Tutorial - A delicate blend
05 March 2008
Edgeblending is an incredibly powerful technique for combining projected images to create large displays of almost any size and shape. Here are the basics of how to go about it, provided courtesy of Robert Drake, Technology Research Engineer at TV One.
Edgeblending is a method used to create a wider, taller or larger video display. This is done by using two or more video/data projectors to create a single image. Each projector being used will output a portion of the desired image, overlap with the outputs from the other projectors and using an Edgeblending process, the image edges are blended together to create one clear and bright, seamless image.
This process has been used for many years with displays often being seen in staging and live performance venues, however it is becoming more popular across a vast range of industries and markets such as churches, government, and even in places such as galleries, art centres, museums, conferences etc. Edgeblending techniques offer the ability to do very large and very clear displays in a simple and efficient manner, and already there are some very innovative displays out there. It offers a real freedom to create anything as you are not limited to size or shape. Even 3-dimensional Edgeblending is now being seen across the world, full 360 degree blends that can envelop a person completely and create a full and lifelike illusion of standing anywhere in the world, or even the universe.
Although Edgeblending is not the only solution for obtaining bigger video displays, it is generally thought to be one of the most efficient and cost-effective. Obviously you could use larger LCD screens to create a larger image, however there is a limit on the size of each monitor, and more often than not, several monitors are required to create the desired size of the image that the client wishes to display, resulting in lots of heavy and expensive LCD screens having to be arranged together. Even large LED walls are not always the best solution as they can also be very expensive, as well as bulky and heavy when you are travelling between events or creating backdrops for live performances.
If Edgeblending is the desired choice for the large display then there are several processes involved in getting it right. The source for your image will firstly have to be divided into the several portions required for your blend, whether you have a 1x4 projector set up, or a 3x3 projector set up. Each projector will require an input that is the relative portion of the image that it is projecting, plus around a 10 to 20% overlap section. This is where the projector alignment plays a vital role in the set up of your blend.
It is critical that your projectors are stable relative to the surrounding area and also to each other, this can sometimes result in complex rigging being assembled to house all the necessary projectors but it is extremely important that it is done correctly. If projectors are housed independently of each other, you can quite often find that one projector could be susceptible to movement which can disrupt your entire blended image. It is also worth noting that if your Edgeblend consists of three or more projectors being used, it is often best to start with your middle projector and work outwards when aligning them. It can be very frustrating to start at the bottom right hand corner or such like, only to find when you come to align the last projector at the top left hand corner, the whole set up needs to be adjusted in order to fit the screen.
Once all of the projectors are aligned and stable to the surrounding area as well as each other, you will notice that the overlap of images is slightly brighter than the rest of the image. This is due to the luminance of the projectors overlapping. Therefore gamma correction will need to be applied to this overlap area to ensure that the brightness is uniform throughout the entire image.
There is another correction that is also used, which is the adjustment of the black level. Projectors in general don’t project true black, as there will always be a certain amount of light that is being projected. This results in a slightly lighter shade of black where two projectors overlap. The black level on the image portion that is not being blended will need to be adjusted in order for its black level to match with the overlapping area. If this is not done correctly and your image goes to a solid black colour (the same is also apparent on a solid white colour), there will be a lighter shade of black in the overlapping areas, making the blended area clearly visible to onlookers.
Another point to note when doing Edgeblending is to ensure that your projector bulbs are emitting the same colour temperature. Projector bulbs can fade over time and there is an obvious difference if using a bulb that is only one day old and a bulb that is three years old. When using a new bulb, the picture should be a clear bright image whereas older bulbs can give off a slightly more orange-brown image. Over time, you may need to adjust the colorometry of the projectors within your Edgeblend to compensate for the degradation of the bulbs that are being used so that the colours and brightness are equal across the blended image.
Edgeblending software is available in many different forms, depending on the source that you wish to blend. There are many PC based applications available if your source is PC based, that will divide the correct portions of your image to separate graphics cards which will then output to each projector in your set up. Some systems will even handle the blending of the image within the software but some require external hardware to control the blended portion of the image.
It is also possible to Edgeblend using video sources such as live video from a camera or the output of a DVD player. Generally this image would need to be fed into a piece of hardware such as a TV One Video Processor. This hardware, using the incorporated Edgeblend feature will divide your image as necessary and allow you to adjust the gamma and black levels accordingly to produce a seamless blended image. It is also beneficial to have the ability to adjust Aspect Ratio as video sources tend to be 16:9 (or a similar aspect ratio). If you are projecting onto a screen that is a different ratio, maybe it is wider or taller, then aspect ratio conversion will also need to be employed. This is best done after the projectors have been aligned. Finally, if your source does contain live (moving) video, then it is worthwhile genlocking each output to the others’ as this will eliminate any line or frame drift, which may cause frame-rate conversion problems.
Used the world-over, Edgeblending is a very cost-effective and efficient method of producing the most innovative and stunning, large displays of clear and seamless images, and with the ever-increasing demand from clients to be bigger and brighter and more unusual than ever it seems that this method will become increasingly more popular.
By Robert Drake
The author Robert Drake BEng (Hons), is a Technology Research Engineer for TV One.
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