By the end of this article you will know what projection mapping edge blending is, why we use it, and how to do it yourself. In previous posts we looked at how to projection map on many different surfaces ranging from geometric to three dimensional. However since all of those examples were small we were able to get away with using only one projector. In the real world we are going to need to deal with situations where we want to cover a large area with projections.

Let's say we have a set that has two flats on either side of the stage. One way we could approach this is by using two separate projectors one for each individual flat. This would be simple to do since the flat fits within the output of the projector beam and we have covered in previous posts how geometric projection mapping works.

But what if we had a large wall in the background that we wanted to projection map on. One way we could do this is to grab ourselves a super bright projector and choose a correct lens that would cover the entire wall and call it a day. And while we could do this, large projectors are expensive and bulky. The most obvious solution would be to use more than one projector. Using multiple projectors has lots of benefits! We can cover extremely large surfaces, have a brighter image, and have a much better pixel density. But how exactly would we approach this?

One method would be to use a technique called image butting, which is when we take the images of the projector and try to align them perfectly along the edge they meet. However this technique is extremely annoying, and can oftentimes be impossible. This is because every projector has different attributes that can cause one to differ from another. For example one could heat up and warp the image slightly differently than the other. This would cause a very noticeable gap in the image if everything is not perfect.

This solution to this is to overlap the two outputs of the projector in the middle. This will allow for a buffer zone of sorts to be created where we don’t need to be super careful when setting up the projectors. But this will cause us another headache. Whenever we shine two light sources on top of one another they get brighter, and the exact same principle applies for projectors as well. Whatever portion of our images are overlapped will be twice as bright as areas that are not overlapped.

To solve this issue in our projection mapping software we can cut off part of the image on both sides to make the images not overlap. However this brings us back to our original problem of things not lining up exactly and being noticed. We need to find a way to seamlessly merge the two images together.

Thankfully there is a technique called edge blending that does just this. We can take the output of one projector and define the region that will be overlapped. We then can create a gradient that will fade out the image so we don't have a harsh edge. Next, on the other projectors image we will do the same thing with the reversed gradient. When we put it all together the image will be seamless and will be more tolerant to changes. 

Since edge blending is a common thing to do your projection mapping software will most likely have it built in. Today I am going to show you how to do it using QLab. If you need a refresher on how to projection map with QLab check out this video here. The only thing we need to begin projection mapping with edge blending is two or more projectors.