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Hey guys. Im rendering spinning wheels, and Im finding strange behavior with geo samples. It seems if I render with something high like 14, 24, 32 it creates an awful stepped look to the render. Like 5 static renders have been blended together in post, all rotationally offset from each other. If I render with 8 samples, it seems to smooth out. Any idea why this number would work but others not?
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Could it just be a temporal sampling issue? And if 8 gets you what you want, why go higher? -
You'll have to explain temporal sampling to me....8 does seem to remove the weird effect. In my head it makes sense that the higher the geo samples, the more data Vray collects about the animation, and therefore the smoother the result. I don't see why 9 or higher would fundamentally change the result in a bad way. Its not that I've got a problem per se, as my render seems ok now, but I did spend an hour or so testing why 10, 12, 24, 36 all gave the same error, till a mate said he always uses 8. Seems confusing. I've only tested this on wheel spin.Originally posted by SonyBoy View PostComment
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Simply put I'm referring to the "wagon wheel effect", which you used to see in old cowboy movies and often made the wheels on a moving wagon appear as if they were rotating in the wrong direction. It's caused by temporal sampling, in that case the frame rate x camera shutter speed x the rotational rate of the wheels. In CG this can theoretically happens as well.Comment
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I guess I should elaborate -- what I mean is that I guess it's possible that the additional frame samples when you go above 8 that Vray is taking are creating some sort of mathematical loop or clamping, producing a weird result.Comment
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It's just bad luck - whatever speed your wheel is rotating at, the wheel keeps landing in a similar place to where it was at another motion blur subsample. Say you have a wheel that has four spokes and it does one full 360 rotation over one frame, if you were using four subsamples then each subsample would get 90 degrees of rotation. Since your wheel has four spokes, each motion blur step would look like the same wheel placed on top of itself 4 times at 25% opacity. If you want you can use higher motion blur steps for some objects and not others, when I was doing helicopter renders I was using lots of samples since the blades were spinning at such a high rotation!Comment
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Well yeah that's exactly the temporal sampling I'm talking about and what creates the wagon wheel effect, i.e. the distance an individual spoke is traveling between frames is not being recorded (in real photography obviously a spoke appears in a particular position in one frame, and then has moved by the next frame). This is what can make wheels appear to be rotating backwards. Seems like there should be an option to calculate motion blur based on actual rotational values.Comment
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As you've said motion blur occurs when an object starts in one place and ends in another. In vray what happens is the object is duplicated at both the start and end position of a motion blur step and the vertices are morphed randomly between these two different positions, then samples are taken. They're pretty much doing a linear slide between one place and another. To get a smooth arc we almost have to treat things like the amount of radial subdivisions in a mesh so that the angle between all of these linear slides is small enough that we see it as a nice smooth arc. It's also one of the reason why motion blur is so heavy, all of the meshes are being copied by the amount of motion blur steps so a tonne of extra geo is being generated (watch out for render time displacement
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Sean are you rendering stills or for animation?Comment
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Im rendering stills. I've had the wagon wheel effect in anims and I'm fine with that since its an observable effect irl. Ill take some screen grabs of this weird hard edge result I get on anything than 8 samples because it feels like the wagon wheel effect would be present but the result would still be smooth if rendered as a still. The result I got on some sample settings were very hard edged and unrealistic imo.Comment
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