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Car perspective

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lou cannCar perspective
by on Apr 11, 2011 at 11:42:33 am

Hi dave!

We are planning to do a 2 shot of a person driving a car. The camera is shooting the windshield so the green screen is behind the rear window.
We went out and tested the shot and to my suprise, my horizon and perspective were off. Simply raising or lowering the background plate didnt do it. I always wanted to orbit/rotate the backplate to line up with the foreground. I felt it looked like a saturday nite live skit.

So my questions is:
How would you plan & film so that the 2 plates line up? Is there a mathematical approach?

For example, If I were to shoot the car on the street, I would have actor & background already composted. Now, if I placed a green screen behind the rear window & filmed it, then moved the car away to film the back plate. I would have the exact, perfect composite because all the elements are in the exact place. So, how would I get those results without doing all that?

Am I making any sense?


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Roland R. KahlenbergRe: Car perspective
by on Apr 11, 2011 at 2:52:29 pm

Markers on the green screen should help if they're sufficiently clean. Camera height for the second shot is crucial. Some optics distortion will also be required. And then a reflection pass. So, nope I don't know if there's a mathematical formula.

HTH
RoRK

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Dave LaRondeRe: Car perspective
by on Apr 11, 2011 at 3:19:18 pm

As Roland says, a lot of things go into matching a shot.

A short list would include camera angle off true horizontal, camera angle off true straight-ahead, camera height and focal length of the lens.

It's really tough -- if not downright impossible -- to determine the focal length on a zoom lens when using a consumer or prosumer camera. You can't read that information of the lens.

You CAN, however, rig a jig to match it by putting vertical poles at the left & right edges of the shot, perpendicular to straight-ahead for the camera, then doing some very careful measuring. To shoot the background, set the jig up just as carefully and zoom the lens to match. It's a big pain.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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lou cannRe: Car perspective
by on Apr 11, 2011 at 5:42:49 pm

Hi, ok I think you guys are onto something.
But justto be clear, I am going to pick your brain.

1st. Camera height:
So let's say the camera (5D) is set to 60" from floor to nodal.
When filming the background, the camera has to be 60" from 'Outside ground" to nodal inside the car or truck?

2nd.
Vertical poles:
Are the poles for tilt angle?

If I understand you, this is what I would do:
Set the camera up & frame up my shot to film.
Measure the height from floor to nodal.

Measure the distance from Nodal to Vertical poles.
Place markers on pole so that when I look in the viewfinder, the markers are at the top & bottom of the frame.

Now, When filming the background, I have to re do all of the steps above?

Althought this sounds very mathematical, why do I get the feeling that I am in for a big suprise?


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lou cannRe: Car perspective
by on Apr 11, 2011 at 5:46:41 pm

Oops, I forgot to add Focal length.
So if the DP zooms in on the shot Is there a way to know how much he zoomed in? The camera will be the canon 5D


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Dave LaRondeRe: Car perspective
by on Apr 11, 2011 at 6:20:56 pm

[lou cann] "When filming the background, the camera has to be 60" from 'Outside ground" to nodal inside the car or truck?"

Yes.

[lou cann] "Measure the distance from Nodal to Vertical poles.
Place markers on pole so that when I look in the viewfinder, the markers are at the top & bottom of the frame."


PLUS the distance from the camera to a point where A line connecting the two poles is perpendicular to the camera as it shoots straight ahead. The vertical poles are at the left & right edges of the shot to determine ZOOM angle (focal length of lens). You'd use the markers on the poles to determine the camera's TILT angle. Nice thought on the markers, by the way.




[lou cann] "Now, When filming the background, I have to re do all of the steps above?"

You sure do.




[lou cann] "...why do I get the feeling that I am in for a big suprise?"

Because it's a boatload of work, that's why. You have to measure precisely, painstakingly record measurements & angles, and then replicate them in a different location. You don't have the benefit of having a Hollywood camera crew with the proper time-tested tools, and which has done this stuff a gazillion times before. You're a guy with a 5D doing it for the first time.

