I'm playing around with some green screen ideas at the moment and one of them is to combine a fancy chessboard and insert my girlfriend walking around on it. Getting the shot for the chessboard and the green screen video clip will be ok, it's just a set up query I have and was wondering if anybody can help?
So the chessboard will be a static photo. The camera will be slightly above the board, slightly angled down and taking in the whole board. I plan to start with a simple piece and just have my girlfriend walk through some gaps on the board and end up in the midle of it. is there a simple rule of thumb or calculation that allows me to position the video camera correctly when filming her walking around on the green screen?
You have to start with the basics: Perspective is determined by camera position, framing by focal length. then you factor in scale.
If yuu shoot the chess board with the camera tilted down 10º, then you shoot your actor wih the camera tilted down 10º. That is the easy part. Here's how to handle the scale issue.
If the squares on your chess board are two inches square and you want your actor to look like they are standing on a sqare that is two feet square then you have to multiply the distance the camera is above the chess board by 12 becaus there are 12 inches in a foot. That means if the cameraa is 10 inches above the chess board and 24 inches from the center then the camera must be 10 feet above the floor and 24 feet from the virtual center of the board.
Another technique. would be to set up the camera in the greenscreen stage first then put the chess board on a C-stand or even just hold it up in front of the camera to get the perspective you want. Take some measurements and then shoot the two shots.