Anything you can write is “shootable”… for enough money… ask James Cameron:-)
Have you looked around online to see if there isn’t already some footage like this that you can purchase? COW member Eran Stern I think has a company that sells nothing but stock-footage green screen elements like this. Also Artbeats and Digital Juice may have something you can just buy off-the-shelf. You would be prudent to research if this is indeed cheaper than or equal in cost to shooting your own.
Assuming you *have* to have a fresh one shot, say, because you need them to be wearing a certain shirt or because it has to be a recognizable person, your choices are to spend money on the shooting outside in the park/athletic field with green screen elements, or to spend more time and thus money in post-production editing, doing what is called “digital rotoscoping”, which is the painstaking process of cutting the guy out from the background by hand, a frame at a time. IF you have never done that before, you are looking at a LOT of time to spend doing that.
But you’re the only one who knows what’s going to be more expensive overall, since in your situation the editing/compositing time might be for all practical purposes “free”, whereas getting the green material shot in useable quality and getting crew to set it up and, most vital, someone competent to shoot it right may cost more.
If the green screen isn’t shot well, you’ll STILL have to resort to the rotoscoping process. Some tricks of shooting can help minimize problems though.
For example, by shooting with a lot of low-angle shots, which are mostly blank sky, you have much less roto work to do in the first place, you might even be able to pull a blue screen key off the blue sky for free, or a luminance key, if the sky is overcast. Shooting with a shallow depth of field makes everything behind the jumper so blurry you might not need to do any keying.
And that gives me an idea for a third choice for you. If you can very narrowly pre-define the few EXACT camera shot angles that will be used in the final version, and you use a very shallow depth of field, then instead of setting up a platoon of green screens like the artist Christo, you could go to Kinko’s and blow up 2 or 3 stock photos of a stadium crowd or whatever, stretch those on large wooden frames as if they were stage theater flats, (yes, these could even BE painted flats) and stand them up outside in the park. Frame the camera precisely. The lens will blur the poster/theatre flat thing enough at that distance that it will “read” on-camera like the real thing.
Remember what I call the “Kurosawa Rule”: Nobody else will know what was or wasn’t there, for real, *outside* of the actual frame of the shot. It doesn’t NEED to be, only to *appear* to be, and if you only set up the believable initial premise and visual cues, the audience’s brains will fill in the rest. Don’t build or pay for anything the audience can’t ever see.
I get that name for it from a story of an interview of the famous Japanese Director. The interviewer was amazed by a certain shot of an antique warrior castle used in the movie, asked him why he placed the camera in that particularly dramatic angle, what the significane was. Akira replied that that was the one *available* angle that showed the whole actual building, and that a fraction to either side would have revealed skyscrapers and telephone poles and etc. The audience of course knows nothing of this, and if you first put up a shot of a huge empty plain, they will assume the following shot of the castle is set on that plain.
Shoot your runner/jumper with all extreme low angles, long zooms from the front, and him running into the lens, with short depth of field, and maybe a high-angle shot from a ladder, looking down onto the landing area after the jump, and maybe you won’t need ANY chromakey or rotoscoping!