Vray camera tracking problem

# 1 29-05-2012 , 03:53 AM

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I'm asking this on behalf of a friend who is trying some camera tracking tests in Maya with Vray.

In short he imported his Syntheyes camera solve and set it up in Maya and all worked fine. But when he switched to the Vray render-er it was all out of sync.

Later he found that if he turned off motion blur the track was okay, but no motion blur, which you'd want.

I realize the forum doesn't deal much into Vray but I figure someone has done a camera track with Vray before and with motion blur.

Any ideas?

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# 2 29-05-2012 , 10:21 AM

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this doesn't really make sense. if the camera is tracked it shouldn't really make a difference if its rendered in mental ray or vray or anything really. matchmove has nothing to do with rendering. i've not used syntheyes but i guess the camera is baked when he exported it?

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# 3 29-05-2012 , 12:33 PM

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That's right, doesn't make sense. I'm seeing him tomorrow so I'll have a better idea how to describe it.

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# 4 29-05-2012 , 05:28 PM

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It can make some level of sense depending on how 'out of sync' the match-move is.

I've been doing a lot of 2D tracking of cell phones to lay in graphics lately, and each time I had to tweak the shutter settings to match. In AE, the defaults would put the image way out of sync because it was sampling from the current frame and after. I'd suggest trying these tweaks:

find out what shutter speed the clip was shot on and set your shutter accordingly (since maya doesn't have exact shutter speed, you'll have to do a calculation. 1 frame of motion blur = 1/[camera frame rate], .5 frames of motion blur = 1/([camera frame rate]*2).

set the shutter phase (maya's equivalent is shutter open/close, but I don't know vrays so you'll have to figure that... a quick google search revealed that it has a physical camera model, which could be useful) to something just before the current frame to just after...

in AE the sweetspot for most of my work was [shutter phase]-180 [shutter angle]360, which translates to motion blur starting from half a frame earlier, and ending half a frame after. With keyframe easing turned on the tracking data (which had a keyframe every frame) favored the center frame with nice smudging on the edges.

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# 5 29-05-2012 , 05:58 PM

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2D tracking is a bit different. matchmove is matching a camera and its move in 3D space. so ignoring maya for a moment (because it really doesn't matter what you render in.) if the track is correct in syntheyes you should have exported a camera and depending on the camera data you have, you should have set the film back, the focal length have a correct frame range and set the fps correctly.
When you export your camera from boujou,3de,pftrack it will have all the frames baked. it will have all the camera information you used to solve the track. this should be correct (or close) or the camera solve will be wrong and you will see objects swimming around or things will no line up in a sensible way (circles in the plate will be oval when modeled for example.)

I think what you were explaining is shutter angle. this determines the amount and type of motion blur. generally this is set to 180 degrees.
it works like this..
https://upload.wikimedia.org/wikipedi..._animation.gif
if you look at a film like apocalypto the DoP/director set the shutter angle to an acuter angle so you have less motion blur (frame exposed for a shorter amount of time) but you get a kind of strobing effect.
RUN THROUGH THE JUNGLE - YouTube
in maya MB is calculated on the half frame. if you render anything with MB you can see the time line move 0.5 left and right of the frame you are rendering.

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# 6 29-05-2012 , 06:46 PM

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In every 2D/3D package I've seen, you have the capacity change two basic motion blur parameters:

Shutter angle, as you described, will affect how long your virtual shutter is open to render.
Shutter phase (often has different names), the ability to shift the timing of the shutter left or right on the timeline.

What I meant to say is setting the shutter angle to the original footage is important, and if the image renders shifty with motion blur, the problem for me has often been a simple matter of shutter phase adjustments, which without seeing the OPs video issue, seems a likely solution.

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# 7 29-05-2012 , 07:50 PM

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ah, shutter opening i imagine.
i suspect the track is wrong, as i cant see how MB would effect it.

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# 8 29-05-2012 , 08:24 PM

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here's how MB can affect a track... all of these blurs can be achieved with different settings in Maya Software motion blur settings (though again, I dunno about Vray's). If you imagine these effects applied to a camera move, can you see how that may cause drifting if you've got the wrong settings for the blur? In my experience, each shot is a bit different, and even if your shutter speed is right, if your shutter settings aren't right, you could come out with one of these below. Which can cause surprisingly bad drift. I'll upload a video showing what I'm talking about later today hopefully. It really surprised me when it happened. Of course, it happened in After Effects, but the end result is similar in all packages.

