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Problem with Perspective

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Ryan Moser
Problem with Perspective
on Aug 19, 2014 at 11:32:41 pm

Below is a test I did with a still image that I digitally zoomed and composited over a motion dolly shot. Being familiar with cameras, even I was surprised at how much of a difference a zoom and dolly can make in perspective, for elements relatively in the same plane. Make special note of the Speed Stick in relation to the iron.







The reason behind this test is, I'm hopefully going to be executing a shot that dollies in through the windshield of a car.

I was planning to composite the green screened windshield onto the dolly shot. The reason the windshield is being replaced in this manner is because the windshield wipers are going to be used.

Unfortunately motion control is not really a suitable answer due to budgetary reasons. Unless if anyone is aware of cheap motion control dolly rentals.

Maybe an invisible cut/morph is the best solution?

I thought I was so close to finally solving this shot. If anybody can help me solve this problem it will be greatly appreciated. Thank you.


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Walter Soyka
Re: Problem with Perspective
on Aug 20, 2014 at 10:51:07 am

As a pure post solution, assuming you have tried to manually get both dolly shots as close as possible, I'd try the following:

1) Grouse about how easy this would have been to fix in production with motion control

2) Use 3D tracking on both shots, talent and windshield, to solve for the camera move

3) Analyze the rate of movement in Z-space of the camera in the talent shot

4) Use time-remapping to match the rate of movement in the camera of the windshield shot to the rate of movement in the talent shot

5) Hope it all works!

Maybe you can test this workflow before shooting?

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive [twitter]   |   RenderBreak [blog]   |   Profile [LinkedIn]


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Dave LaRonde
Re: Problem with Perspective
on Aug 20, 2014 at 2:47:33 pm

Here's a vote of support for Walter's advice. Especially the testing part. I'm just going to add a few tidbits:
  • Check out hollywoodcamerawork.com. They have stock shots with tracking marks on green screens you can download... plus tutorials on using these shots.
  • Check out videocopilot.net. Andrew Kramer has a lot of EXTREMELY useful keying tutorials you can use to your advantage... and you'll note that almost all of the footage in those tutorials is extremely well-shot. He's not fixing poorly-shot footage unless it's a tutorial on fixing poorly-shot footage.
  • If you're using AE exclusively to build the shot, bear in mind that Keylight works better if the green screen is a little on the dark side. The green screen backgrounds on Hollywoodcamerawork are a little too hot for Keylight. If the brightest part of the green screen shot is at 100 IRE, the background should be at 65-75 IRE.
  • Light the green screen evenly -- no falloff at the edges or hot spots -- and make sure the screen itself is featureless with as few seams, wrinkles, cracks etc. as possible.
  • Use the same camera/lens combo for all shots involved.
  • Make sure all shots have viable tracking points in them. They don't have to be swatches of green in a non-green-screen shot, they can be non-moving, distinct items; for example, things on a car dashboard and in the back seat.
  • You'll probably match one shot to the other. On the shot that is matched to the other, make sure the zoom is a lot slower. It will give you a better chance at a successful match.

I don't envy your task, sir. I wish you luck.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Ryan Moser
Re: Problem with Perspective
on Aug 21, 2014 at 12:10:53 am

Big thanks to both you and Walter for taking the time to help me with my current struggle.

I've had some success with using a Bezier Warp to fix the perspective. I currently think the best solution may be to dolly with both shots. Time out the green screened windshield plate as close as possible and fix any sizing issues etc. via using a Bezier Warp. That way I can maintain the most resolution possible.

Thanks for the taking the time to share your knowledge and thoughts on the subject!


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Dave LaRonde
Re: Problem with Perspective
on Aug 21, 2014 at 1:43:26 am

[Ryan Moser] "I currently think the best solution may be to dolly with both shots."

Hmmmm..... if you can use a track, AND if you can match camera focal lengths, AND you can motorize the dolly somehow AND you can match the dolly distance, you might stand a fighting chance.

To cover a certain pink posterior, it would be good if this motor had clearly-defined speed settings, and you found one that worked for the.... well, hero portion of the effects shot, and that there were additional and lower settings for the shot to which you would match the hero portion of the shot. By doing this, you could at least manually match the frames of the second shot to the hero shot using very careful scaling and positioning.

