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BLOSPRAY - OSPRay as a Blender render engine

Blender scene, with proxy object for the volume Final render by OSPRay through BLOSPRAY

BLOSPRAY aims to provide high-quality rendering of scientific data in Blender 2.8, with a specific focus on volumetric data and use in an HPC (High-Performance Computing) context. To accomplish this BLOSPRAY integrates the OSPRay ray tracing engine from Intel in Blender as an external renderer.

See here for some example renders.

Currently, this software is somewhere between alpha and beta quality, as it can already be used to produce useful visualizations, but lots of things are still incomplete and/or missing. Also, not all errors are robustly and gracefully handled yet.

In case you give it a try and/or have remarks or questions, let us know at visualization@surfsara.nl! Bugs and feature requests can be entered in this github project.

Goals

  • Flexible and high-quality rendering of large scientific datasets using OSPRay from Blender
  • Provide a way to add user-specific types of data as scene elements without having to import that data into Blender
  • Make OSPRay features available to Blender users

NON-goals:

  • Compete with Cycles (the built-in renderer of Blender). Cycles has a different focus, as it provides production rendering of animations and stills, aimed (more or less) at artistic workloads. Instead, BLOSPRAY focuses on rendering scenes containing (large) scientific datasets, where efficient handling and rendering of the data is usually a more important goal and challenge than production of photo-realistic imagery.

Features & limitations

BLOSPRAY is still in its early stages of development, but the following basic functionality and integration with Blender features is already available, with certain limitations :)

Features (and known issues)

Please check this list before reporting a bug for a known issue

  • Scene elements
    • Polygonal geometry, i.e. Blender meshes
    • Subdivision surfaces: these are handled by Blender's dependency graph mechanism, which triangulates the geometry. This in contrast to passing the subdivision control mesh to OSPRay (which also has subdiv support).
    • Currently no specific support for other non-polygonal objects (e.g. text, curves, ...) is present. If these objects render correctly it's due to the dependency graph mechanism
    • Object transformations and parenting
    • Instancing, mostly for smaller number of instances. For large numbers of instances there currently is quite a high overhead in terms of scene setup and memory usage
    • Point, sun, spot and area lights. There's no support for OSPRay's HDRI light yet
    • Vertex colors on Blender meshes
    • Node-based material editing for a subset of OSPRay materials, using nodes from the OSPRay sub-menu. Note that connecting nodes to input ports, like a color, will not work as expected when a connection is made (and there is no way to hide the sockets to signal this). Also, although most of the Cycles nodes are still available in the shader editor almost NONE of those nodes will work.
    • Modifiers that work on mesh geometry should work, as they get handled by Blender's dependency graph before being passed to BLOSPRAY
    • Volume and isosurfaces rendering (see tests/volren.blend)
  • Rendering
    • Supports OSPRay's SciVis and Path Tracer renderers, and most of their settings
    • Final render (F12), including canceling
    • Interactive preview render works with respect to viewpoint changes, but most other scene changes will NOT update in the rendered view. Switching out and back into rendered view should sync to the updated scene
    • The view will match in orthographic (top/left/...) and "free" 3D view mode, but it is currently incorrect when viewing through a camera
    • Border render (i.e. render only part of an image) currently only works for final render, not for interactive render
    • Simultaneous rendering modes in Blender are not supported. E.g. multiple 3D views each in interactive rendering mode will not work as the render server does not support this
    • Material/light/... preview rendering does not work
  • Rudimentary support for BLOSPRAY-specific volume, geometry and scene plugins
  • Rudimentary transfer function editing for volume data (by mis-using the ColorRamp node)
  • Camera
    • Perspective and orthographic cameras, plus OSPRay's panoramic camera. The latter is similar to Cycles' equirectangular camera, but without any parameters to tweak
    • Depth of field
  • Blender UI
    • Integration within UI, mostly panels for editing properties and such, is not very advanced
    • The addon provides some UI panels to set OSPRay specific settings, but in other cases we use Blender's custom properties to pass information to OSPRay. These can even be animated, with certain limitations, but are not a long-term solution.
    • Note also that some builtin UI panels are disabled when the render engine is set to OSPRay as those panels can't directly be used with OSPRay (e.g. they contain Cycles-specific settings)
  • This project is developed for Blender 2.8x, the latest stable release. Blender 2.7x is not supported

