A portal effect relies on matching camera position and angle in both shots so the perspective works and the ability to separate the actor from the background. It's all layers and timing. The effect is a lot easier to pull off if you don't move the camera.
Basically you put your actor in the scene they are walking into first, set up the shot taking note of the distance from the camera and the angle from the camera. Then you shoot a clean plate without your actor. No actor in the scene. Now you go to the second location, where the actor walks into the portal and you set up the shot so the actor is at the same angle and the same distance they were in the first shot. You set up the scene so that it will be easy to cut the actor out of the background. Nearsow you shoot another clean plate - without the actor, then you shoot the plate (shot) with the actor in the scene.
In post you put the footage with the location where the actor walks into the scene on the bottom layer, put your the scene where the actor walks through the portal on the top layer, and the shot of the portal on the second layer. That's all there is to it. If you carefully match angles and distance from the camera the shot will work. If you don't it will look fake and you'll have to do some distorting to sell the effect.
If the actor is supposed to walk in front of objects at the end of the shot you'll need to rotoscope these objects so that you can put them on a separate layer. It's all about layers, angles and lighting.
I don't have a single tutorial that I can point you to because this is more complicated than something you can shoot and edit in an afternoon unless you have years of experience doing visual effects. If it were an assignment in a class that I was teaching I would give the students about 10 days to complete the project.
Hi Rick, thanks for your reply.
I understand the method you're explaining, but what I'm curious; wouldn't it be hard to match up the actor exactly in both scenes so he does the exact same movement? There will be no distortion, no cutting, it will literally look like the actor comes out of a door from one environment to another. Is there no way to use a form of green screen and then use a plain shot from the first environment as the background?
Here's somewhat of what I thought of, I would just record one piece of footage with the actor, whereas there's a door between him and the camera, and a green screen behind him. Initially, the camera will be close enough to the actor so that the door isn't visible, but will slowly move back as the actor slowly comes through the door. I will then record some footage in the first environment and use that as the green screen, and colour correct the actor to look like he's in the first environment. Once he's through the door, I will remove the colour correction and simply use the raw footage, while still maintaining that green screen as the first environment. The actor can then close the door and the green screen will be removed.
I think the above idea would work well, but I'm not really good with After Effects, I'm still just a beginner-intermediate user.
The actor will not walk in front of objects, and will simply walk in a straight line by the way. The shot will only be about 20 seconds long, with him talking before he goes through the door, and talking more after he's through the door.
Andrew Kramer does a portal tutorial. It doesn't quite cover the complexities you or Rick have mentioned, but might help you out in finishing the look of effect.
First thought, 20 seconds is an eternity in a film. Most shots are under 7 seconds. A walking through the portal shot will work just fine if it is only 2 or 3 seconds long. The rest is in the editing.
Second thought, Separate your actor from the background any way you can. Green screen is OK if you have a decent camera, but shooting against black or even shooting at night with light only falling on the actor will work. There are dozens of ways to fairly easily separate an actor from the background. If his position relative to the camera is about the same you can remove the background and then move the layer to line him up precisely with the master shot (plate). You don't try and match the actor's position and action in the new location by shooting him actually in the new location. You composite the actor into the scene, then you edit to a different angle to continue to tell the story.
Third thought, CUT... As I said in the previous paragraph your actor through the door using a composite, then cut and continue the shot from a different angle. The portal could still be in the background or maybe just a glowing light effect with the portal just off screen. Most walking through a portal shots you see in movies are only a couple of seconds long before it cuts to a different angle.
Last thought, you are over thinking this a bunch. Don't move the camera until you get some experience because if you move the camera you'll need to move the camera in exactly the same way in the master and the insert shot. The insert is the shot with the actor walking. The master is the clean plate of the new location that you want enter. Clean plates are clean - no actors. Shoot your actor so it is easy for him to be separated from the background. Match the lighting and camera angles and you are 90% of the way to a believable composite. The easiest way to set up the clean plate is to set up the camera on a tripod and frame the location that the actor is going to enter. Now put some tape on the ground and mark where the actor starts his walk. Then have the actor walk into the shot to the place where you want to cut and put another piece of tape on the ground. Now mark a spot in the background that is exactly in the center of the cameras field of view and take some measurements. Measure from the center of the camera to the starting point for the actor by measuring from the camera's center line to a point perpendicular to the starting point. Then measure from the starting point to the center line. Now do the same with the ending point. Take these measurements to the new location and put tape marks on the floor and have your actor walk the walk.
Another technique wold be to use a portable green screen held by two assistants. You still give the actor marks, but after you shoot the clean plate you bring the actor into the scene, add the two assistants with the green screen, and then have them all walk to the camera. If you are careful with your lighting you're all set to separate the actor from the background and get your shot.
I would probably shoot the walking through the portal scene with the actor walking toward the portal and away from the camera, then cut to him coming into the new location walking toward the camera. If the camera is to his left when he walks toward the portal then it must be to his left when he walks through the portal. (Read up on the action axis) Design or create artwork for the portal that you can layer between the shots. Learn about track mattes and masks.
Take a look at Andrew Kramer's tutorial. That will give you one approach to the technique. His portal shot is about 2 seconds. It also does not show the room the actor is coming from.
Thank you both for your answers.
I think I should explain a little more in-depth into what exactly I'm doing, as although both your answers were helpful, I still feel very confused as I think the effect would be very complex to achieve.
Alright, so the short movie I'm making is more of an explanation short movie, whereas the actor talks to the camera and thus the audience, explaining about certain topics in technology. Now in this particular scene, he'll be talking about how immersive curved screens can be. Whilst doing that, I wanted to sort of highlight the point immersive, and so I was planning on making the actor suddenly go through a big hollow picture frame, from a nice garden into his house. The entire movie will be sort of based around some "magic", if you may, to make it more interesting and creative, and so I thought doing this "portal" effect would be a nice way to make things look surreal.
Now, moving on, I'm starting to think of scrapping this idea and trying something else instead, so do you guys have any idea of some way to highlight how immersive something can be, in a similar concept as the portal effect (with the actor being in a nice garden and then going into his house, sort of making the garden look like what you'd see on a screen), but perhaps through a different method rather than the portal.
Any help on this is appreciated.
Thank you very much.
Film masters on youtube has a Dr Strange portal effect tutorial.
Anything you do to simulate moving from one space to another is going to require separating the actor from the background and carefully matching camera angle and distance from the camera. You can create surreal transitions by zooming in or zooming out on an actor while he is walking toward the camera, or by moving the camera smoothly on dolly track toward or away from the actor while changing the framing with a zoom lens. This psycho effect has been used a lot in movies and eliminates the need for compositing. If you want to change locations then you need to separate the actor from the background and place him or her in a new scene. The transition is the key. Something as simple as a bulge effect may do it for you. Something as simple as keeping the actor the same size while smoothly zooming or moving the camera may do it for you. The design is up to you.
I still maintain that a single 20 second shot is an eternity in a film. It take a tremendous amount of planning and camera movement to sell an effect for 20 seconds in a single shot. You'll be better off doing your transition and then cutting to different angles. Each of the angles may be a composite but, especially for a new film maker, creating and successfully pulling off a magical transformation into another experience is going to be a lot easier to achieve if you do it in a series of short shots carefully planned and edited together.
I would try some experiments. Make sure you can get a good green screen (or blue) for keying and that you can make your lighting match. Use clean plates. Shoot 4K if you can for HD projects so you have more footage to work with when you are bending reality. If you cannot shoot with a green screen then make sure that your actor is shot on a simple background so your rotoscope will be as simple as possible. Good luck...