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Arnold for Houdini User Guide > OpenVDB Workflow > image2015-7-29 12:0:20.png

In this tutorial we will go through how to setup and use the Volume Sample Float shader to adjust the color temperature effect of an OpenVDB file, using the temperature and density VDB grids.

The scene file can be downloaded here.

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

Start off by downloading this VDB file which contains an explosion with VDB grids for density and temperature. You can find more VDB files available on the OpenVDB download page.
Create a Volume from the Arnold shelf.
Select the Volume and select the Volume Tab. Make sure the Type is set to OpenVDB.
Under the Filename attribute, open the .vdb file.
Enter 'density' and 'temperature' into the Grids field.

Arnold for Houdini User Guide > OpenVDB Workflow > image2015-7-29 12:26:36.png

^{_{Arnold volume settings. Grids set to 'density' and 'temperature'.}}

Standard Volume

Create an Arnold Shader Network. Inside that, create a Standard Volume shader and connect it to Volume of the OUT_material.
Change the Emission Mode to Density and the Emission Channel to temperature as this is the grid that we want to change.

Arnold for Houdini User Guide > OpenVDB Workflow > image2017-9-7 10:36:11.png

Ramp RGB

Create a Ramp RGB and connect it to the Emission Color input of the Standard Volume shader.

Change the colors of the ramp to look like the color temperature of an explosion.

Arnold for Houdini User Guide > OpenVDB Workflow > image2015-7-29 12:9:9.png

^{_{Ramp RGB recreate's color temperature of explosion}}

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.

Create a Volume Sample Float shader and connect it to the input of the Ramp RGB.

Select the Sampling tab of the Volume Sample Float shader and enter 'temperature' in the Channel field. This will remap the temperature grid of the Volume using the colors from the Ramp RGB.

Arnold for Houdini User Guide > OpenVDB Workflow > image2017-9-7 10:35:13.png

Select the Remap tab. Change the values of the Volume Sample Float shader to that in the image below:

Arnold for Houdini User Guide > OpenVDB Workflow > image2015-7-29 12:8:7.png

^{_{Final Volume Sample Float attributes}}

Arnold for Houdini User Guide > OpenVDB Workflow > image2017-9-7 10:37:35.png

^{_{Final shader network}}

Volume Sample Float Attributes

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

Standard Volume

Ramp RGB

Volume Sample Float

Volume Sample Float Attributes

Input Max

B ias

Output Max

Standard Volume

Ramp RGB

Volume Sample Float

Volume Sample Float Attributes

Input Max

Bias

Output Max

B ias