Known to be missing and/or buggy:

  • When scaling a volume much smaller it appears that the rendered volume is "thinned out", i.e. there's much less opacity accumulating See ospray/ospray#380
  • Slice rendering on volumes is not working yet
  • In interactive render mode when showing a camera view the rendered view does not match the scene
  • Texturing is not handled
  • HDRI lighting is not available
  • Motion blur is not available
    • This is not supported by OSPRay itself, but could be added on the BLOSPRAY side
  • Parallel rendering mode through MPI is supported by OSPRay, but not used in BLOSPRAY yet
  • Many errors that can happen during scene sync between Blender and the render server are not caught and/or not reported correctly. Restarting either the render server and/or Blender might be needed in these cases.
  • In many cases only a subset of OSPRay parameters can be set from Blender, either using UI elements or using custom properties
  • Scene management on the render server is not optimal yet. I.e. memory usage might increase after each render.
  • Only a few OSPRay materials can be set through the shader editor. They also don't work on all types of geometry yet.
  • Command-line (batch) rendering isn't supported in a nice way yet, as the lifetime of the BLOSPRAY server needs to be managed manually, although this isn't a big issue.
  • Volumes can only use point-based values, not cell-based values (XXX this is a limitation of the volume_raw plugin)
  • All Blender meshes are converted to triangle meshes before being passed to BLOSPRAY, even though OSPRay also supports quad meshes. This is partly due to the way the new Blender depsgraph export works.
  • Tracking identity of Blender scene objects in a robust way is a challenge, as an object's name is about the only thing available for this. Therefore, renaming scene objects while a render is ongoing might trigger funny effects.
  • The upcoming Blender 2.81 includes a new Denoise compositor node that is baed on Intel's OpenImageDenoise, but BLOSPRAY does not provide the needed extra render passes yet (albedo and normal)

OSPRay (more specifically 2.0.x alpha) also has some limitations:

  • OSPRay's SciVis and Path Tracer renderer do not have the same set of features and performance characteristics:

    • Only the Path Tracer supports advanced materials, like the principled material
    • The lighting in the SciVis renderer is very basic, limited only to ambient occlussion
    • Volume rendering differs between the two renderers: the SciVis renderer only supports simple ray-marching, while the path tracer XXX
  • Volumes are limited in their size, due to the relevant ISPC-based code being built in 32-bit mode. See this issue.

  • Unstructured volumes can only contain float values (i.e. not integers). See here

  • Intersecting/overlapping volumes are not (always?) rendered correctly

  • Lights generated in a scene plugin (see below) can not be transformed currently on the OSPRay side.

  • OSPRay meshes are either pure-triangle, pure-quad or subdivision meshes. The latter can mix polygons of different numbers of vertices, but might suffer from being less efficient (XXX although this needs more testing).

  • The OSPRay orthographic camera does not support depth-of-field

  • Blender supports multiple colors per vertex (basically one per vertex per face loop), while OSPRay only supports a single value per vertex. During export the vertex colors are reduced to a single color per vertex

  • OSPRay does not support motion blur through its API, although the underlying Embree library does

  • All rendering is done on the CPU, because, well ... it's OSPRay ;-)

Workflow

The overall idea is to use Blender to set up the scene to render in the usual way. Regular Blender meshes are handled mostly correct, although Blender's Cycles and EEVEE materials are currently not exported. A subset of the materials supported by OSPRay is available, though.

To make use of (BL)OSPRAY-specific features a "plugin" can be enabled on a normal mesh object. This plugin generates OSPRay-specific scene elements that are handled independently from Blender. Examples of such elements are volumetric data or point sets.

There are currently 3 types of plugins in BLOSPRAY:

  • Geometry plugins: these generate a single OSPGeometry, such as OSPRay's "spheres" "streamlines" or an "isosurface".

  • Volume plugins: these load (or generate) volumetric data in the form of an OSPVolume. Note that OSPRay supports regular volumetric grids, but also ununstructured grids and AMR meshes.

