PROJECTOR INFO AND TIPS
The following is just some basic info I've learned about projectors and some considerations regarding placement and operation for various effects.
What to Look For
There are so many options for projectors now, and many of them relatively inexpensive. Before jumping on Amazon and ordering the cheapest unit you can find, save yourself some disappointment and ensure the projector specs are going to meet your needs. The primary characteristics you should pay close attention to are brightness, throw ratio, native resolution, and to some extent contrast ratio. Some secondary features that may be important are zoom/scaling, vertical and horizontal keystone adjustment, and an ability to play media directly from a USB drive.
Brightness is especially critical when it comes to projecting onto a translucent screen for a convincing "holographic"-style effect. Unfortunately, the brightness specifications given for a lot of projectors are not all that accurate or reliable. Brightness might be specified in ANSI lumens, lux, or just lumens. My best projector for our main 12'x8' effect is rated as 3300 lumens, and it's far, far brighter than a couple of my others that claimed to be 3500 lumens. Read reviews by verified purchasers carefully. Know that generally, LED projectors aren't going to put out as much light as halogen-based units at an equivalent price point.
Throw ratio is an important factor that will determine how far away your projector will need to be positioned from the screen or surface to produce the image size that you want. This ratio is determined as the throw distance (distance between the projector lens and the screen) divided by the width of the projected image. A throw ratio of 1 or 1:1 means that these measurements should be equivalent. If the projector is 10' from the screen or wall, the resultant image will be 10' wide. A ratio of 2 or 2:1 means that the image width will be half of the throw distance. At a 10' throw distance, the image will only be 5' wide. Typically, lower throw ratios (less than 1 to 1.5 or so) are considered "short throw" projectors. Some modules have an optical zoom feature that will allow a limited adjustment of this ratio.
Native resolution can greatly affect the visual quality of your effect. Native resolution is the actual number of pixels displayed by the projector and should not be confused with "supported resolutions". Be warned that almost every projector's description puts "supported resolution" up front, but sometimes doesn't make the true, native resolution obvious. Plenty of models say they support HD/1080p, but they actually scale the source video/image down to the projector's actual resolution. A model with a native resolution of 853x480 is not going to produce the level of crispness and detail that another with a native resolution of 1280x800 or 1920x1080, even when both are displaying the same HD source file. The larger your projection is going to be, the more a higher resolution will make a difference in your image quality. If you are creating a small effect, like projecting on a tombstone or on pumpkins, a cheaper, low resolution projector will probably be fine. A 1280x800 projector works very well for my 12'x8' effect. If I were going to do a very large projection mapping effect on my house, I would almost certainly want a true HD, 1920x1080 model.
Contrast ratio is another specification that can be misleading and unreliable. Generally, the higher the contrast ratio, the better the range of colors and, typically, the deeper the blacks in your projected images. This is another case where it is best just to read reviews carefully.
Keystone adjustment will give you flexibility in how you place and position your projector. Some amount of vertical keystone adjustment is pretty common in most projectors, even cheap ones. This will allow you to correct the distortion that occurs from placing the projector above or below the screen or surface. Horizontal keystone adjustment seems to be a feature rare to inexpensive models, but it can be important if you need to position your projector off to the side of your screen, especially in rear-projection effects, to make the source less visible. Some higher-end models will allow digital fine-tuning of the image corner positions to overcome almost any distortion.
Masking: Projecting onto objects or surfaces, but you can still see the rectangle or black area around your subject? If the contrast ratio of the projector isn't sufficient to make the black areas of your image really black, then you might need a way to mask the projector beam to better disguise the fact that a projection is in use. You can do this by cutting a hole in a piece of cardboard and taping the card over the lens. When the mask is that close to the lens, it won't produce a clean, focused shadow, but rather a soft, diffused gradient from around the projected image to true black as the beam is obstructed. If you want to crop the projection to a crisp shape for some reason, just mount the cardboard at a distance from the lens. You'll need to experiment with the size and shape of the hole to produce the desired masking of the projection.
Placement: For rear-projection effects, you don't want the bright "hotspot" of your projector lens to be visible behind your screen. This can be difficult to accomplish unless you engineer the whole effect to only be viewable from a specific angle and distance. A projector with horizontal keystone adjustment can be an enormous help, as the projector can be positioned far enough off to the side that guests won't see the source of the projection. You can also get creative and obscure guests' view of the projector by placing props in the line of sight to hide it, as long as those objects don't interfere with the projected image. For window displays, this problem is much easier to solve, as you can just place the projector below the height of the bottom of the window. Placing the projector way above the screen can also be an option, but make sure to look at your projection from childrens' height to make sure they aren't staring into the beam from their low angle of view.
One more thing to keep in mind if you are projecting onto a highly translucent screen (like when using tulle, chiffon, or other similar materials) is that the majority of light from the projection is going to pass through the holes in the fabric's matrix and continue until it lands on another surface. For this reason, be careful not to position your projector and screen in a way that the rear-projected image will be cast onto a visible surface between the screen and your guests.
Lighting: Just a couple of notes regarding lighting and projection effects. First, be aware that bright projections, even on translucent screens will change the light levels in the whole area around the effect. This can be a good thing! Spirits and skeletons can produce a ghostly glow over the area that improves the illusion. Just be sure it's not lighting up anything you don't want to be seen.
Rear-projection "holographic" effects tend to be more convincing and eerie when there is scenery visible behind them. Use and position lighting judiciously to make the background visible, but no so bright that your projection gets overpowered or washed out. Ensure that no light from your accent lights, floods, or spotlights falls on your projector screen!