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Animated Snow Trails

By Andy Nicholas
Web: Open Site
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Date Added: 9th December 2009
Software used:
LightWave

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

1164_tid_tut04_03.jpg
Result (Frame 91)

You can download the completed scene file including objects here.

Processing The Image Sequence

We're now going to process the rendered image sequence in Digital Fusion.

Add two Loaders (LD) to a new flow and load the rendered image sequence into both of them. For one of the loaders, set Clip Time Start and Clip Time End to 0, and set Extend First Frame to 180. This will ensure that that loader only ever outputs the first frame of the image sequence.

Add a Channel Booleans (Bol) and set the background to the main Loader, and the foreground to the single frame Loader. Change the settings of the Channel Booleans tool to:

Operation: - Subtract
To Red: - Red FG
To Green: - Green FG
To Blue: - Blue FG
To Alpha: - Do Nothing

1164_tid_tut04_04.jpg
Digital Fusion Flow

You're probably asking what's going on here. Well, all we're doing is subtracting the first frame of the image sequence (which doesn't contain the ball), from each frame of the rest of the sequence. Take a look at the image below for a more visual explanation:

1164_tid_tut04_05.jpg
Image maths!

The cool thing about this, is that we're left with only the displacement due to the ball. What's more (and this a fairly subtle point), since we've subtracted the displacement of the surface from the entire image (including from where the ball is), the image represents the difference in height between the lowest point on the ball and the snow surface, i.e. we can apply this to our displaced snow surface and it will deform to perfectly accommodate the ball.

Since this displacement map will work on our height varying snow surface, we've already satisfied the fourth point of the problem that I originally stated on page 1 of this tutorial.


Snow Trails

So far we've managed to make an animated displacement map that will deform the surface to make a hole for the ball to fit in. This doesn't solve our problem however, since we need to make the ball leave a trail. As it happens, there's a tool in Digital Fusion called "Trails".

Add a Trails tool (Trls) to the output of the Channel Booleans tool. Leave all the settings for the tool at their default, apart from the Apply Mode which you should change to "Lighten.

Change the current frame to 0, and press Restart on the Trails tool. Drag the tool into one of the viewport windows and press play to view the result.

1164_tid_tut04_06.jpg
After applying Trails





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