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# 9 29-05-2012 , 10:37 PM

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But aren't you still talking about a 2d track and 2d motion blur? I think the problem is to do 3d matchmove and 3d mb.
I just find it hard to believe as I've never had anything remotely like this problem in any of the jobs I worked on.

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# 10 30-05-2012 , 01:17 AM

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My intuition says it can apply to 3d as well, but you have cast doubt in my mind. So I'll be rendering some test spheres and posting the results user added image

I am willing to be wrong. I am not, however, willing to remain ignorant of my wrongness! Here goes.

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# 11 30-05-2012 , 01:47 AM

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this is the setting I'm referring to. it's present in all the packages I'm familiar with, whether 3d or 2d/3d hybrid, I've seen a similar/comparable setting in each. The blurred shots were all rendered from frame 1 with the settings underneath.

The first blur had -1, 0. The second had -.5, .5. The third had 0, 1.

Since the renderer was sampling from different parts of the timeline to generate the blur, each shot the ball appears to be in a different part of the frame. Applied to tracking, assuming the track looks fine rendered w/o blur, and looks out of sync when rendered with, my thought is simply that for this shot, it may lock back into sync by adjusting those settings. The theory can be partially tested by comping the matchmoved shot with the blurred render and shifting it 1 frame earlier and or later, and seeing if it improves. If it does, then it's gotta be that setting. If it doesn't, it could still be that setting, since 1 frame could have been too much of a shift (sometimes I need to only shift motion blur sampling by 1/4 of a frame depending on the nature of the shot, but I haven't learned yet how to intuit WHEN that's the case).

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# 12 30-05-2012 , 09:30 AM

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thanks i understand shutter opening and i see what you are saying. The motion blur can make the track look off, but the track is still correct.
I'd be interested to see the difference with some tracked footage. usually you matchmove something to place an object on the ground etc, so this fast motion MB shouldn't really apply to this situation... unless the camera is moving really fast.
if its an object track (sort of like the phones in your example but talking 3d not 2d.) i can see how this would effect it greatly.

i think we ought to see the shot to know what it is.

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# 13 30-05-2012 , 09:40 AM

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I agree. All we can do right now is speculate till we see the shot.

I haven't actually tried a legit matchmove with a lot of motion blur (usually, given I have one computer and stupid short turnaround times, and very little experience, I do tricky directional post blurs for my 3D matchmoves instead). And, of course, the closer the object to the camera/the slower it moves, the less it'll be affected by this particular issue. Anywho, I've really gotta get better at matchmoving. Just got a shot in today that I had to opt for a 2d track and scaling the values to get the shot because I just don't have time to relearn boujou user added image

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# 14 30-05-2012 , 10:09 AM

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Thanks everyone for your thoughts and tests. I had my friend send me the scene and I am rendering it and checking how it syncs with the back-plate in After Effects. I didn't touch the motion blur settings other that turning it on. So far, it looks okay but it does also appear to float a touch, seeming to pr-empt the camera motion on certain jerky frames. So finding the correct motion blur setting is some kind of art it seems, and Vray has quite a few MB settings.
I think the tests above show how crucial this can be, however I would actually render without MB and add it in post for the type of shot this is, the Re-vision plugin is doing a very nice match from what I can tell, I'll let this render go to the end and then render out a pass with no MB and apply the post motion blur, I have a feeling it will do a better job and be quicker.
It's a shame Vray doesn't interpret the correct blur settings based on the camera settings automatically. Post motion blur isn't always the way to go.

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# 15 30-05-2012 , 10:23 AM

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It is often the best way when on a budget. Legit 3D blur is computationally expensive. Once I was like "hmmm, so I'm rendering out only about 200,000 polys, 12 second shot with some detailed maps. I have 28 hours till delivery. Oh. this render will take 34 hours. Time to do post blur instead. Ah ha! 12 hour render! Now I can do a second render if this one has any issues."

But yeah, in an ideal world, we'd be able to do everything with beautiful 3d blur....

What you're describing (IF the move is rock solid otherwise), definitely sounds like shutter phase/opening issue. Esp. if it's heavily noticeable on jerky moves. Luck to ya!

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Vray camera tracking problem

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