Making part of model that is in the shadow dissapear?

Atm im messing in Sader Forge trying to make that effect, i think i need to do some thing with light attenuation and connect it to opacity channel, but i just can't get it to work. Can anyone explain to me how to get that effect properly.

 

I can't help explain it, but I recently saw this and thought it might be kind of relevant.

 

You'll actually run into lots of trouble with that, because it is same kind of problem as casting shadows onto semi-transparent object.

If you want that to work with dynamic shadows and non-transparent object, that pretty much can't be done automatically due to unity's ridiculous approach to shadows and semi-transparent objects.

You could try to make a shader no ambient pass AND zwrite-enabled per light pass. In light pass cut off pixels that are far away, but you'll get all kinds of weird effects with it.

The best idea would be to track some obejct in scene, pass its position into shader, and cut off pixels that are too far away from the object. If you need "dynamic shadows", then you could setup additonal render target, make it act as shadowmap, and cut off pixels based on it.

If you haven't dealt with shaders in detail, that'll be quite complicated for you.

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In general, you can only make this kind of effect to work with object that is lit with single point light. Just implement custom shader with no ambient pass, in per-light pass render without alpha blending AND with zwrite/ztest enabled, and cut off geometry based on shadow value of that light (see shader and surface shader tutorials for details).

That won't work with primary directional light and won't work well with multiple lights.