About Dop cache:
How to capture cache is kind of a new concept to me. So here is to reinforce the concept, I used a Dopfile node and a file cache node to cache the simulation in the Dop network. And I clicked the save to disk button to save the simulation into a geo file. Then I use it in a file node by navigating to the geo file.
How to make a loop:
The basic idea is to mix two volumes. And key each of the blends in and blends out.
How to make a smoke ball:
First I need to understand two whys.
Why need to map the whole volume to a smoke ball?
Because each frame will be rendered as a texture on a atlas import to unreal engine for vfx development.
What is VDB and Why use VDB can generate faster?
This explains the differences between a standard volume and VBD volumes. Different kinds of volumes can handle different simulations.
When searching for VDB info found a good website of cool stuff in Houdini http://www.tokeru.com/cgwiki/?title=Houdini#Volumes
Detail about Render to texture:
Set the camera resolution to 512x512
Set the Rendering Engine from Ray Tracing to Micropolygon Rendering and that will eliminate the noise in the final render image.
Render to MPlay can help preview the texture, the render view can only show the ray tracing preview somehow.
Check the alpha channel of the MPlay renders preview, a good flipbook texture requires an obvious contras fallout of the alpha. Or it will cause problems when it comes to an alpha erosion.
render to disk, it's in the render folder.
Here is the difference between ray tracing render result and micropolygon render the result. Ray tracing has noise around the smoke.
Render out the Atlas:
use the .exr sequence file in the last step as an import.
use mosaic node to form an 8x8 atlas.
use a premultiply as a protection of the atlas
using a rop output node to render the final atlas out.
Because I 'm using Houdini non-commercial version I can only render out things with a low resolution and Houdini mark on the texture.
the format has to be .tga to be used in Unreal