3D track motion and aliasing.

farss wrote on 6/13/2004, 6:01 AM
Having a brief spare moment I decided to have a play around with this. Created a rather horrid spiral in PS, used event rotation to spin it and added velocity envelope just to jazz it up a tad. Pretty stomach churning stuff, bit like bad acid from the 70s.
Anyway, then I took the 3D demo of the box with the opening lid and replaced the generated media with my twirly spiral thing, offset them 12 frames or so, so each face ramped speed at different times and rendered the whole thing out and burnt to DVD just to see how bad it would look on a big screen.
Well there does seem to be a problem ( apart from my appalling sense of color), having Vegas do 3D mapping of interlaced DV does seem to introduce some nasty aliasing, I would have thought the results should look pretty damn good, I started with a 2Kx2K png, OK that got cropped down to 720x576 and rotated but no sign of any problems until it's remapped onto the planar surface, result get pretty bad when the surface is at an oblique angle.
Anyone else seen this, I do appreciate that this may not be Vegas's fault, I can sort of see a way it could happen no matter how clever Vegas is. But then again not much point having these clever tools if they only work well with generated media.

Comments

farss wrote on 6/13/2004, 3:37 PM
Bump, just in case the rest of the world was asleep while this trickled to the bottom.
Spot|DSE wrote on 6/13/2004, 5:21 PM
Farss, I see this with any media that isn't generated, depending on how far forward or back I bring the media. I don't know how Vegas could resample it better, because it's somewhat like a pan/crop thing when you're bringing the media forward or back. I also see it with generated media if the media isn't sized large. Try setting a generated title at 720 x 576, then using 3D to roll one edge all the way forward and enlarged. Even in Best mode, you'll see aliasing. Now resize the media to twice as large. No aliasing.
farss wrote on 6/13/2004, 6:21 PM
Interesting problem. I'm just curious because I see this kind of thing done in old analogue systems and it looks perfect so there should be a way to do it. In my test I was even getting vertical sheering which is very noticeable. I'm hoping it has something to do with the limitations of DV25 in which case maybe rendering the footage to a HD format first might help.
Other reason I'm interested is client has had similar problems with FCP, admittedly a way old version of FCP and so far the results from Vegas look better, hoping to find a way to make it perfect.
I'll try some more experiments a bit later, client just dropped another job on me, needless to say it's urgent.
farss wrote on 6/18/2004, 5:57 AM
This has been in the back of my brain for a few weeks now and suddenly the light bulb went on, right in the middle of a big audio project.
From what I'd seen happen it seems that the image is being mapped by first scaling it and then applying the 3D mapping. What happens then is that no sub pixel calcs are taking place. What should happen, I think is first the 3D transform should be done and then perhaps the scaling.
Let me try to explain. When you scale an image down you need to create a new, smaller number of pixels. To do that I assume some form of guassian calc is applied to the pixels from the original frame to detemine the appropriate value of the new pixel. This seems to work fine, scaled frames come out perfect.
However when you skew an image, like onto the oblique side of a box, the way the distribution of pixels are sampled to generate the new ones needs to be different, the algorithm needs to take into account the skewing factor else you will get apparent sheering of the image along the original scan lines. In a square image each adjacent pixel is 45 deg apart from the current one, but when the image is being highly skewed those originaly adjacent pixels can end up much further away.

Sorry if this isn't a very clear explaination, the days when I might have been able to better explain the maths are long gone, trig was never one of my strong points. Hopefully though someone will tweak to what I'm talking about or else maybe I'm way off the mark!