gif recorded from the author's website

I had previously forayed into the world of ray tracing, but it never really worked out well. However, I soon discovered Ray Marching, a method of generating graphics which can be easily adapted to 3D ASCII animations. There was no way I wasn’t going to try this out.

Ray Marching

The one key ingredient in ray marching is the Signed Distance Function (SDF). An SDF is a mathematical function that calculates the shortest distance from a vector to some 3D surface. Using an SDF, we can march rays forward by the maximum distance before they potentially hit a surface. This is more efficient than ray tracing, which marches all rays at a small, fixed value. Once the SDF for a ray becomes close to 0, we know we have hit a surface. To calculate shading on that surface, we need to know the surface normal, or the vector pointing perpendicular from a surface. With an SDF, this is easy - the surface normal is simply the gradient of the SDF at its surface. This is because the fastest way to increase an SDF at a surface is to travel perpendicular to it. Then, we can convert the shading information into ascii characters to generate a frame. For more information, you can check out this article.

Now, for the results!

A rotating rectangular prism A sphere with a revolving light above it.

Some Technical Stuff

Julia made it very easy to work with math expressions and get good performance out of it too. With stack-allocated arrays and multithreading both an import away, the ray marcher could easily render at 60 frames per second.