  • Scene plugins: these can generate any set of OSPInstance elements, plus zero or more lights.

Meshes that have a plugin attached can be transformed in the Blender scene in the usual way. Meshes with a volume plugin have a few more options specific to volumes:

  • Note: the two options below are currently broken in 0.1!

  • One or more isosurfaces can be rendered through the volume

  • Slicing through the volume data with a separate set of slicing meshes. These meshes must be attached as children to the mesh data that has the volume plugin attached

All types of plugins can provide a proxy mesh, such as a bounding box or simplified mesh, to be used as proxy in the Blender scene.

See the Plugin section below for more details.

Render server

BLOSPRAY consists of two parts:

  1. A Blender addon (directory render_ospray) that implements the Blender external render engine. It handles scene export to OSPRay, retrieving the rendered image from OSPRay and other things.
  2. A standalone render server (bin/blserver after building) that receives the scene from Blender, calls OSPRay routines to do the actual rendering and sends back the image result.

The original reason for this two-part setup is that there currently is no Python API for OSPRay, so direct integration in Blender is not straightforward. Neither is there a command-line OSPRay utility that takes as input a scene description in some format and outputs a rendered image.

Plus, the client-server setup also has some advantages:

  • The separate render server can be run on a remote system, for example an HPC system that holds a large scientific dataset to be rendered. This offloads most of the compute-intensive rendering workload and memory/storage requirements of the data to be rendered away from the (desktop) system running Blender.

  • It makes it feasible to use OSPRay's Parallel Rendering with MPI mode, by providing a variant of the render server as an MPI program. Again, this parallel version of the server can be run remotely on an HPC system. Note that this mode is currently not implemented yet.

  • The network protocol is currently not strongly tied to Blender, so the render server can be used in other contexts as well.

  • BLOSPRAY development becomes slightly easier as both Blender and the render server can be independently restarted in case of crashes or bugs.

Of course, this client-server setup does introduce some overhead, in terms of network latency and data (de)serialization and transfer. But in practice this overhead is small compared to actual render times. Caching of data on the server between renders helps reducing the overhead even further. However, for scenes with thousands of objects, or where objects are instanced in large numbers the network overhead will become noticeable (and sometimes even prohibitive).

Note that the render server currently doesn't support loading multiple different Blender scenes or serving different users at the same time. There is also a manual action required to start/stop the render server.

Plugins

There is a rudimentary plugin system that can be used to set up custom scene elements that are represented by a proxy object in Blender, but whose full representation and rendering is stored on the server side in OSPRay. The original use case for plugins (and even BLOSPRAY itself) was to make it easy to use OSPRay's high-quality volume rendering of scientific data in a Blender scene. Blender's own volume rendering support is geared towards the built-in smoke and fire simulations and isn't really a good fit for scientific datasets. Plus it is very hard to get more advanced volume data (e.g. AMR meshes) into Blender.

Apart from volumes, plugins can be used for other types of scene content as well, like polygonal geometry or OSPRay's builtin geometry types like spheres and streamlines. Basically any scene element that can be created with the OSPRay API can potentially be used during rendering through plugins.

The plugin system is especially useful when working with large scientific datasets for which it is infeasible or unattractive to load into Blender. Instead, one can use a proxy object, such as a cube mesh to represent a rectangular volume, in a Blender scene and attach a plugin to it. During rendering BLOSPRAY will call the plugin on the server to load the actual scene data associated with the proxy. In this way Blender scene creation, including camera animation and lighting, can be done in the usual way as the proxy object shows the bounding box of the data and can even be transformed and animated.

A nice benefit of writing a plugin in C++ is that it allows one to load a large dataset directly into memory, without having to go through the Blender Python API in order to accomplish the same. The latter is usually less efficient.

Note that BLOSPRAY plugins are different from OSPRay's own Extensions, that are also loadable at run-time. The latter are meant for extending OSPRay itself with, for example, a new geometry type. BLOSPRAY plugins serve to extend Blender with new types of scene elements that are then rendered in OSPRay.

