Volumes#

This section covers the different types of volume data sources included with Mitsuba. These plug-ins are intended to be used together with the medium plugins and provide three-dimensional spatially varying density and albedo fields.

Constant-valued volume data source (constvolume)#

Parameter	Type	Description	Flags
value	float or spectrum	Specifies the value of the constant volume.	P, ∂

This plugin provides a volume data source that is constant throughout its domain. Depending on how it is used, its value can either be a scalar or a color spectrum.

<medium type="heterogeneous">
    <volume type="constvolume" name="albedo">
        <rgb name="value" value="0.99, 0.8, 0.8"/>
    </volume>

    <!-- shorthand: this will create a 'constvolume' internally -->
    <rgb name="albedo" value="0.99, 0.99, 0.99"/>
</medium>

'type': 'heterogeneous',
'albedo': {
    'type': 'constvolume',
    'value': {
        'type': 'rgb',
        'value': [0.99, 0.8, 0.8]
    }
}

# shorthand: this will create a 'constvolume' internally
'albedo': {
    'type': 'rgb',
    'value': [0.99, 0.8, 0.8]
}

Grid-based volume data source (gridvolume)#

Parameter	Type	Description	Flags
filename	string	Filename of the volume to be loaded
grid	VolumeGrid object	When creating a grid volume at runtime, e.g. from Python or C++, an existing `VolumeGrid` instance can be passed directly rather than loading it from the filesystem with filename.
data	tensor	Tensor array containing the grid data. This parameter can only be specified when building this plugin at runtime from Python or C++ and cannot be specified in the XML scene description. The raw parameter must also be set to true when using a tensor.	P, ∂
filter_type	string	Specifies how voxel values are interpolated. The following options are currently available: `trilinear` (default): perform trilinear interpolation. `nearest`: disable interpolation. In this mode, the plugin performs nearest neighbor lookups of volume values.
wrap_mode	string	Controls the behavior of volume evaluations that fall outside of the \([0, 1]\) range. The following options are currently available: `clamp` (default): clamp coordinates to the edge of the volume. `repeat`: tile the volume infinitely. `mirror`: mirror the volume along its boundaries.
raw	boolean	Should the transformation to the stored color data (e.g. sRGB to linear, spectral upsampling) be disabled? You will want to enable this when working with non-color, 3-channel volume data. Currently, no plugin needs this option to be set to true (Default: false)
to_world	transform	Specifies an optional 4x4 transformation matrix that will be applied to volume coordinates.
accel	boolean	Hardware acceleration features can be used in CUDA mode. These features can cause small differences as hardware interpolation methods typically have a loss of precision (not exactly 32-bit arithmetic). (Default: true)

This class implements access to volume data stored on a 3D grid using a simple binary exchange format (compatible with Mitsuba 0.6). When appropriate, spectral upsampling is applied at loading time to convert RGB values to spectra that can be used in the renderer. We provide a small helper utility to convert OpenVDB files to this format. The format uses a little endian encoding and is specified as follows:

Volume file format#
Position	Content
Bytes 1-3	ASCII Bytes ’V’, ’O’, and ’L’
Byte 4	File format version number (currently 3)
Bytes 5-8	Encoding identified (32-bit integer). Currently, only a value of 1 is supported (float32-based representation)
Bytes 9-12	Number of cells along the X axis (32 bit integer)
Bytes 13-16	Number of cells along the Y axis (32 bit integer)
Bytes 17-20	Number of cells along the Z axis (32 bit integer)
Bytes 21-24	Number of channels (32 bit integer, supported values: 1, 3 or 6)
Bytes 25-48	Axis-aligned bounding box of the data stored in single precision (order: xmin, ymin, zmin, xmax, ymax, zmax)
Bytes 49-*	Binary data of the volume stored in the specified encoding. The data are ordered so that the following C-style indexing operation makes sense after the file has been loaded into memory: `data[((zposyres + ypos)xres + xpos)*channels + chan]` where (xpos, ypos, zpos, chan) denotes the lookup location.

<medium type="heterogeneous">
    <volume type="grid" name="albedo">
        <string name="filename" value="my_volume.vol"/>
    </volume>
</medium>

'type': 'heterogeneous',
'albedo': {
    'type': 'grid',
    'filename': 'my_volume.vol'
}