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LED Basics for TouchDesigner Pros

The first time you use any kind of hardware can be daunting. There are new terms, new techniques, new bottlenecks to your workflow, new people to deal with, and all kinds of crazy things that you maybe never even expected. LED walls are no exception to this. While they are amazing to use and can provide us with stunning canvases, they come with their own headaches and your first time using them can be quite daunting. Let’s dive in!

Raster maps

The first big concept you need to know about are raster maps (or pixel maps or LED maps… or anything with the word map in it…). This is an image that shows you where from your 1920×1080 or 4K output will ends up on which of the LED panels. The reason we need this is because in between every computer/media player and LED wall is an LED controller. These controllers take the incoming video signals, crop out the specific areas needed, translate all the data, and send it out to the actual LED tiles. Because LED walls are usually non-standard aspects and can be quite creative, you’ll almost always have to prepare your content on a raster map. Here’s a really simple example I made:

What you see in the above will be in almost every raster map. You’ll have a black or white or checker background. Then you’ll have shapes drawn on it that denote where the LED controller will crop the content from. And finally you’ll have pixel coordinates on the corners of the shapes that will help you when you need to position your content on the raster. This example would be for an LED wall with a resolution of 590 pixels by 440 pixels. The 10,10 in the top left tell us that the content should start 10 pixels in from the left and 10 pixels down from the right.

When you’re sending your output from TouchDesigner to the LED controller, it would look something like this:

The LED processor/controller would then crop out that part and you’d see a nice looking image on your LED wall. These can be as simple as I made above, or they can become quite complex, but the fundamentals are still the same. Take your content, crop and transform it around onto the right parts of the raster, and then output them. Nothing to be afraid of.

Refresh rates

Refresh rates shouldn’t be too complicated but it is something to be aware of. Often times LED walls refresh at a much faster rate than traditional computer screens. I’ve seen everything from 60 hz, 75 hz, 90 hz, 120 hz, and more. It’s good to ask the LED technicians/company what refresh rate their LED wall is and if it’s adjustable. Your ideal scenario is to either be at the same refresh rate, or a number that evenly divides the refresh rate. For example if the wall is at 90hz, maybe 30 fps or 45fps are where you should aim your project. That way frames are either tripled or doubled equally. Problems could arise if you have for example a 75 hz wall and your content is running at 60 fps. Somewhere between these 2 systems, 15 extra frames need to appear per second. Often times either the GPU or the LED controller will just double random frames to fill the extra space per second. This is a common issue called judder. It’s when there’s a mismatch between the refresh rate of the screen and the amount of frames generated by your application, which looks like stuttering on the screen. Again, it’s not a hard solve, just something to ask about ahead of time. If the refresh rate is 120 hz, either 60 fps or 30 fps will work fine. if it’s something like 75 hz, you may want to ask the LED tech’s if they can change it to either 60 hz or 90 hz to avoid judder. Ask often and confirm early in the project!

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Previz Dangers

We all love previz. It’s the new cool thing that everyone is doing for their clients. It makes all of our lives easier by making client approvals smoother and faster. But there can be some downsides. If we previz something incorrectly, you might get buy-in on one thing, and then have the client completely stumped and surprised when it’s on-site time (a terrible time to have surprises…). A common previz failure often happens with LED walls. Because of the nature of most LED walls (not including some of the latest and greatest ~1mm pixel pitch walls), is that there is a visible black space between each pixel. If you stand right up in front of an LED wall you might not even be able to tell what’s on the screen! The lower the resolution, the farther you’re recommended viewing distance is to get a good experience. Now imagine if you went into TouchDesigner, took your movie file, textured it on a Grid SOP, and then rendered it and called it the LED wall. Your client (who also might not have experience with LED walls) will think it will look as crisp and clean from any angle…which may or may not be true. I’ve seen all too often clients get surprised when they see the LED wall for the first time and they see all the spaces between the pixels that weren’t represented in the previz.

Solutions? There are a lot of different solutions that depend on how deep into the weeds you want to get. You could instance circles in TouchDesigner and make the measurements between them accurate to the dimensions of the LED wall. Then sample the colours from your texture and use those to colour the instances. If you’re looking for a quicker fix, you could make a texture grid of many small squares, then multiply your texture against it so that it looks closer to an LED wall. In most cases, the 2nd is more than enough, but if you want to get really technical and accurate then the first is the way to go. Either way, just be aware of the fact that you should try to incorporate the reality of the LED tiles into your previz.

Wrap up

LED’s can be daunting to work with if it’s your first time getting to work with them. But following some simple rules and knowing about the terms and specs can make your life a lot easier. Hopefully between raster maps, refresh rates, and pre-viz warnings, you should have most of the important bases covered so that you can relax and have a blast with those bright and high resolution displays! Enjoy!