2. The Stereo Stage Volume
The Stereo Stage Volume
Now that we are making stereo movies we have to consider that the stage presented to the audience is no longer the flat screen but a virtual volume.
It has some particular characteristics and it is of course just an illusion!
This is why we have to be so careful in creating the footage otherwise we will break the spell.
First let’s define what this stage volume looks like.
Imagine a conical shape starting at your eyes and projecting out to the edges of the screen and beyond:
The closer you get to the observers eyes the narrower the stage volume gets. It is all still very usable – up to the chosen divergence limits – what needs careful attention is where objects break the frame.
The stereo stage volume in front of the screen needs to be handled very differently to the volume behind the screen.
Negative Stage Volume
You can treat the stereo stage volume behind the screen just like you are looking through a window. The screen is where the glass would be and you can see beyond it into the distance.
As long as you stay within divergence limits it is relatively easy to maintain the stereo illusion in this space. Even if an object breaks the frame it just looks like you cannot see it because the window frame is in the way. We are very used to objects blocking our view of others behind them so this is all very normal.
In this example we can see the actors torso but not his legs. Configured as shown it will not look strange at all – just like he was standing behind a barrier that obscures our view of his legs.
Positive Stage Volume
If we place our actor in front of the convergence plane then he will appear forward of the screen to the audience.
What happens if we break the frame now?
It looks weird because parts of him are missing!
In the diagram below we have created the illusion of his torso floating in space in front of us, but where are his legs?
There are things we can do to make this look less strange – for example if he’s wearing dark trousers then this reduces the contrast against the black surround on the screen and makes the break less noticeable.
It also helps if our attention is not near where the break occurs. If we are concentrating on dialogue and looking at his face then we are less disturbed by his legs being gone.
Us Humans only see clearly on a very small patch on our retina and we tend to look right at the object of interest – this is why in the above example if we concentrate on his face and the dialogue, we basically stop seeing the missing legs and his face is nicely forward converged coming out of the screen towards us.
If you do look at the edge of the image – where you expect his legs to be your brain tries to figure out how what your are seeing might be possible.
The best explanation it can come up with is that his legs must be behind the screen plane right ? thats’ why we can’t see them… but hang on – he’s converged in front of the screen – what is this stereo mess?
We’ve broken the illusion by doing this, the 3D illusion is weakened or destroyed AND you cause strain for your viewers as they try to sort out what is going on.
So if you must break the frame on a forward converged object please keep our attention as far away as possible from where the break occurs.
Here’s a really bad example !
Here we are breaking the frame on two sides at least – maybe more if we have framed his face off the side of the frame as well as the top.
This is a real mess in stereographic terms because the illusion will be really broken. There are depth conflicts everywhere you look in the frame. This will NOT deliver a convincing stereo effect and it will cause unnecessary strain for your viewers.
What you see here is the top of his head and legs obscured by the edge of the screen – but you also see him converged forward of the screen – so how can it be obscuring our view of his head and legs? A visual conundrum for which there is no explanation and so the illusion is broken.
The further forward you get the harder it becomes to maintain the illusion for two reasons:
1) The stage volume is getting smaller and smaller and therefore it becomes harder and harder not to break the frame.
2) To create the illusion of an object projecting a long way out of the screen we need lots of parallax offset. Be careful not to exceed overall divergence limits within the scene too much or for too long or the illusion will break.
The screen we are projecting on is no longer a solid barrier, visually it just isn’t there! The volumes behind and in front of the screen are continuous and in fact it is very dramatic to see objects break through this “wall” and span from negative to positive convergence.
Here our actor is reaching through the screen towards the audience. His arm is fully connected to the stereo stage volume. There are no occlusions in conflict with the stereo presentation, even his legs being cut off is acceptable as he is behind the screen plane.
This will produce a very dramatic stereo effect, his hand really will appear to be reaching out of the screen towards us.
Better still it will be very low strain vision for our audiences.
We can converge floating objects forward of or behind the screen plane without introducing stereoscopic problems.
Dust, ash, smoke etc all hang in the air and will appear to hang in space in the stereo illusion. They can be very effective in enhancing the stereo effect.
The physics of them is plausible to us – we are used to seeing a wisp of smoke float in front of us.
Again – if you break the frame in the positive stage volume it breaks the illusion – we are used to seeing objects disappear behind a barrier – say the edge of the screen window – but if they float to the side and just vanish its a bit weird… Try to frame floating object wholly within the frame if they are forward of the screen. Behind the screen plane they can enter or exit from the sides all they like without disturbing us.
A lot of the effects we are used to in 2 dimensional vision also work in 3D.
Point of View is one of them. If a presenter looks at camera they seem to be looking directly at you no matter where you sit in the audience. The same is true in stereo – if they look at camera they are looking right at each person. If they point a stick at camera that stick will be directed right at every person in the audience, the experience is the same for everyone.
If you sit way off axis in the cinema you will be looking not at a rectangular screen but at a trapezoidal shape. Although this is not an ideal viewing position in 2D or 3D you will still see the full stereo effect and just like in 2D you gradually lose yourself in the story (if it is any good!) and stop seeing the geometry of the screen.
It is not wise to push this too far however, and certainly a lot less than in 2D. This is because our main depth queue in stereo is horizontal parallax so we want the left/right images delivered into the audiences eyes to be offset left/right with no vertical component at all. Provided you’re not way off to the side and if the parallax offsets in teh images are not too great the vertical distortion introduced will be small and your viewers will accommodate it easily enough.
Correctly utilising the stereo stage volume is key to producing effective stereo vision. Remember that the stereo effect we a re creating for the viewer is an illusion. We are deprived of many of the depth queues we perceive in the real world so it is really important not to break the ones remaining to us.
It is not difficult to set up each shot to work with rather than against the stereo effect but it does require that you plan for it in every shot. This is nothing new – our DP’s have always composed each shot to maximum effect, it’s just that the rules have changed a little in 3D.
Breaking the frame is perfectly acceptable in negative stage volume but is very damaging to the effect in positive stage volume.
This is not to say that you cannot have positively converged objects in your shots – they work great and can be some of the most powerful and dramatic visuals. All we are aiming for here is to work with the stereo illusion rather than against it – which is to say to maximise its effect.