If you want to use chroma key effect in a streaming or video conference app, but you can’t get a physical green screen background in place, you can fake it. All by using Background Replacement in macOS.

Whether it is streaming gameplay to Twitch or improving your Zoom calls, sometimes you want to replace the background with something else. It’s unprofessional to have a background that includes piles of dirty clothes, random dinner plates, and other unsightly items.

When it comes to using tools like Zoom or FaceTime, you can get around that by blurring the background, or even replace the background with a picture.

However, not every program that accepts a webcam feed will deal with this in an intelligent way.

The problem this time is the free OBS (Open Broadcaster Software), a tool for compiling sophisticated graphical setups for streams. Chiefly, this is used for game streams, such as those on Twitch, but it can also be used as a virtual webcam for other software, like Zoom.

While OBS does have a green screen effect, it’s the previously established chroma key effect. It’s not the modern background replacement of FaceTime and Zoom, but it can be used to great effect.

That is, if you have a green screen available, or if you have one, you have the physical space to pull the effect off properly.

You can work around the limitations of OBS and physical space, by combining the two systems together.

Chroma key

The two techniques that we will use here fundamentally work in completely different ways. But one can be used in the service of the other.

Chroma key is an old technique, best known to people who have seen a weather report or a behind-the-scenes video of a Hollywood movie. In short, it is the removal of a specific color, or a range of colors, from a video and replacing them with blank pixels.

This is keying-out the color, giving chroma key its name.

Chroma Key removes a green background, so you can create a composite of a subject with a different background – Image Credit Lisha Dunlap/Pexels

A person walking in front of a green screen background would result in a video of them walking in a black void. Unless that footage is overlaid on top of another image, in which case the background would change behind the subject.

This has been an effective technique for quite some time, and has been used to great effect by game streamers. Using a green screen, they can appear on screen with the game in the background, but with no real-world objects appearing behind the streamer’s head.

However, to use chroma key, you do need a background. This could be a simple green sheet, or you could invest in a pop-up chroma background that tensions the surface flat.

By the way, while chroma key usually deals with a bright green background or sometimes a blue as an alternate, you can use other colors as well. Just don’t use a color that is too close to your skin tone, otherwise you may go partially invisible on stream.

If you have a background, the next step is to light the background, as well as yourself. This can be hard, as ideally, you need sufficient space between you and the background so that you don’t cast shadows onto it.

Shadows are tough to key out, so you want to avoid them.

Ideally, you should evenly light the chroma background, so that there’s only one small range of green that needs to be keyed invisible.

The reality of the situation is that there will be things getting in the way of you creating a perfect key. If you don’t have enough lighting or enough physical space, you’ll find it difficult to get the screen up and lit without shadows, while also lighting yourself up properly.

In my case, while I have the lights and the backgrounds in various forms, I do not have enough physical space behind my chair to set it all up. I could move my entire computing setup across a bit and free some space, but a combination of electrical cable lengths and a hint of laziness makes that a non-starter.

Background replacement

The other technique, background replacement, is what users of Zoom and FaceTime will be familiar with. It’s a computer-driven method of keying out the background with different imagery, like a beach scene.

However, instead of keying out a color, it’s a system that relies on detecting the subject of the video feed. Everything that is not the subject in the middle of the video is replaced by a chosen background.

It is by no means a perfect cut-out. The edges of your hair or your body could be cropped a bit too closely, and sometimes things could pop in and out of view if they are moved suddenly.

Using macOS Background Replacement to make a fake green screen source.

If you have small animals that like to jump onto your lap, viewers may see them suddenly appear out of nowhere.

But if you’re using it for a small webcam image in the corner of a game stream or for an impromptu effect, it works pretty well.

It’s a feature that Mac users have had for some time. With the introduction of macOS Sequoia, Apple introduced Presenter View as well as Background Replacements.

When you activate a webcam in macOS, you can access a green camera icon in the menu that includes various options, including setting the Portrait background blur effect, studio lighting, reactions, and microphone modes.

There’s also an option for Backgrounds, which triggers Background Replace. Selecting it instantly forces the Mac to work out what the background elements are of your webcam view, and immediately changes them over to a background image.

All together now

You are able to get the benefits of using the green screen background-free Background Replace in macOS in OBS. The key (pun not really intended) is to use them in concert.

Applying the Chroma Key effect to the fake green screen feed.

The way to do it is to create a fake green screen background in Background Replace. That webcam feed, which appears to be you in front of a green background, is then ingested by OBS

OBS, not knowing the truth, then keys out what it thinks is a green screen background, resulting in a chroma key effect and an invisible background for your webcam footage.

Yes, we are using a background replacement tool in macOS to fool a background replacement tool in OBS.

How to use macOS Background Replace with OBS for a clean chroma key effect

• Open OBS and add a webcam feed to your scene.
• Create an image of a solid green square in an image editing program. Alternatively, you can search Google Images for a more correct chroma green image that does the same thing.
• In the macOS menu, select the green or purple icon to show the webcam settings options.
• Select the webcam, then the Background circle icon to enable it.
• Select the rectangle to the right of Background to bring up the Background image picker. Select the third tab and click the Plus icon to add a custom background.
• Add the created or downloaded green screen, and then select it in the Background image picker.
• Back in OBS, select your webcam, right-click it, and select Filters.
• Under Effect Filters, click the plus and select Chroma Key.
• Use the settings to select the Green key, or whatever color your Background Replacement is using. You can adjust the Similarity, Smoothness, and Key Color Spill Reduction to make the key better.
• Once happy, select Close.

At this point, you will have a webcam view of yourself with clear pixels around you. Place a background behind the resulting image, and you’ll see the key in action.

Be advised that this relies on computer vision analyzing a flat image instead of something with a depth map, and therefore, it’s not going to be perfect. But, if it’s a short-term solution or one where you simply can’t go down the proper chroma key route, this is a good alternative to get the same effect.