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 have '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.

It is recommended that you first follow the Writing Volumes To VDB tutorial as we will be using the VDB from this tutorial. The original Houdini file can be downloaded here. The VDB file can be downloaded here.

The scene can be downloaded here.

 

Volume

  • Start off by creating a Volume from the Arnold Shelf.
  • In the File path of the Volume, open the file - default_flame.114.vdb
  • In the Grids field type 'density'This is the name of the dynamic field group that the Volume VDB was saved with.

VDB volume with grids 'density'

 

Standard Volume

  • Select the Material tab of the Volume. 
  • Increase the Volume Density to around 200. Don't worry about this value too much. We will be using the VolumeSampleFloat to adjust the amount of scattering in the volume.

Shading

  • Open the arnold_vopnet in the Arnold Volume.
  • Create a VolumeSampleFloat shader and connect the float attribute of the VolumeSampleFloat shader to the Density parameter. 

 

Volume Sample Float

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

Enter 'density' into the channel attribute

 

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

Contrast
Bias

 

 

The final corrections used in this example involved adjusting the Contrast, Gain and Output Max attributes as per the image below:

Final attributes for Volume Sample Float

 

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, you may need to reduce the exposure of the scene lighting to compensate for the additional multiple scattering of illumination in the volume.


 

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