.. _integrator-piecewise_volpath:

Piecewise volumetric path tracer (:monosp:`volpath`)
-------------------------------------------

.. pluginparameters::

 * - max_depth
   - |int|
   - Specifies the longest path depth in the generated output image (where -1
     corresponds to :math:`\infty`). A value of 1 will only render directly
     visible light sources. 2 will lead to single-bounce (direct-only)
     illumination, and so on. (Default: -1)

 * - rr_depth
   - |int|
   - Specifies the minimum path depth, after which the implementation will start
     to use the *russian roulette* path termination criterion. (Default: 5)

 * - hide_emitters
   - |bool|
   - Hide directly visible emitters. (Default: no, i.e. |false|)

This plugin provides a piecewise volumetric path tracer that can be used to
compute approximate solutions of the radiative transfer equation for plane
parallel geometry. Its implementation makes use of multiple importance sampling
to combine BSDF and phase function sampling with direct illumination sampling
strategies. On surfaces, it behaves exactly like the standard path tracer.

This integrator only works with piecewise media. It uses assumptions on the
medium's geometry to accelerate the sampling and transmission calculations.

This integrator has special support for index-matched transmission events (i.e.
surface scattering events that do not change the direction of light). As a
consequence, participating media enclosed by a stencil shape are rendered
considerably more efficiently when this shape has a :ref:`null <bsdf-null>` or
:ref:`thin dielectric <bsdf-thindielectric>` BSDF assigned to it (as compared
to, say, a :ref:`dielectric <bsdf-dielectric>` or :ref:`roughdielectric
<bsdf-roughdielectric>` BSDF).

.. note:: This integrator does not implement good sampling strategies to render
    participating media with a spectrally varying extinction coefficient. For
    these cases, it is better to use the more advanced :ref:`volumetric path
    tracer with spectral MIS <integrator-volpathmis>`, which will produce in a
    significantly less noisy rendered image.

.. warning:: This integrator does not support forward-mode differentiation.

.. tabs::
    .. code-tab::  xml

        <integrator type="piecewise_volpath">
            <integer name="max_depth" value="8"/>
        </integrator>

    .. code-tab:: python

        'type': 'piecewise_volpath',
        'max_depth': 8