- [❔ other question] How to fill a volume grid with sigma_t values corresponding to a certain material

[vucenovic]

Hello,

In your paper you showed a volume optimization using smoke. I was wondering if there is any code available showing how you setup the scene, especially the .vol file's data to correspond to the material of smoke. I am trying to recreate a similar setup, and have spectral data of my material available, which can be loaded into my scene with the tag like this: <spectrum name="sigma_t" filename="$matobj"/>, where $matobj is my filename. I would like to port this data to my volume grid.

[merlinND]

Hello @vucenovic,

The smoke plume reference data comes from OpenVDB's sample volumes: https://www.openvdb.org/download/
It was then converted to Mitsuba's simple binary format using @dvicini's converter: https://github.com/mitsuba-renderer/mitsuba2-vdb-converter

If you are trying to supply 3D spectral data and rendering it in a volume, I believe you will need to extend the Grid3d plugin to support it. Currently, it can handle either 1 or 3 channels:

mitsuba2/src/textures/grid3d.cpp

Lines 106 to 117 in 858509e

	switch (m_metadata.channel_count) {
	case 1:
	result = m_raw ? (Object *) new Impl<1, true>(m_props, m_metadata, m_data, m_filter_type, m_wrap_mode)
	: (Object *) new Impl<1, false>(m_props, m_metadata, m_data, m_filter_type, m_wrap_mode);
	break;
	case 3:
	result = m_raw ? (Object *) new Impl<3, true>(m_props, m_metadata, m_data, m_filter_type, m_wrap_mode)
	: (Object *) new Impl<3, false>(m_props, m_metadata, m_data, m_filter_type, m_wrap_mode);
	break;
	default:
	Throw("Unsupported channel count: %d (expected 1 or 3)", m_metadata.channel_count);
	}

You will probably need to achieve something like the regular or irregular spectrum plugins, depending on your spectral data.