From Zoom calls to SciFi effects films -- you can do it all with a DIY chromakey background made from retroreflective fabric, lit with a lens-mounted NeoPixel light ring. A QT Py microcontroller running CircuitPython allows you to control the light ring color and brightness with a push and twist of a rotary encoder knob.

Create chromakey mattes to place your subject in front of any background image or video you like using a retroreflective greenscreen/bluescreen -- you decide the color you need with the press of a knob.

Traditional greenscreen/bluescreen backgrounds are tricky to light evenly, and require a lot of space to prevent your subject from casting shadows and receiving green/blue spill. This retroreflective screen solves all of these problems -- and costs less, too!

This rig works equally well with realtime keyers (built into Zoom, OBS, and others) as well as offline compositing software, such as DaVinci Resolve or iMovie. 

Parts

Chromakey Essentials

The goal of a chromakey is to create an opacity matte (or alpha channel) that isolates a subject being filmed from their background so that they may be composited in front of a different background.

This happens both in realtime applications and in offline composites for rendering.

Realtime chromakeys are used for weather people to appear in front of their maps on the local news. They're used in Zoom conferences to make a participant appear in front of a Tahitian beach, or in OBS streams to composite a gamer's head over their Mario speed run.

Offline chromakeying is done in compositing and video editing software such as Nuke, After Effects, Premiere, DaVinci Resolve, and iMovie for compositing over film plates, video background, and stills in everything from VFX feature films to indie music videos to home movies.

Traditional Greenscreen Challenges

The entire premise of shooting on a greenscreen relies on having a very evenly lit greenscreen. The more variation in the lighting from shadows cast on it by the subject, the more difficult it is to use.

Additionally, it's important to avoid having green spill light bounce from the greenscreen onto the actors.

These issues are typically solved by creating a lot of space between the actors and the greenscreen, by lighting the greenscreen with dedicated lights, and by avoiding frontal lighting for the actors to prevent shadow casting.

These techniques work well in a dedicated filming environment such as a soundstage, but can be a huge challenge when shooting at home or in other constrained locations.

In this image, our living room has been turned into a greenscreen stage. Note the pair of side lighting rigs dedicated to the greenscreen.

Retroreflector

Since the requirement of a greenscreen is to have an evenly lit, saturated color backdrop, there must be a way to achieve this that doesn't require a lot of space and dedicated lighting, right? There is! Enter the retroreflector!

A retroreflector is a specialized material (or arrangement of mirrors) that reflects light directly back at its source, regardless of incoming angle. This is why you see a runners shoes, or a safety vest, or a stop sign at night so brightly reflect the light from your car's headlights the entire time they're in view -- there isn't just one flash of light if you get lucky and the angles align for a brief moment. The retroreflective material ensures that.

Here's a terrific video on retroreflectors that my friend Josh shared with me.

Retroreflective fabric is covered in tiny spherical lenses which direct incoming most light back at its source, regardless of entrance angle. They do this by refracting incoming light beams toward a mirrored surface below the lens, which reflects the light, and then the glass beads refract the light a second time, aiming them back toward the same direction from which they came.

This image adapted from one on the 3M Scotchlite website may help explain:

Retroreflective Chromakey

If we shine an even, saturated green (or blue) light at the retroreflector from the point of view of a camera, all of that lovely green light gets reflected right back at the camera lens. The light does not need to be very bright at all, which means a ring of LEDs will do the trick of turning the backdrop into an even chroma screen but will not cast any colored light onto the actors. It's science, but the results seem magical!

Also, note how the subject in the following examples is just inches from the screen without any problems from shadows that we'd see with a traditional greenscreen.

The chromakey light ring code requires some CircuitPython libraries to work. Complete the following steps to get your board ready.

CircuitPython Library Bundle

You'll need to install some libraries on your QT Py.

Carefully follow the steps to find and install these libraries from Adafruit's CircuitPython Library Bundle.  Our CircuitPython starter guide has a great page on how to install libraries from the bundle.

Library Install Troubleshooting

If you get an error indicating you have run out of space while copying files over, please refer to the Prevent & Remove MacOS Hidden Files section of the Troubleshooting page for information on resolving the issue.

Text Editor

Adafruit recommends using the Mu editor for editing your CircuitPython code. You can get more info in this guide.

Alternatively, you can use any text editor that saves simple text files.

Code

Copy the code shown below and paste it into Mu, then save it to your CIRCUITPY drive with the name code.py

How It Works

Libraries

First, the libraries are imported -- time for debouncing pauses, board for pin definitions, digitalio to set pins to inputs or outputs and to use the encoder knob button, neopixel for RGB LEDs, and rotaryio for reading the encoder.

NeoPixel Setup

Next, the dim_val variable is created and set for the starting brightness value of 20%.

Then NUM_PIX variable is set to the number of NeoPixels in the ring you're using, in this case 24.

Depending on the type of NeoPixel ring you use, you'll set it to either "RGB" or "RGBW".

A list for colors is created so you can cycle between them using the push encoder button, then the ring is set up and turned on to the first color in the list.

Pin Setup

You'll set up the encoder pins next. Based on the physical layout of the encoder legs, you'll need to set pin A2 as a "fake" ground by setting it to be an output pin. Thanks again to Tod Kurt for this setup.

The encoder pins are on A3 and A2.

The state of the encoder position is stored in the last_encoder_val variable to compare it to the currently read value, thus detecting if the knob is being turned and if so in which direction.

Main Loop

The main loop of the program does the following things:

Compares the encoder value to the last encoder value.

If the encoder value is increasing (knob is turned clockwise), the brightness is increased. Conversely, if the encoder value is decreasing, the brightness is decreased.

When the push encoder button is pressed, the color is switched to the next one in the colors list.

There are various ways to attach the light ring to the camera, dependent on the camera used. The lens of the camera must be within the LED circle, and you want to avoid blocking any moving parts or sensors.

Below is a 3D printed solution for attaching to a number of different cameras including a lens barrel and a webcam.

First, we'll mount the rotary encoder to the QT Py. This is based on a method created by Tod "@todbot" Kurt as documented here.

Light Ring Mount

This light ring mount is designed to fit a variety of lenses on mirrorless cameras and DSLR cameras.

First, download the 3D model files linked below. Print the ring and four of the adjustment blocks. Settings may vary from printer to printer. Below are my key settings, but these are pretty forgiving models, so you should be able to print without too much noodling.

• PLA filament
• 0.15mm layer height
• 30% infill
• 210º C extruder temperature
• 60º C heated glass build plate

Attach to Lens

To attach the chromakey light ring to your lens, simply pull the four gripping blocks away from center, press the ring to the end of the lens barrel, and release the gripping blocks.

Next we'll set up the retroreflective screen and light it up!

Hang the retroreflective fabric using spring clamps, magnets, or even gaffer's tape. You will want it to be very close to the subject for best performance. This helps to avoid a halo effect.

In this shot I've got an LCD monitor showing me what the camera sees -- this is a great way to see how the greenscreen looks, since you can't see the effect with your naked eye unless you're positioned right behind the camera.

Here's a behind the scenes video of the system in action: