At this point, we need to make a few small adjustments.
Firstly, making sure that you have the final output shown (as in the image above), add a Color Corrector (CC) and press the Levels button. Pull in the upper right hand side control to fit to the top of the histogram. This maximises the use of the color space and will give us a smoother result.
Next add a Blur / Sharpen tool (Blur). Set the Blur Size to about 11 or 12 pixels in Gaussian mode. This will help smooth the displacement and prevent any jaggies appearing in the displaced mesh.
Finally, add a Saver (SV) to the output of the flow, and save your image sequence as 24 bit Targas. Set Process Mode to Full Frames and turn off the Save Alpha option under the Format tab, as we don't have any alpha information in the flow.
The Final flow
Press Render to save the image sequence
The final displacement map
You can download the completed Digital Fusion flow here
Displacing the Snow
This is the fun part where we apply the animated displacement map back on to the snow surface:
Make a copy of the original Lightwave scene that you rendered the depth map from and load it into Lightwave Layout. Apply some sort of basic surface to the ball and snow surface objects so that they appear shaded in the viewport (note: if you downloaded my scene, you may need to turn the lights back on). Then go to File->Save->Save Object Copy and save copies of the objects so that you don't lose the new surfaces (or overwrite the objects of the old scene).
Go to the Image Editor and load in the image sequence that you saved from Digital Fusion. Make sure you select "Sequence" from the Image Type drop down box under the Source tab
Open up the Geometry properties of the snow surface object and set the Display SubPatch Level to 10. Do the same for the Render SubPatch Level. Now go to the Deform properties and add a Textured Displacement plugin modifier. Double click on the plugin name entry to view the settings. In the Texture Editor that appears, set the layer to the following settings
Layer Type: Image Map
Texture Amplitude: -1
Image: (select your image sequence here)
Width Tile: Reset
Height Tile: Reset
Texture Axis: Y
Scale X: 10 m
Scale Y: 1 m
Scale Z: -10
Note that we need to set the Z component of Scale to a negative number so that the image is flipped. This is a result of the fact that we rendered the image from below, and are now projecting it from above.
Exit the Texture Editor and close all other active Lightwave panels. If you now press play, you should see the ball moving across the surface and correctly deforming the snow surface as it goes.
You can download an animation of this here
There are all sorts of improvements that can be made to this method which I haven't gone into during this tutorial for the sake of simplicity and portability. For example, it would probably be a good idea to use a floating point image format for the image sequences, since this would result in much better accuracy compared to 8 bit RGB. It would also be a good idea to use a snow surface with randomised positions for it's vertices to prevent the artifacts that appear as a result of using a uniform grid.
Something else you may want to try is adding a slight bulge that moves in front of the ball as it makes each trail in the snow. This can be achieved with a separate displacement map generated by subtracting one frame of the original displacement map from the previous frame. I've implemented this with some success, and I might write a tutorial on how to do it if this tutorial should prove popular.