The Basics
Edge Blending and Stacking are a projection methods that allow one to combine the brightness of multiple projectors into a single seamless image. This is often useful in large-format projections such as 3D projection mapping on large buildings, or ultra-wide content on large screens. As video projectors are limited in brightness by pure physics, this method allows one to overcome this limitation and project on much large areas than would have been possible with a single projector. In some cases this can also save costs by allowing the combination of smaller and cheaper projectors, although more time and effort required is required for alignment compared to a standard projection.
Edge Blending
Edge-blending can be thought of as a “tiled projection” in the sense that each projector covers a different area of the surface. A technician will aim each projector to cover a unique area of the final canvas in rectangular tiles. However an obvious problem arises here – as now instead of a single large-format image, you would now see repeated “tiles” of the same image. To solve this issue, a projection technician will use specialised software to “route” only a small corresponding portion of the image/video to each projector. This can take time to align, skill and planning but the final result should be that each projector only beams a portion of the final image. These portions all combine to make a much larger, brighter final image.
There is one final detail in the process that needs to be noted, which is the “blending” of the “tiles”. The projection technician will naturally allow a small overlap (typically about 115%-20%). Left unattended, these overlaps will often have hard, unnatural edges and ruin the illusion of a single large image. The audience will see the image broken into the edges of each projector, and the visual result will be negatively affected. To solve this, the technician will “soft edge” the edges of each “tile”. Done correctly, this allows a seamless blend between all the tiles – it should be impossible to see that the image is made up of multiple smaller areas.
Stacking
Stacking is also used to increase brightness, but instead simply overlays two or more projectors on top of each other. An analogy could be to think of shining one vs two torches as the same spot – the brightness would effectively double. Typically the projectors would be positioned as close to each-other as possible – if they were far apart the distortion of the image would make it almost impossible to perfectly align. The same duplicated image would be sent to each projector, and aimed at exactly the same spot. However, just moving the physical projectors will seldom get the image perfectly aligned, as even a 0.1 degree difference between the two would ruin the image. The solution is a method known as “keystoning”. There are two different keystone methods:
Software Keystoning: Some lower-end consumer projectors do not have built-in keystoning options. In this case, specialised software is used to ever-so-slightly keystone the video content sent to one of the projectors. This is processed on the computer’s graphics card, and must be sent as a separate video feed as you are now effectively sending two different videos (although they would appear virtually the same to the naked eye).
Hardware Keystoning: Pro projectors often come with built-in keystoning options, and can process these adjustments within their video processing hardware. This is usually the preferred method as it lowers the processing needed on the computer driving the content. Often a remote will be used to do this more easily.
The result in both the above cases, should be two or more perfectly aligned images.
Sensitivity to Movement
Unfortunately, with all this fine aligning the opportunity for misalignment from unintended movement is easy. Even the smallest movements can throw alignment off, so the mounting structure for the projectors must always be stable and secure. Footsteps, wind and even changing humidity can misalign projectors. Often metal cages will be engineered for projectors to bolt onto each-other to mitigate this.
Summary: Stacking vs Edge Blending
Both methods have allowed truly magnificent displays on large buildings and other surfaces. Although it can be complicated and sensitive to movement, it allows us to overcome the physical limitations of a single projector. Each method has its perks and disadvantages:
Sensitivity to misalignment: Stacking is more sensitive to edge-blending as 100% of the image must be perfectly aligned, compared to only 10%-20% for edge-blended.
Processing power: Stacking typically does the adjustments on the projector’s hardware, reducing processing load on the computer or media server.
Lenses: Stacking requires wider lenses to cover and entire area, and edge-blending requires narrower (longer) lenses.
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