I believe you understand that this is going to be no picnic, whatever you eventually do. Again, good luck. Write back if the shot succeeds....
...or you decided to do it in one elaborate take, with a hose substituting for rain, carefully-placed lights for the interior, and a large backdrop. In other words, a practical effect.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Ryan Moser
Re: Problem with Perspective
on Aug 21, 2014 at 2:40:09 am

The reason the entire shot is so difficult is because it is a combination of practical effects. This is just the tip of the iceberg of the problems I've had to overcome. Thanks again for your feedback, Dave!


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Daniel Waldron
Re: Problem with Perspective
on Aug 21, 2014 at 2:03:03 pm

I am very curious how this will work out. I'd love to see the end result and how it was achieved.

For an inexpensive motorized rig, check out Revolve Camera's dolly. I've used it mainly with DSLRs, but I've seen larger rigs on it as well. I'm not sure how accurate the motion is from shot to shot, but I imagine it would be much easier to match than dollying manually.


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Matthew Woods
Re: Problem with Perspective
on Aug 21, 2014 at 3:47:55 pm

Would it maybe be easier to 3d track the pivot points for the wipers in your dolly shot, then animate the windshield wipers in 3d space? It doesn't seem like it should be too hard to animate the wiper movement. You would have a side benefit of having more control over the timing of the wipers, so that it doesn't conflict with any of the action in the car.

-Matt

Need a quick break from motion graphics?
Try my game Constellation at:
http://www.paperdragongames.com


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Ryan Moser
Re: Problem with Perspective
on Aug 22, 2014 at 2:16:38 am

I think your suggestion is probably the easier route. However it would conflict with the current practical effect that we have perfected.

One of the aspects of the entire shot is a bug splatters on the windshield. The driver uses the wipers to clean up the mess. The practical effect looks flawless because it's basically as real as you can get. The changing of the color of the water and the smear of the bug as the wiper goes across it.

The problem with the practical effect has been tying it all together with the performance dolly move.

Thanks for yours and everybody's feedback in helping me conquer this task. I'm happy to realize that it isn't just me struggling to solve the solution to this. I would have been relieved, but also embarassed, to realize a really easy solution. If we are able to pull this all off at the level I'd like, I believe it will be truly impressive.

I'm definitely going to take your suggestion to heart and try some tests with it.


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Ryan Moser
Re: Problem with Perspective
on Aug 22, 2014 at 2:06:50 am

Thanks for the suggestion, Daniel. Unfortunately I don't think the Revolve Dolly will be of help for this particular project. We need a large dolly that can support the wait of both a tripod that has a jib or some sort of extending arm attached.

Thanks again! I'd be happy to share the final project whenever it's actually complete. Unsure when that will be.


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Raza Ahmad
Re: Problem with Perspective
on Aug 22, 2014 at 4:53:11 pm

I know this is probably photographically non-ideal, but:

The simplest option I see here is to only move physical (as in in the real world camera) in one shot, the one where you are dollying in to talent.

The second shot, camera with wipers, would just shoot it with a static camera, if you are using RED or similar you could leverage extra resolution here.

You 3D track your dolly shot, making careful note of lens length and height and marker distance (AND FOCUS POSITIONS!) to get a spacially accurate 3D Solve, then inject your windshield as a 3D layer between the virtual camera and your physical dolly shot, and use the 3D motion track + focus data to realistically blur out the windsheild imagery as your camera moves, ultimately moving past the layer via a very quick dissolve or single frame transition.

Not sure... that would be the $0 film school idea.


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Ryan Moser
Re: Problem with Perspective
on Aug 23, 2014 at 1:01:24 am

This is the solution I was planning to use. However in the test I did (the video with the iron that I posted) the change in perspective between a dolly move and a zoom move within a single composite looks unnatural. Time to test out some other ideas. If I knew how to use Nuke I believe this would be a fairly easy composite using their f_align plugin.


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Dave LaRonde
Re: Problem with Perspective
on Aug 25, 2014 at 5:25:23 pm

Oooh, don't try to mix a zoom and a dolly. It won't work. With a zoom, the elements of the camera lens simply magnify the image -- there's no perspective change.

On a dolly, the lens focal length remains constant, but the image gets bigger because the camera moves, revealing & hiding portions of the image as it moves -- a perspective change. Not to mention that the actual shapes of the images change.

You gotta pick one -- a zoom, or a dolly shot using THE SAME LENS FOCAL LENGTH FOR EACH PART OF THE SHOT -- or you're just asking for trouble.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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