In this tutorial, we will cover how to radically change the appearance of a volume using the VolumeSampleRGB shader to produce a thick, 'oily', smokey-looking volume. This shader along with the volume_sample_float shader has 'post-production' style attributes such as contrast and gamma. These attributes are very useful for 'fine tuning' the appearance of a volume. In this example, the VolumeSampleRGB shader has been used to convert a typical fire volume into something that looks more like thick smoke. We will use a default fire simulation from the Pyro FX shelf in Houdini that has been written to disk as a volume VDB.

The VDB file can be downloaded here.

The Houdini scene file can be downloaded here.

Volume

• Start off by creating an Arnold Volume.
• In the Filename of the Volume, open the file: default_flame.114.vdb
• Ensure there is a Grids field type 'density'. This is the name of the dynamic field group that the Volume VDB was saved with.

Standard Volume

• Create an arnold_vopnet and connect a Standard Volume shader to the Volume parameter of the OUT_material in it.
• Change the Transparency Color to a light orange. You may want to adjust the Scattering Intensity, depending on the scene lighting.
• Try reducing the Transparent Depth. This will help to thicken the volume.

Volume Sample RGB

• Create a VolumeSampleRGB shader and connect the output parameter of the VolumeSampleRGB shader to the Density attribute of the Standard Volume.

• Select the Volume and under the Material tab, select the arnold_vopnet.

Volume Sample RGB

• Now that we have got the technical details out of the way, its time to have some fun playing with the various parameters of the Volume Sample RGB shader. However, before we continue we must ensure that we enter the word 'density' into the channel attribute of the Volume Sample RGB shader otherwise it won't be able to see the density channel in our volume.

_{Enter 'density' into Channel}

Below are some other examples of changing the Volume Sample RGB shader's attributes:

Contrast

Exposure

Multiply

Add

The final corrections used in this example involved adjusting the Gamma, Exposure, Multiply and Add parameters as per the image below:

_{Final attributes for Volume Sample RGB}

Volume Ray Depth

Increasing the volume ray depth can also have a dramatic affect on the appearance of the volume. Remember to increase the number of Volume Samples to compensate for any resulting noise. In this example, 6 Volume Samples was used.

When increasing the Volume Ray Depth, y ou may need to reduce the exposure of the scene lighting to compensate for the additional multiple scattering of illumination in the volume.