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A black hole raytracer written in Haskell

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# Blackstar
A black hole ray tracer written in Haskell. There's [an article](https://flannelhead.github.io/projects/blackstar.html) about this on my homepage. I've also written a [theoretical writeup](https://flannelhead.github.io/posts/2016-03-06-photons-and-black-holes.html) on Schwarzschild geodesics.

![An example image](https://raw.githubusercontent.com/flannelhead/blackstar/master/example.png)

## Features
* Fast, parallel ray tracing
* Rendering [Schwarzschild](https://en.wikipedia.org/wiki/Schwarzschild_metric) black holes
* Rendering accretion disks
* Drawing the celestial sphere using a star catalogue
* Bloom effect
* Antialiasing by 4x supersampling for smoother images
* Easy, YAML based configuration
* A simple CLI
* Batch mode and sequence generator for creating animations

## What about the name?
It is a tribute to David Bowie, referring to his last album.

## Building
Use [`stack`](http://docs.haskellstack.org/en/stable/README/) to build this. First clone the repo, then run `stack build` and follow the instructions given by `stack`. You should be able to build `blackstar` on any platform where you can install `stack`.

You will have to build the star lookup tree first. Download the [PPM star catalog](http://tdc-www.harvard.edu/software/catalogs/ppm.html) [this archive](http://tdc-www.harvard.edu/software/catalogs/ppm.tar.gz) and extract the file `PPM` to the root folder of this project. Then run `stack exec generate-tree PPM stars.kdt` and the tree will be generated and saved to the file `stars.kdt`.

### Speeding it up with LLVM
When doing large or batch renders, it is recommended to build `blackstar` using GHC's LLVM backend. GHC produces LLVM bytecode and LLVM produces fast native code from GHC's output. In my tests I've noticed ~1.5x speedups.

The LLVM backend isn't used by default since one needs to install (and usually build) a specific version of LLVM separately. Moreover, the build time is significantly higher with LLVM, so one doesn't definitely want to use it while hacking on the code.

To successfully build with LLVM, you need to:

* Download and [build](http://llvm.org/docs/GettingStarted.html#getting-started-quickly-a-summary) [LLVM 6.0.1](http://llvm.org/releases/download.html#6.0.1). You can skip the Clang parts. After the build, you should make sure the tools `llc` and `opt` are found in your `PATH`. Notice that these aren't included in the prebuilt LLVM binaries, that's why you'll need to build it.
* Build `blackstar` with `stack build --ghc-options -fllvm`. (If you've just built it, run `stack clean` first to ensure it really gets rebuilt with LLVM.)
* Wait patiently
* Enjoy the result!

You don't necessarily have to use LLVM at all. However, if you can acquire binaries of the right LLVM version, that will give you some speedups.

## Usage
When `blackstar` has been built with `stack`, you can run it with
```
stack exec blackstar -- [-p|--preview] [-f|--force] [-o|--output=PATH] [-s|--starmap=PATH] SCENENAME
```
Notice the two dashes (`--`) which are required to terminate `stack`'s argument list.

`cabal` users can run `blackstar` by executing
```
cabal run -- [OPTIONS] SCENENAME
```
in the root folder of the project.

Scenes are defined using YAML config files. Look in the `scenes` folder for examples. To render the `default` scene to the directory `output`, run
```
stack exec blackstar -- scenes/default.yaml --output output
```
in the root directory of the project. The `--output` flag specifies the output directory. By default, `blackstar` searches for a starmap in the path `./stars.kdt`, but a different path can be specified using the `--starmap` flag.

The rendered files are named `scenename.png` and `scenename-bloomed.png`. The `--preview` flag can be used to render small-sized previews of the scene while adjusting the parameters. The `--force` flag will cause `blackstar` to overwrite output images without a prompt.

If a directory is given as the input scene path, `blackstar` searches non-recursively for YAML files in that directory and tries to render them. The scenes are placed in the specified output directory.

There's also a help text which can be seen by running
```
stack exec blackstar -- --help
OR
cabal run -- --help
```

Better images can be achieved by rendering larger than the target size and then scaling down (some antialiasing is achieved). This is called supersampling and is implemented in `blackstar`. It can be enabled by setting `supersampling` to true in the YAML config file &mdash; see `scenes/default-aa.yaml` for an example.

## Animation
There is a separate YAML config format for specifying animations. For example, see [default-ani.yaml](animations/default-ani.yaml).

In the first pass, the animation file must be rendered into separate config files for each frame. The `animate` executable takes care of this. First, create a directory where the frame config files will be put.
```
mkdir frames
```
Then run `animate`:
```
stack exec animate -- animations/default-ani.yaml -o frames
```
Now you should find quite a bunch of `.yaml` files in the folder `frames`.

Make another folder for the output frames:
```
mkdir frames-out
```
Now you will be able to run `blackstar` in batch mode to render the frames:
```
stack exec blackstar -- frames -o frames-out
```
This will take quite a while.

After the frames have been rendered, generate a video from the `*.png` still with your utility of preference. You can also use my script `scripts/ffmpeg-animate`, which uses `ffmpeg`. You only need to give it the prefix of the numbered frames:
```
scripts/ffmpeg-animate frames-out/default-ani
```
The output video will be rendered to `out.mkv`.

## Profiling
Thanks to `stack`, profiling is incredibly easy. Rebuild `blackstar` by running
```
stack build --profile
```
and then run it with
```
stack exec blackstar -- scenes/default.yaml -o output +RTS -p
```
The profile will be generated to `blackstar.prof`.

## TODO
As always, there's a plenty of room for improvement. For example:

* Animation: mathematically rigorous non-stationary observer
* Arbitrary textures for accretion disk (or some cool noise generator)
* Redshifting of the accretion disk
* Preview / scene planner GUI ([fltkhs](https://hackage.haskell.org/package/fltkhs))

Pull requests are welcome! If you find some cool scenes, I'd appreciate if you contributed them to this repository.

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