In this tutorial, we will go through how to set up and use the Volume Sample Float shader to adjust the color temperature effect of an OpenVDB file.

A tutorial on how to write volumes to OpenVDB from Houdini can be found here.

 

  • Start off by downloading a VDB file such as the explosion.vdb or fire.vbd file, for example, from the OpenVDB download page.
  • Create an Arnold Volume - Arnold> Volume
  • Select the Volume and ensure that the Type is set to OpenVDB.

  • Under the Filename attribute, open the .vdb file.

 

Arnold volume settings. Grids set to 'density'.

 

Standard Volume

  • Render the scene. It should look like the following image.

 

Volume Sample Float

We can use a Volume Sample Float shader to control the appearance of our fire volume effect. This shader will remap the values of the 3D voxel grid in the same manner as tone-mapping and remapping filters to an image in Photoshop, which operates in a 2D grid.

  • Break the connection of the 2D texture node to the UV Coordinates in the ramp texture attributes near the bottom. 
  • Create a Volume Sample Float shader and drag and drop it onto the U attribute of the UV Coord as indicated in the image below.

Volume Sample Float shader connected to Uv Coord (U) attribute

 

  • The Connection Editor will open. Connect the outValue of the Volume Sample Float to the uCoord of the ramp.

 

 

  • Select the Volume Sample Float shader and add density to the Channel attribute.

 

Emission

Volume Sample Float Attributes

We can use the Volume Sample Float shader to alter the appearance of the fire. The following images show different adjustment attributes and their effect on the fire volume.

Input Max

Bias

Gain

Output Max

 

 

 

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