Differences to Mitsuba 0.6

Mitsuba 2 strives to retain scene compatibility with its predecessor Mitsuba 0.6. However, in most other respects, it is a completely new system following a different set of goals. The previous section on variants provided an initial overview of a number of these differences. This section briefly summarizes other relevant changes.

Missing features

A number of Mitsuba 0.6 features are missing in Mitsuba 2. We plan to port some of these features in the future and discard others. The following list provides an overview:

In the following, we list the main missing features:

  • Samplers: only the most basic independent sampler is currently supported. In the future, we plan to reimplement stratified and Quasi-Monte Carlo samplers.

  • Shapes: the basic shapes (PLY/OBJ/Serialized triangle meshes, rectangles, spheres, cylinders) are all supported. However, instancing and assemblies of hair fibers are still missing.

  • Integrators: Only surface and volumetric path tracers are provided. The following are missing:

    • Adjoint particle tracer: to be added.

    • Bidirectional path tracing: to be added.

    • Progressive photon mapping: to be added.

    • Path space Metropolis light transport / Manifold exploration / Energy redistribution path tracing:

      Path-space MCMC techniques were an incredibly complex component of the previous generation of Mitsuba. We do not currently plan to port these and recommend that you stick with Mitsuba 0.6 if your application depends on them.

  • Animation: specification and handling of temporally varying transformations, e.g., for motion blur is not yet fully functional.

  • Diffusion-based subsurface scattering: currently missing, status undecided.

Scene format

Mitsuba 2’s XML scene format is almost identical to that of Mitsuba 0.6. Most plugins have the same name and same parameters, and we made sure that parameters behave in the same way as in Mitsuba 0.6.

One significant change is that Mitsuba 2 uses “underscore_case”-style instead of “camelCase”-style capitalization. This is part of a concerted change to the entire renderer that also touches all C++ and Python interfaces (see the developer guide for reasons behind this). To given an example: a perspective camera definition, which might have looked like the following in Mitsuba 0.6

<sensor type="perspective">
    <string name="fovAxis" value="smaller"/>
    <float name="nearClip" value="10"/>
    <float name="farClip" value="2800"/>
    <float name="focusDistance" value="1000"/>
    <transform name="toWorld">
        <translate x="0" y="0" z="-100"/>

now reads

<sensor type="perspective">
    <string name="fov_axis" value="smaller"/>
    <float name="near_clip" value="10"/>
    <float name="far_clip" value="2800"/>
    <float name="focus_distance" value="1000"/>
    <transform name="to_world">
        <translate value="0, 0, -100"/>

The above snippet also shows an unrelated change: the preferred syntax for specifying positions, translations, etc., was shortened:

<!-- old notation -->
<point name="position" x="0" y="0" z="-100"/>

<!-- new notation -->
<point name="position" value="0, 0, -100"/>

All of these changes can be automated, and Mitsuba performs them internally when it detects a scene with a version number lower than 2.0.0. Invoke the mitsuba binary with the -u parameter if you would like it to write the updated scene description back to disk.