C64 Programming

Perhaps this also falls under "Raytracing / Image Synthesis", but I decided it deserved its own section. :-) I have also started work on a raytracer that can run on a C64. If you don't know a lot about raytracing, it may surprise you that it requires very little memory to calculate a very interesting image. In fact, it is possible to use procedural objects and textures to avoid large data sets. Since I haven't seen a "real" raytracer on the C64, I decided to write one.

The C64 has an 8-bit 6510 processor (same instruction set as the 6502). It runs at just under 1Mhz!! I thought that I might be able to write the raytracer in C64 Basic, but I scrapped that idea because the raytracer would be extremely slow, and C64 Basic doesn't even have functions! It would be very difficult to write a raytracer in the language even if speed wasn't an issue. So, I decided to write the raytracer using cc65 (an almost C compiler).

My next problem was figuring out how to perform floating point math operations. The cc65 compiler doesn't support any floating point number formats. So, I had to perform these operations without the compiler. Also, the 6502 doesn't handle floating point numbers like most current processors. In fact, the processor doesn't even have a multiplication instruction. But, the Basic language does support floating point numbers. So, I wrote functions in C/Assembly that "stuff" the areas in memory that Basic uses as floating point registers. Then I call the Basic ROM routines to calculate the result for me.

Initially my program just rasterized a sphere (i.e. no rays were cast). As you can see, I must dither the image so I can display it on the C64. It takes almost 30min for the sphere to render on a C64! Luckily I can run the program much faster on my PC using a C64 emulator. I am also able to run the raytracer natively on my PC. I placed all the machine specific code in different modules, so I am able to compile the raytracer for the C64 or under Linux.

Here is a much newer image I generated. This time I am actually ray tracing instead of rasterizing the image. The raytracer now supports two types of primitives; spheres and planes. The raytracer also supports different material types. At the moment it supports a base material that contains diffuse colour, specular colour, and reflectivity attributes. I also have a checkerboard material that is almost the same as the base material, but it has two different diffuse colours.

Here you can see a true-colour version of the scene show above. I have modified the raytracer so it writes images out to disk as true colour PPM files. The image here was actally rendered on the c64. I didn't precisely time how long this image took to render, but I think it would take about five hours to render on a c64. My 800Mhz Athlon renders the same image in about two seconds!

Here is another image showing the latest features of my raytracer. Now, the raytracer supports shadows properly. I also changed the scene I was raytracing a bit (so that the FOV looks better).