As filmmaking continues to adopt digital tools and techniques into its creative framework, additional elements in the creative process have to be more cognizant of technical specifications that could impact the way content is enjoyed.
In the past this was relegated mostly to post-production studios where the director’s vision had to be translated effectively for different screen sizes, this could range from large-format IMAX style theatres to personal mobile devices.
As digital cameras became the industry standard cinematographers were faced with new workflows, seemingly unlimited creative decisions, and higher levels of detail than ever before.
Similarly, animation and VFX sequences that were dependent on computer processing power and rendering technology now had space for more involved CG processes.
With a recently invigorated push for the development of virtual production techniques in film, we are seeing many of these digital tools being pulled into pre-production workflows, and, in the case of in-camera VFX [ICVFX], or the use of an LED wall, these techniques are being adopted on set in real-time.
This shift in film technology and application comes with its own lexicon of terms and processes as new workflows and creative applications of this tech further redefine what it means to be a filmmaker.
So, all of that said, what is pixel pitch and why does it matter?
A pixel is the smallest building block used to assemble a larger image, a unit of measure for digital imaging. Many of us are probably familiar with it in terms of photo resolution in raster imaging or from 8-bit art popularized in early gaming.
On a display, each pixel is represented by a light or small cluster of lights, defining the size of the pixel, small commercial screens like that on your laptop or phone which rely on OLED technology have Red, Green, and Blue LED making up a cluster which together make-up one pixel.
The number and size of pixels your display can produce, or the resolution, will have differing levels of quality as the physical dimensions of the display are adjusted. This scaling also affects the way the eyes or the camera read the screen from different distances. This is where pixel pitch comes into play.
Pixel pitch is a measurement of the density of pixels within a screen or pixel based device. This measurement, in millimetres, is calculated as the space from the centre of one pixel to the centre of the next closest pixel. Pixel pitch itself is a measurement of resolution, the smaller the pixel pitch the higher your resolution is going to be.
The pixel pitch of an LED wall is going to be comparable to live broadcast and billboard applications that are widely used in advertising or events already. These large format screens are great for viewers who are really far away from the screen itself, and ideal for the naked eye.
Every LED panel is going to calculate the ideal viewing distance a little bit differently for their product, but a good rule of thumb is to consider the minimum viewing distance for your screen to be 8 metres for ever 1 millimetre in pitch.
The higher the resolution of your screen the closer to the screen the viewer can be before the image becomes unreadable — to put this into perspective a digital billboard with dimensions around 12’x25’ will lean towards a pixel pitch of 20mm, meaning there is approximately 2cm of “dead” space between each pixel. This ensures that the image is readable at great distances and little to no information is lost to the human eye. This results in a low resolution screen around 360x760 pixels. Considering the use case and the readability goals for a billboard, this resolution maximizes the size of the screen while minimizing the LED and energy costs to operate.
When you’re using and LED wall for film or video production there are other factors that come into play — for one, the viewer is no longer the human eye but is instead the camera, as well, the ideal viewing distance isn’t 100s of metres but is going to need to encompass a set space that the camera will operate within while also allowing the camera to get as close as possible.
Of course you can cheat perspective with an LED wall by forcing the background to render the background in a way that makes it look like the gap between the subject and the background is different than what it is in reality — You can check out more about this effect in our post How Parallax Works in Virtual Production.
But typically, when you’re filming in a soundstage with an LED Wall the camera is operating within 10-15 metres of the LED wall, which means your pixel pitch is going to need to be much smaller.
Different cameras and lenses read images on an LED surface differently than others, so your pixel pitch needs are going to differ depending on what equipment you have and how close you are getting to the wall.
At this point, the standard for film production ranges between a pitch of 1.6 and 2.6.
Options are already being revealed that close the gap even further, but for most use cases a pitch of 1.6 is already going to be excessive and any pitch smaller than that is going to come with an astronomical price tag and virtually no visual improvements on camera.
When matching up an LED panel with your filmmaking workflow there are other concerns to keep in mind. Moire for example, is when the grid created by dead space between pixels on your LED wall conflicts with sensors in your camera creating visual artefacting and aliasing in your footage — You can check out more about how screens and cameras interact in our post The Silent Killer of Virtual Production.
In theory, this can be avoided with a smaller pixel pitch, but as touched on before, the cost to up the resolution of an LED wall further would be astronomical and provide less in benefits, in addition the rendering power required to output images at the new resolution will require more complex image mapping and more expensive rendering machines.
At the end of the day, no matter if you’re building your own LED Volume or accessing a studio that already has one, knowing how these things work will save you a headache during tech rehearsals, understanding your creative needs will define your technical needs.