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Contents 1 Image3D file format 1.1 What is i3d 1.2 What can I do with it? 1.3 How do I use it? 2 How to output arbitrary channels to an I3D file? 3 Point Clouds

Image3D file format

What is i3d

i3d (Image3d) is the volumetric file format for Houdini. This format allows you to store arbitrary data in a 3 dimensional array. The supported VEX DataTypes are float, int and vector.

You can also have an arbitrary number of pieces of data called "channels" which you can think of as layers of data.

What can I do with it?

Commonly the i3d format is used to store a volume of density information to be rendered like clouds or dust by VolumetricRendering, but some other uses might include storing VectorFields (by storing vectors instead of scalars in the file) or perhaps lighting data for VolumetricRendering. As mentioned previously, you can store more than one piece of information so you can put density, lighting, colours, etc in the same .i3d file.

How do I use it?

i3ds are generated by the Houdini renderer called "i3dgen", which is available in Outputs as 3DTextureGeneratorROP. i3dgen executes an Image3dSHOP for every voxel which calculates the output data. Compression can be applied during generation to keep the file size down. VEX provides functions for reading these files in the form of commandline like either sample into the 3d texture (texture3d()), functions to integrate across the volume (integrate3d() and integrate3dClip()) and functions to trace across the the 3d image until some condition is met (intersect3d()).

You can visualize the .i3d data in Houdini directly using the IsoSurfaceSOP. Another thing you can do is convert it to regular geometry files using the program i3dconvert. Tutorials

• Here is a tutorial on how to generate i3d files and render them to look like clouds.

How to output arbitrary channels to an I3D file?

Create an Image3D VOP, add to it a 'parameter' tile with the "Export Parameter" toggle enabled and connect the tile's "input" (that appears when you enable the toggle) to something. You might also want to enable the "Invisible" toggle as well, so that it won't show up in the parameters pane when you create a SHOP from it. File:I3d arbitrary vars.png File:I3d arbitrary vars-prompt.png

Point Clouds

i3d files are formatted in a regular grid of cells. Houdini supports an alternate manner of representing volumes of information using PointClouds. PointClouds are best stored in the tbf format for maximum efficiency.