bdaniel7 / fsharp-pov

A WIP FSharp DSL for creating POV scenes

FSharp DSL for POV

Persistence of Vision is a raytracer that has been around for 30 years, producing images or animations from a text based scene language.

This is my entry in the F# advent calendar 2021, and is proof of concept of writing a DSL for FSharp to produce POV scene files. I've never tried using the custom operation features of computation expressions, and wanted to have a play after seeing how much the likes of Farmer can achieve.

I would have liked to achieved much more, but I work in the health IT sector, and the Covid Omicron situation has meant work was much busier than expected.

An simple example POV scene is below:

camera {
    location <0,0,0>
    look_at <0,0,10>
}

// Yellow ball
sphere {
    <-6, 0, 20>, 5
    pigment {
        color rgb <0.99, 0.83, 0.40> 
    }
}

There are some great example POV scenes files and resultant images in this repo.

Using my hacked together computation expressions, the scene above can be writen in F# as

render [
  camera {
    location (0,0,0)
    look_at (0,0,10)
  }

  // Yellow ball
  sphere {
    location (-6,0,20)
    radius 5
    pigment (Rgb(0.9,0,0.06))
  }
]

The POV language has macros and lots of features, I've only done the bare minimum to get a simple scene represents in F# and rendered. In particular I was interested in exploring how the following new features of F# 6 helped in writing the DSL:

• Implict conversions
• As patterns
• Custom operator overloads

The implicit conversions were really helpful in not having to worry about overloads for ints vs floats, very nice !.

I also used marker and optional interfaces on F# records which worked out well.

Code

It's all in one script file with a couple of different modules definited:

• Model is the core POV types + a couple of interfaces
• Builders is the computation expressions for pov types
• Serialisation is responsbile for converting F# types to the POV scene language, it's pretty hacked together
• Povrary just shells the pov executable

Windows users can download the POV binaries, Mac users can brew install pov. I've only tested on a Mac, but Windows should work fine if the binary is found on the path.

It can be run with dotnet fsi Example.fsx. It will render an example scene (see image below)

TODO:

• Some sort of Christmasy scene 🎄
• Add lots more objects
• Add common povray command line arguments like image size, anti-aliasing, animations
• have a pigment builder that works inside other builders (if possible), supports textures, etc
• tidy up the serilisation overrides