I came across two situation that urged me to find a way to combine or let's say flatten vdbs so instead of having multiple densities you will end up with only one, and those situations are:
- If you are using the Houdini Cloud Lighting Baker tool I created earlier, so if you have a couple of clouds that you merged and wanna bake the light on to them, only the first one will store the color data.
- Another important use of this technique is when you are exporting VDBs to be used with vray volume grid again only one density will be loaded.
As you can see in the picture below I merged a couple of vdb converted spheres and in the primitive tab you can see there is a density for each of them.
When this file is loaded with the vray volume grid you will immediately notice only one sphere in the viewport and when you investigate this more you will find that vray is only loading one of the density and it will allow you to choose which.
So the solution (in case you considered this a problem) is to flatten them using an easy trick, in the picture below you will find that I added an empty vdb and wired it to the A port of the vdb combine node (note that the res of this vdb will determine the flattened ones) and in the combine node you only have to tick "Flatten All B into A" and set the operation to Maximum that's it. Now you will end up with only one density primitive.
And when loaded into vray volume grid all of them will be displayed.
It was a short one, I hope you find it useful, if you do please consider supporting me on Patreon where you will also find the scene file to download.