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.
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 - Plugins> C4DtoA> Arnold Volume.
Select the Volume and select the volume in the File path attribute.
Arnold volume settings. Grids set to 'density'.
- Assign Volume Collector to the Volume and change the following attributes:
- Set the Scattering Source to channel (this will allow us to change the name of the scattering channel).
- Add 'density' to the Scattering Channel. This is the name of the grid that the volume was saved with.
- Change the Attenuation Source to scattering. This will give us a realistic light scattering within the volume.
- Ensure that the Attenuation Mode is set to absorption. In this case the volume looks better using absorption.
Volume Collector Scattering and Attenuation settings
- Render the scene. It should look like the following image.
Now we want to use a colored ramp to affect the emission parameter to give the smoke some fire.
- Add density to the Emission channel and increase the Emission intensity attribute. This value will depend on how bright you want the emission to look.
Volume Collector 'Emission' attributes
- Connect a ramp_rgb to the Emission Color attributes of the Volume Collector shader.
- Change the colors of the ramp to look like fire.
We can use a Volume Sample Float shader to control the appearance of the 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 drag and connect it to the input of the ramp_rgb.
Volume Sample Float shader connected to Uv Coord (U) attribute
- Select the Volume Sample Float shader and add density to the Channel attribute.
- Render the scene. You should see that the emission attribute is rendering with the colored ramp texture. However, it may look a little dull initially. Try increasing the Emission attribute in the Volume Collector to make the colors of the fire appear brighter.
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.