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'Tracer bullets' expression?

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Antony Buonomo
'Tracer bullets' expression?
on May 19, 2009 at 8:53:39 am

I have been asked to add some tracer fire to footage of World War I biplanes dogfighting. The scenes will be shot in France this summer and will be on standard def. Can anyone think of a simple way of tying tracking to the direction of Particular? Ideally I would just like to track the plane (one or two points) and then feed this information into Particular to give the direction of the particles, possibly using a light to simulate a 3D path.

Thanks

A



Vertigo Productions

http://www.vertigo.co.uk


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Kevin Camp
Re: 'Tracer bullets' expression?
on May 19, 2009 at 5:37:37 pm

it's easier than you think...

particular can use a light as an emitter. if you choose a light that is a spot light , the emitter becomes directional. you can adjust the cone angle to tighten the spread of the particles, the intensity adjusts the number of particles and the light's point of interest adjusts the direction.

just create a new light, name it 'emitter', set a narrow cone angle and make sure that 'casts shadows' is off. then set particular to use a 'light' as an emitter.

you can attach tracking data to either light's position or the light's point of interest, or attach tracking data to both. i would apply the tracking data to nulls, then parent the light's position/point of interest to the nulls. the tracker only works in 2d, so this would allow you to make some tweaks to the z positions.

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Kyle Hamrick
Re: 'Tracer bullets' expression?
on May 19, 2009 at 9:11:58 pm

^^^^^ What Kevin said.

If you're the kind who likes tutorials, Andrew Kramer has a couple that, while looking totally different from what you're going for, use a very similar technique to accomplish it.

http://www.videocopilot.net/tutorials/

Check out
#18 "Light Streaks"
#25 "Light Streaks 2"

Kyle Hamrick

Editor/Motion Graphics Artist

http://www.kylehamrick.com


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Antony Buonomo
Re: 'Tracer bullets' expression?
on May 20, 2009 at 6:10:26 pm

Thanks guys, I will check out that tutorial. And thanks Kevin for that tip about the light, I knew I could use a light as a motion path for Particular but I didn't know about the other controls. As it turns out it is proving difficult to use that technique on the piece of test footage that has been given to me, it seems to be difficult to 'aim' the particles so that they looked like they are disappearing into the distance and still follow the natural direction of the plane by using two trackers, nose and tail. In principle it ought to work, and be easy, but I haven't quite nailed it yet.

The shot I have been given has the camera mounted on the side of the plane, looking forward and showing most of one wing. I have simply tracked one point near the nose and stuck the emitter (using 'Directional') to that. I am manually fiddling with the XYZ rotation parameters until it looks right. Can you see any way of using another tracker, further back on the fuselage (to give me a straight line along the plane), somehow taking this information and plugging it into the rotation parameters so that the particles always shoot along the planes axis?

Cheers

A



Vertigo Productions

http://www.vertigo.co.uk


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Kevin Camp
Re: 'Tracer bullets' expression?
on May 20, 2009 at 6:40:10 pm

i think the problem is that you can't track in 3d... so you can't get the data to properly line the bullets' path up with the shot

you might try just attaching the spotlight emitter's position to the wing, but link the light's point of interest to the position value with and expression like this for poi:

[transform.position[0],transform.position[1],value[2]]

this will link the x and y poi values to the position. now you can use the light's rotation values to 'aim' the emitter into 3d space. with the poi and position linked, i think it should follow the track fairly well with some tweaking of the rotation of the light.

one thing i omitted yesterday, but you probably figured out, that even though the spot light can control direction and such, you'll still need to set the emitter to 'directional' to use the spotlight's controls.... sorry for any confusion.

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Antony Buonomo
Re: 'Tracer bullets' expression?
on May 22, 2009 at 6:02:27 pm

Thanks very much Kevin! That worked really well. I duplicated the emitter and then parented them both to a null. I now have a pair of front mounted guns that I can move around together and attach to the appropriate tracking point. I guess it's not quite how I envisaged doing it but it works and looks good.

Thanks again.

A



Vertigo Productions

http://www.vertigo.co.uk


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