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A (not too) small physically based volumetric renderer

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SmallUPBP is a small physically based renderer that implements the unified points, beams and paths algorithm described in the paper

Unifying points, beams, and paths in volumetric light transport simulation
Jaroslav Křivánek, Iliyan Georgiev, Toshiya Hachisuka, Petr Vévoda, Martin Šik, Derek Nowrouzezahrai, and Wojciech Jarosz
ACM Transactions on Graphics 33(4) (SIGGRAPH 2014)

as well as a number of other algorithms, notably including progressive photon mapping, (progressive) bidirectional photon mapping, bidirectional path tracing, vertex connection and merging, beam radiance estimate, and photon beams.

The code compiles to a command line program that can render images of a number of predefined scenes using the provided algorithms as well as of custom scenes supplied by a user.

It is based on the SmallVCM renderer developed in 2012 by Tomáš Davidovič and Iliyan Georgiev. SmallUPBP heavily modifies it to add support for participating media, point and beam-based volumetric estimators, and loading scenes in OBJ. Its main authors are Petr Vévoda, Martin Šik and Jaroslav Křivánek.

Two third-party libraries are incorporated. Namely Embree for ray tracing acceleration and OpenEXR for reading and writing OpenEXR images.

SmallUPBP is released under the following license:

This code was used to render the images in the aforementioned paper and all scenes and batch files necessary for reproducing the presented results are provided (see the scenes folder). It is released for educational and research purposes and neither the code nor its documentation claims to be complete and bug-free. We apologize for any errors and confusion.

Thorough description of the renderer, including its usage and implementation details as well as its theoretical background, is given by Petr Vévoda in his thesis

Robust light transport simulation in participating media
Petr Vévoda
Master's thesis. Charles University in Prague, Prague, Czech Republic, February 2015.

Here we present its slightly modified version with minor fixes and improvements. Note that the thesis originally refers to a DVD. Content of the DVD is almost the same as of the archive you can download from this page.

If you have any questions or ideas for improvement, feel free to contact Petr Vévoda ( or Jaroslav Křivánek (


The archive you can download from this page contains following folders and files:

Compilation and run

The source code is divided into two Microsoft Visual Studio 2013 solutions. The first one, OpenEXR.sln in the OpenEXR\src folder, contains single project of the same name with the OpenEXR library. The second one, SmallUPBP.sln in the SmallUPBP folder, contains three projects: embree, sys and SmallUPBP. The first two are parts of Embree and need to be compiled with the OpenEXR project as static libraries and linked with the main SmallUPBP project. The easiest way to do this is to use the provided solutions. Firstly, build the OpenEXR.sln solution. It creates the static library OpenEXR.lib (or OpenEXR-dbg.lib depending on the selected configuration) in the OpenEXR folder. Then build SmallUPBP.sln. It creates the remaining embree.lib and sys.lib static libraries as well as the executable SmallUPBP.exe file in the Build\SmallUPBP\x64\Release folder (or in the Build\SmallUPBP\x64\Debug folder depending on the selected configuration). Note that the build configurations of both solutions must match. The SmallUPBP.exe file is completely independent of any other files and can be moved and run freely (however, the batch files in the scenes folder expect it in the original location).

As mentioned earlier, the resulting executable file is a command line program. Type SmallUPBP.exe -h or SmallUPBP.exe -hf to see a short and full help respectively. The short help contains only basic arguments sufficient to render your first images while the full version lists all arguments the program knows. For more information about the program please see the Petr Vévoda's thesis.