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.
- Start off by creating a Volume - Arnold> Volume. Change the Type to OpenVDB.
- In the File path 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.
- Create a Standard Volume shader and assign it to the volume.
- Create a VolumeSampleRGB shader and connect the output attribute of the VolumeSampleRGB shader to the Density attribute of the Standard Volume.
- Change the Scattering Color to blue and the Transparency Color to orange. You may want to adjust the Scattering Intensity, depending on the scene lighting.
- 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 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 the channel attribute
Below are some other examples of changing the Volume Sample RGB shader's attributes:
1 2 10
0 1 8
1 10 25
-5 0 5
The final corrections used in this example involved adjusting the Gamma, Exposure, Multiply and Add attributes as per the image below:
Final attributes for Volume Sample RGB
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 in order to compensate for the additional multiple scattering of illumination in the volume.