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Front Projection vs. Rear Projection: In-Camera Compositing

Why use rear projection? Aside from the retro-novelty look of rear projection, cost and ease of use are most compelling reasons. It’s the cheapest means of in camera compositing. However using a rear projection screen is an exercise in compromise. For any given screen material used in rear projection, the opacity of the screen determines performance. The greater the opacity of the material, the sharper the image will appear, but there will be a sacrifice in brightness. Our major concern during the shooting was  making sure the background image would appear bright enough against the lit actors. After testing a few materials we settled on drafting paper as having the best balance of opacity. Vinyl sheet caught the image in much sharper focus, but was significantly dimmer than the drafting paper. Not least of these concerns was cost. A roll of drafting paper is relatively cheap and fairly strong, so it was easy to work with.  The production schedules for no-lo budget movies are very abbreviated, any special effect or technique you use must be easy to work with and set up fast. The rear projection screen fit this requirement.

The rear projection screen is not all gravy though. Rear projection screens of any significant surface area take up a lot of space. The space from the projector to the screen forms a frustum that can not be interrupted by any object. The larger the screen you wish to use the greater the volume of space the frustum will take. This limits your shoot locations to large empty spaces, sealed from outside light. Such locations can be very difficult to find for lo-no budget films. Fortunately, we had such a location with The Palo Alto Media Center, a television production studio which graciously hosted our shoots.

An additional consequence is the loss in brightness of the image as the projector moves farther away from the screen. The projector is  after all still just  a light source and is subject to the same “drop-off” as any traditional light source. So if you want  big rear projection screen, the distance from the proctor will increase causing the projected image to dim.

Light spill is a major issue with rear projection screens too. Any light that falls on the screen from the camera side will be visible to the camera. Trying to avoid light spill on the screen caused us to illuminate our actors from a high angle, and throwing a very narrow beam. This high angle lighting is not always flattering to your actors and severely restricted the areas that they could perform while lit properly. As we shot a fight sequence against the rear projection screen,  the restriction of movement imposed by the size of the screen and narrow lighting caused us to shoot the sequence as a series of short takes, which in turn required a high editing workload.

Stanley Kubrick was the first to use front projection successfully in "2001: A Space Oddyssey"

Front projection is functionally superior to rear projection in nearly every way. It is far less sensitive to light spill and requires less space than rear projection, but it is a bit more sophisticated system that requires special equipment and materials.

Dalite retrorflective screen. Used in this picture as a chroma key for compositing. The actual color if the screen is grey. The blue color seen in the photo is reflected from a blue LED ring light mounted to the camera lens.

Let’s start with the screen. Almost anyone who talks about front projection refers to the screen as Scotchlite. This is 3M’s brand name for their retroreflective material and is most familiar to us as the highly reflective strip worn by firefighters. The retroreflective character of the screen means that light which hits the screen is bounced back at the same angle it came in on. To get a handle on the nature of retroflection, imagine that you are throwing a tennis ball against a magic wall. No matter what angle you are to the wall every time you throw the tennis ball at the wall, it always bounces back to you. That’s retroflection.

However, Scotchlite is hard to find by the sheet and it isn’t cheap. Retroreflective paint, (used for highway markings),  is available but I haven’t found any reference or accounts of it’s use specific to this kind of special effect. The use of retroreflective paint would be experimental, but I’m confident it will probably perform as well as any Scotchlite screen. In fact it may perform better than a commercial Scotchlite screen as there will be no seams on the screen surface. The use of retroreflective paint also brings the cost of the screen down to the level of a rear projection screen.

The Scotchlite screen is really only half the story. Front projection requires that the both projector and camera be aligned coaxially. The camera and projector are aligned with use of a half silvered mirror, (or beam splitter), very similar to a 3D rig. Once again though, the glass is not cheap and constructing the frame that aligns the glass, camera and projector requires decent machining skills. Fortunately there are Scarycow members with experience building and using such rigs, and I hope to leverage their knowledge.

To actually use front projection will require some experimental test shoots. My plan is to test the basics of the technique with a tabletop set up. I’ll paint a small surface, (4 square feet), with retroreflective paint to serve as the front projection screen, and place a puppet or one or my daughters’ dolls as an “actor” in front of the screen.  Small half-silvered “sample” mirrors are available for nominal cost, and the size of these mirrors might be perfect for my DSLR. The optics involved in such a set up are no different than with a full scale set, so there’s a lot of opportunity to learn with minimal hassle.

At this point, the question should be asked, “why not use green screen compositing?”. The obvious advantage is the characteristic look that front/rear projection adds texture to your movie, and I have read some arguments that having all the composited elements exposed together in camera gets you a superior  finished product.  But the reality is that this look could be achieved by skilled visual effects guy using chroma keyed footage. Practically, green screen is incredibly flexible and has enormous advantages in simplicity and execution, but doing it right is not as thoughtlessly easy as we’re often led to believe. Light bouncing off the screen can often appear on your actors as “green spill” which makes the compositing process more difficult. To avoid spill your actor must be placed a necessary distance from the screen. To pull a really good chroma key, the screen must be lit evenly to create a fully saturated color field behind the actors. Not least of all this is the need for a skilled VFX artist in post to construct a really good looking final composite. I don’t think there is any way to cheat the amount of work involved in creating good composite effects. With projection screens the work is in production, with chroma key the work is in post, but no matter what method you choose you owe it to your s/v effects guy to plan and prepare as best you can before you shoot. In that regard, projection screens and chroma keying both require the same amount of work in preproduction.

Coming soon: Zoptics, Introvision and other advanced uses of front projection.