Re: Ski Trails

Here are two approaches that might work:

1. Create two renders of your scene; one without any ski trails, one with trails - even in front of the skier! In a compositing program like After Effects, place the animations on top of each other and use a mask to reveal the ski trails behind the skier. It really is cheating but I'm pretty sure this is how many scenes of this type actually is done - and you don't need to be an After Effects expert at all to perform this 'magic'.
Now, how you create your ski trails in the first place is another matter(!). You can use a bumpmap for the trails, or make a 'displaced' object from a similar bitmap.
You can also create 'negative' ski trails, i.e. more like railrod rails, from a couple of splines in the Extrude tool. Then create a boolean object out of the downhill minus the 'rails' which would create trails... The challenge here is to draw splines that follow the terrain perfectly, but an advantage is that you can use this spline as a guide for the skier going down. The skier and his trails would lock perfectly! Remember you can also add a particle emitter to his boot, pouring some random snow dust out behind him.

2. If you don't want or can't do compositing, you can still use the above methods. Material channels like the bumpmap can be video instead of bitmap, so you could make a simple 2D animation with the trails going downhill. You'd then need to align the skier to the front edge of the trail as it is revealed; a process which probably is cumbersome, but possible.
Boolean objects are also dynamic, i.e. you can animate the 'hole' in an object. Here you need a sort of double boolean: first take the ski tracks (the railroad rails) and let these be cut off by a huge box object, inside one Boolean object. It is the box that you want to animate: as you move this down the rails will appear. Now take this animated object and move it inside a new Boolean with the downhill; the rails will gradually cut trails in the hill.
This may look complex but it is the most dynamic and flexible. You only need to align the upper edge of the box to the skis; should be pretty easy - and you can do it all in Cinema4d!





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