A raytracer written in C++, with features not unlike those of /usr/bin/yes
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README.md

tRayce

Introduction

tRayce is a basic path tracer (ex-raytracer and photon mapper) written in C++, inspired by various articles on the Internet (such as this one).

It supports basically nothing, including, but not limited to:

  • Not having a scene definition language, unless we call C++ a scene definition language.
  • Not supporting primitives other than spheres, planes and axis-aligned boxes.
  • Not using CUDA/SSE.
  • Not being able to export the rendered image into anything but an uncompressed BMP file.
  • Not supporting textures, procedural or not, unless we call a plain colour a texture.
  • Not being realtime, unless we call 30 FPS at 160x100 on a 2.1 GHz dualcore realtime.
  • Not having a good enough photon mapping (it kind of casts them and stores them, but it only looks nice with a final gathering step with horrendously large amounts of final gather rays, which is rather slow since I don't do any irradiance caching (at the time of writing, it's been rendering a 320x240 scene with 175000 photons, 500 photons used in an irradiance estimate and 512 final gather rays per pixel) for about a couple hours now)
  • Not being a raytracer or a photonmapper anymore, the raytracer and photonmapper-enabled code can be found in the with-pt-and-rt branch.

Compilation

make (or Visual Studio)

I wrote tRayce on a Windows system and then managed to compile it on Linux without any major changes (except for rewriting the bitmap export code). The Makefile in the root folder seems to work, though I haven't tested it on anything but my system.

You do need a compiler that supports the C++11 standard.

Usage

There is no scene definition language as of yet: the scene is defined in the source code and compiled with the pathtracer. The sample scene already in main.cpp essentially showcases everything tRayce can do for now.

Some features that tRayce does support:

  • Adjusting the camera: Scene.camera sets up parameters such the dimensions of the image plane the rays would be cast through (Scene::camera.height and width), the focus distance (Scene::camera.planeDistance), the position (Scene::camera.position) and look-at direction (Scene::camera.direction), as well as some depth-of-field effects for the pathtracer (Scene::camera.lensRadius and Scene::camera.focalDistance)

  • Simple post-processing: Scene::render takes a function that is applied to every pixel in the resultant bitmap. It is given the colour of the pixel (unnormalised, 0..infinity) and the depth, which allows for some effects such as light falloff or playing around with colour intensities.

  • Multithreading!

  • Spheres! Triangles! Planes! Axis-aligned bounding boxes!

  • Importing OBJ files (only triangles so far, but can also import and interpolate their normals, as well as texture UV coordinates)

  • Textures! A BMP image can be used as a texture (Material::isTextured, Material::texture), so far, only triangles support texture loading and UV interpolation

  • Specific to raytracing:

    • Soft shadows (Scene::softShadowSamples): when using area lights, instead of placing only one point light, approximates the area light with several point lights, each contributing a fraction of the total shading, resulting in soft shadows.
  • Photon mapping: Not in the main branch temporarily (?) After me skimming through dozens of various articles and papers, it kind of works and looks nice and even uses a kd-tree to store photons and perform nearest-neighbour queries in logarithmic time. What bothers me is that it doesn't actually look logarithmic, since it appears that the amount of kd-tree nodes visited per pixel grows linearly with the number of stored photons. I believe it has something to do with inefficient median splits during the construction (since the tree is balanced at build-time and the lookup routine has to access both branches of the tree fairly often, meaning that the median is closer to the target point than our current best and we are not sure there isn't anything better in the other branch). Also, there is something wrong with the brightness of the image: using classic raytracing results in much brighter images (by a factor of a thousand) than the same images rendered using a photon map. Final gathering is quite slow (which is expected) and sometimes has weird artifacts (like white pixels that could not have appeared from anywhere).

    Irradiance "caching" is now supported: instead of computing irradiance every time during the final gather, the mapper now looks for the nearest irradiance photon (a photon on the position of which the radiance estimate has already been performed). This considerably speeds things up. It also looks like a cool Voronoi diagram when rendered directly.

    Using Monte Carlo integration with stratified samples and a Mersenne Twister RNG, I finally managed to achieve an almost noiseless 320x240 image with 50000 initial photons, 500 photons used for the irradiance estimate and 64x64 final gather samples. The image was rendered in ~2 hours. That seems much slower than what is claimed in the papers (on their rather ancient hardware)

    Halton sequences, which provide a low-discrepancy sequence that sort of looks like stratified random sampling, are also implemented. Setting Scene::samplingMode = HALTON enables them, with photonGatherSamples now being the number of elements in the sequence that are used (unlike the STRATIFIED mode that uses the square of that number of samples). With low amounts of samples, it looks quite bad, with blobs of light smeared all over the objects, but with ~1000 it begins to fade away and only be noticeable on close inspection.

    Photon visualisation is also supported, but that's mostly for confirming they are landing in the right areas.

    Photon maps can be saved and loaded, but they are not checked for validity during load time.

  • Pathtracing:

    • Where main development efforts are focused right now
    • pathTracingSamplesPerPixel does what it says on the tin. pathTracingMaxDepth limits the trace depth. Note that when a ray goes through an object, that's 2 "bounces": when it comes in and when it comes out.
    • Camera::focalDistance determines the position of the focal plane, whereas Camera::lensRadius determines the radius of the circle from which the ray will be cast through the focal plane. This simulates the depth-of-field effect.
    • Can get some noisy caustics!

TODO

  • Speedups:
    • Photon mapping:
      • irradiance caching; figure out why I need so many final gather samples. Also, there is a nice paper called Balancing Considered Harmful that proposes a different way of organising the kd-tree to avoid unnecessary visits to the other branch. Another way is proposed in the paper called It’s okay to be skinny, if your friends are fat.
      • somehow is very different from a pathtracing reference render (places that are supposed to be dark are too bright etc)
    • kd-tree for triangles has buggy lookups, so I need to check the bounding boxes of both branches.
  • Improve the pathtracer: bidirectional path tracing, then probably move on to Metropolis Light Transport

References