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Animated Snow Trails

9 September 2004 31,388 views No Comment

Animated Snow Trails continued…

Now that we’ve got the geometry set up, we need to sort out how we’re going to render this scene to generate the information we need.

What we want to obtain is a depth map for each frame showing how much the object has penetrated the snow surface. If it were a flat surface, this would be simple, but since our surface undulates, we’re going to have to do something a little more sophisticated. For the moment however we’ll ignore this, since we can solve this problem later in Digital Fusion.

So first of all, we need to set all of the objects to white and 100% luminosity. You must also set diffuse, specularity, and reflection to 0% as otherwise this will adversely affect the result.

Now modify the camera position, rotation, and zoom factor to the settings shown below. If you switch to camera view, the whole viewport should be covered exactly by the snow surface.

&nbsp&nbsp Camera Settings

X  =    0 m        H  =   0 deg
Y  = -500 m        P  = -90 deg
Z  =    0 m        B  =   0 deg

Zoom Factor    100

If you’re not using the objects that I’ve provided and you’ve used a different shaped surface, then adjust the zoom factor so that you can only just see the entire snow surface (be exact, it makes life easier later). You should also set your image size to use the same proportions as the snow surface, in this case square. I set my image to be 800 pixels x 800 pixels. This minimises image wastage on the rendered output.

We are moving the camera far away from the surface so as to minimise perspective distortion, since the camera in Lightwave doesn’t provide orthographic projections.


What we want is for surfaces that are closer, to appear whiter. Basically, we want to create a depth map and we’re going to use standard Lightwave fog to achieve this, since it gives us the most control.

Working out fog range

We want the final image sequence to use as much color range as possible to maximise accuracy. The image above shows the ball at it’s lowest position in the animation (Y =-0.494 m). The furthest distance visible from the camera will be at the highest position of the snow surface (Y = 1 m approximately).

Since the radius of the ball is 0.75m, then the lowest position of the lowest point on the ball during animation will be -0.494m – 0.75 m = -1.244 m. We also have to remember that the camera is at Y = -500 m. So the shortest distance from the camera will be 500 m – 1.244 m = 498.756 m. The furthest distance from the camera is 500 m + 1 m = 501 m.

Go to the volumetrics tab and change the fog settings to the following:

Fog Type:       Linear
Min Distance:   498.756 m
Max Distance:   501 m
Min Amount:     0 %
Max Amount:     100 %

Fog Color:      000  000  000  (Black)

I’ve rounded down the Minimum Distance setting just to ensure that we don’t clip the color range. This guarantees that the colors will always be darker than white in the final image sequence.

Go to the Camera Properties window and set your anti-aliasing to Extreme, and set motion blur to Dithered with Blur Length set to 100%. If you don’t have much time, you might want to set the anti-aliasing to a lower setting.

In the Render Options panel, set the RGB output to save a 24 bit Targa image sequence. Press F10 to render the sequence.

Result (Frame 91)

You can download the completed scene file inluding objects below:

Download: SnowDisplace.zip (8.1 Kb)

Next, I’ll show you how to process the image sequence in Digital Fusion to get something a bit more useful for solving our initial problem.

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