And get that stinkin' H.264 footage made by the 5D converted into a decent codec before you start cutting and doing effects.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Dave LaRondeRe: Car perspective: just thought of something else
by on Apr 11, 2011 at 7:26:08 pm

Here's another thing to take into account: road motion for the car.

Vehicles rarely travel on silky-smooth roads in a straight line. The hit potholes, ruts, sewer grates and the like. They accelerate and decelerate. They make turns. All of these things affect the people riding in them, and the people move accordingly to compensate.

But you're shooting in a parked vehicle; they'll have nothing to react to. The exterior background will have some minor bumps in it, but the passengers won't move. They won't even react to the motions, because they'll have nothing to react to.

This is why the most successful chroma key shots inside cars are shot when the car is parked.
It's also why when shooting car interiors in motion, film makers mount cameras to cars, place cars on trailers, etc.

This effects stuff gets pretty complicated, doesn't it?

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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lou cannRe: Car perspective: just thought of something else
by on Apr 12, 2011 at 1:46:46 pm

DAVE And get that stinkin' H.264 footage made by the 5D converted into a decent codec before you start cutting and doing effects.

Ok dave, now you're scaring me. In the past i've worked with Canon XHA1 & 35 mil converted to proRes . Next Week, we will be using the 5D. When I said to the DP isn't there a problem with the 5D on Linear programs or After effects? He replied, The entire School at NYU uses it and NO ONE complained.

So dave the old mighty & Wise,
What am i in for when this DP hands me the Flash card and says..."Quick ! Key this footage and email it back to me by tonight! "



DAVE SAID" Vehicles rarely travel on silky-smooth roads in a straight line. The hit potholes, ruts, sewer grates and the like. They accelerate and decelerate.

That was my other issue.
When we brought the test footage back, it was VERY BUMPY. We had it on a tripod in the car.
So I tried 2 things.

1. I stablized the motion (which turn out to be a nightmare because I had nothing stable to track)''

2. I applied the motion of background to the interior of the car. So now the driver is moving to the background.

Since the background was bumpy. so was the driver and this was too hard on the eyes so I semi stablized it using PRODAD. I then tracked the stablish footage and applied that to the driver and got nice results.

SO my question to you is:
What ways would you go about keeping the 5D from Virbrating , jumpy up & down while filming? Any thoughts?

And did I mention that this is my first time with the 5D? So I have no Idea what to expect when importing into AE.

Thanks for your help,


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Dave LaRondeRe: Car perspective: just thought of something else
by on Apr 12, 2011 at 2:47:17 pm

[lou cann] "What ways would you go about keeping the 5D from Virbrating , jumpy up & down while filming? Any thoughts?"

Put the car on a low trailer and drag it around town. Get your shots. Forget the green screen.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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lou cannRe: Car perspective: just thought of something else
by on Apr 12, 2011 at 6:10:44 pm

Well dave,....we would have done that.

We need to close down the hwy and roads to do that for 1.

2nd. We need 1 million dollar liability insurance for the actors in the car.

3rd. We need Permission from the town of Albany.

Or

Shoot green screen an call it a day.

So basically,...no ideas to absorb shock?


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Dave LaRondeRe: Car perspective: just thought of something else
by on Apr 12, 2011 at 6:17:46 pm

Can you rent a Steadicam? It's probably the only way you'll get the shot. The nice thing: you don't really need a Steadicam operator, because you're just getting a stabilized moving shot. Once you have the shot set, there's no reason to move the camera.

Don't cheap out: don't go the home-built Steadicam route. You'll be disappointed. Furthermore, because that 5D is so light, you might actually have to add weight to the camera.

And watch out for rolling shutter problems with the 5D. Very few DSLRs-- if any -- can handle fast horizontal motion very well; vertical surfaces tend to get curved.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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