Dependencies

For building:

For the plugins:

For running the BLOSPRAY addon in Blender:

  • Numpy, which must available in Blender (try import numpy in a Python console area)
  • Google protobuf (Python modules), see Installation below

Building

Note: BLOSPRAY is only being developed on Linux at the moment, on other platforms it might only work after some code tweaks.

BLOSPRAY uses CMake for building in the usual way. There's currently only a very small set of BLOSPRAY specific switches that can be tweaked.

The current way to build BLOSPRAY is to make a build directory within the source directory and build there. A make install will then place all files (server, plugins, test files) in a bin subdirectory of the sources:

$ cd blospray
$ mkdir build
$ cd build
$ cmake -GNinja ..
# Configure, Generate
$ ninja install
$ cd ../bin
$ ls
blender-ospray-engine$ ls bin
blserver               geometry_triangles.so        libfaker.so                      scene_rbc.so
geometry_assimp.so     geometry_vtk_streamlines.so  plugin.h                         t_json
geometry_hyg_stars.so  libblospray.so               scene_boxes.so                   volume_disney_cloud.so
geometry_plane.so      libblospray.so.0.1           scene_cornellbox.so              volume_hdf5.so
geometry_ply.so        libblospray.so.1             scene_gravity_spheres_volume.so  volume_raw.so

Installation

Part of BLOSPRAY consists of a Blender add-on, but this is not being distributed separately currently, as the development focus is getting to a releasable state in terms of features.

Currently, the way to install the add-on is to clone this repository and then make a symlink to the render_ospray directory in the Blender addon directory. It depends on the installation path of Blender itself what is the best way to add this symlink:

  1. You have Blender installed in a system-wide directory that is not writeable (or you prefer to not alter the system-wide installation)

    In this case the user-specific directory can be used. On Linux this is $HOME/.config/blender/2.81 (see here for the location on other platforms).

     $ cd $HOME/.config/blender/2.81
     
     # Create addons directory, if needed
     $ mkdir -p scripts/addons           
     
     $ cd scripts/addons
     
     # Create symlink to blospray addon
     $ ln -sf <blospray-repo>/render_ospray render_ospray
    
  2. You have Blender installed in a user-writable location. In this case you can create the symlink directly in the installation location:

    $ cd <blender-2.81>/2.81/scripts/addons
    $ ln -sf <blospray-repo>/render_ospray render_ospray
    

In most cases Blender needs an extra Python module for the protobuf dependency. This is most easily done using PIP and Blender's included Python interpreter:

$ <blender-2.81>/2.81/python/bin/python3.7m -m ensurepip --user
$ <blender-2.81>/2.81/python/bin/python3.7m -m pip install -U protobuf --user

Finally, enable the Render: OSPRay add-on in Blender (Edit -> Preferences -> Add-ons).

You should now have a new OSPRay entry in the Render Engine dropdown on the Render properties tab. If not, check for error messages in the console window where Blender is running.

Usage

  1. Start the server (./bin/blserver and keep it running)
  2. Start Blender
  3. Set the Render Engine to OSPRay

Licenses

BLOSPRAY is licensed under the Apache License, version 2.0 (the same license as OSPRay). See LICENSE-2.0.txt for more information.

BLOSPRAY uses the JSON for Modern C++ header-only library, which is included in this source distribution as core/json.hpp. This file is not part of BLOSPRAY and has a different license, see its contents.

Some files, mostly separate test files, are not part of BLOSPRAY itself. These files have license information included in their text, where appropriate.

The file tests/teapot.obj is based on the Teapot model from the McGuire Computer Graphics Archive. The separate meshes in the file were combined into a single one.

The file tests/ss002.128_3.bin.vtk is derived from the Cosmogrid 512^3 simulation by Ishiyama et al. available at https://repository.surfsara.nl/datasets/cosmogrid/648. One shapshot of the original particle data was mapped to a density volume (log10 of 1+#particles) at 128^3. The original data is covered by an MIT license for which the following holds:

Copyright 2011 Tomoaki Ishiyama, Steven Rieder, Junichiro Makino, Simon Portegies Zwart, Derek Groen, Keigo Nitadori, Cees de Laat, Stephen McMillan, Kei Hiraki, Stefan Harfst

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.