They've already got snow that can settle on certain obstacles. From a programming standpoint what your talking about is a nowhere near as difficult, except the bit about it also interacting with fire but I wouldn't notice I barely use magic anyway.
Snow APPEARS to settle and there is STILL no evidence that snow "piles up" in more than two dimensions. Every single bit of footage of ground snow is obviously a 2D pixel shaded effect, like Crysis's snow shader, where a snow-like effect essentially gets wrapped onto an object using the objects normal map to tell the snow where to "stick". So just because there is snow in the air, and snow on the ground, doesn't mean it's actually interacting with the ground in any way. Even if the air snow is in world space, which I have predicted since the beginning, there is still a chance snow or rain will go through overhangs. The reason is because collision detection between that many particles is expensive. The reason we KNOW the particles aren't going to be sticking around is because there is not enough computational power to do this in real-time. If a snow shower releases 500,000 particles a minute, the engine would have to keep track of millions of particles. Clearly, if anything, the particles will terminate on contact, and this is why the snow doesn't "settle" on objects. The snow just disappears. The ground snow shader just appears to have snow build up over time.
we know that weather effects are going to act like particles so it would be possible to there to be a die on hit modifier on it, not many video games actually do this
We don't *know* that snow/rain particles are in world space, but otherwise you're correct. I have predicted and assumed since my very first post on snow, before we had any tangible evidence, that snow would indeed be moved to world space, but I have remained skeptical on whether they'd implement particle termination. The reason is it's a lot of work to do, for one, and also has very little benefit. Maybe only 1-5% of the world is under an overhang, if Oblivion is any indication.
dynamic snow piles up on rocks....im pretty sure objects stopping rain wont be an issue
Nobody has ever said anything about "piling" snow on rocks. There is no evidence for this in a single image or video clip either. I've looked extensively through the trailer and nobody official has said the snow "piles" in three dimensions... Just people on these forums who have stated such without sufficient evidence. The "dynamic" (hate that word... as nobody gets the meaning) snow in all the trailer scenes and in all the screenshots is very obviously a 2D pixel shaded effect like the snow shader in Cryris. The difference? Crysis's snow didn't change. The ground remained the same throughout the level. Hence, Skyrim's snow is "DYNAMIC" in comparison. They're both procedural effects, but only Skyrim's varies with time. So far that has been the ONLY feature advertised.
I have only seen two spots in the trailer with possible volumetric snow... In the city at night, there appears to be a snow drift or two. These are just on the ground and could be static. In the end shot, with the dragon taking off from the ruins, there is evidence that the crevasses between mountain peaks are filled in with snow. This could also very well be static, baked right into the mountain mesh. The only reason I noticed is because as the camera gets closer, the LOD of the mountains change... and the shape of the snow "filler" changes.
And if you're curious how snow can "accumulate" in only two dimensions, you have to be thinking in game terms. For one, bump/normal mapping is made specifically to give the illusion of an extra dimension of data (height) on surfaces. The illusion breaks down at the silhouette of the object. I have looked extensively and all of the "dynamic" snow on every object is clearly a 2D, normal mapped effect. The silhouettes of every object appear to be unchanged by the presence of snow. Rocks and other things retain their, hard, sharp, straight edges. If this "dynamic" snow is as robust as everyone on here claims it is, why isn't the snow on these rocks volumetric, soft, fluffy, rounded? On the contrary, some magazines have quite literally used the term "dusting" to refer to the "dynamic" snow system.
It's pretty much a given at this point that rain will be in game space just like Snow is (ultimately confirmed beyond a doubt in the demo footage we got), so yes, Skyrim has already met that challenge and beyond.
No, it's not confirmed beyond a doubt. Like usual, your anolysis is skewed... You spend too much time "confirming" things and not enough time being skeptical. You're also not a developer on the project so you can't go around confirming things. I also think the term you're looking for is "world space" as "game space" is meaningless. Before we had any real evidence, I predicted a move to world space for rain and snow effects, but there still isn't "beyond a doubt" evidence for it.
The 1-2 seconds of footage you're speaking of do nothing to tell us for sure. It's certainly a prettier effect, but it doesn't go on long enough to confirm anything about snow. It's rather simple why the evidence is insufficient. To expose whether the snow is screenspace or camera space, you have to either strafe, or walk forward/backward. The brief glimpse of the snow has the character TURNING, which is insufficient data to come to a conclusion. In Oblivion, you can turn the camera left or right, and it appears just fine, because the particle effect is being rendered all around you in camera space. Basically, the camera has to change worldspace position for it to become obvious.
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With all that said, I've always thought snow would be put into world space. It still doesn't say anything about snow collisions, which I remain skeptical on. It's extra work, and maybe 5% of the world or less is under an overhang. Actually, given the snow animation from that demo footage (if it's even snow fall, could just be snow blowing around while it's NOT snowing), it would be less of a problem in this game if they didn't fix the collision issue... It makes sense to see snow like that even under an overhang. It flows more naturally, and blows more horizontally... totally unlike Oblivion's "Windows Screensaver" snow animation.
As for rain... Who says there is even going to be rain? There may only be a few areas warm enough... and I doubt they're employing any kind of season system, since they've named an entire area "Fall Forest". Not to mention they didn't bother with rain at all in Fallout 3... and snow was only in an expansion?
I'm not even sure if Crysis employs collisions on snow... And I think Crysis is a good template for comparison, as Bethesda has been very clearly influenced by Crysis's snow technology. The ground snow is nearly the same method, and now we have seen in video that they've employed a similar snow particle system in Skyrim.
http://i55.tinypic.com/ng1pfp.jpg vs Skyrim http://pcmedia.ign.com/pc/image/article/114/1149185/elder-scrolls-v-skyrim-20110211094610642.jpg / http://i801.photobucket.com/albums/yy297/b_harrison/misc/skyrim_demo_capture001.jpg