Fun Fact: This same sort of thing also happened on the Classic Macintosh Quadra 840AV, when running in 8-bit (256 color) mode. Playback of realtime video capture reserved color index #243 (a very dark green in the system palette), and ANYWHERE that color was used, it would be replaced with the live video. I created some cool effects using this back in the 90s.

I discovered this by accident and won a few beers back in my university days by betting I could play a video in MS Paint. :)

Long time back I'd play PS2 games in a chat window in EverQuest while waiting for mobs to spawn. I had a capture card that would overlay over a particular shade of purple that I discovered while trying to screen shot something from a game. I made an empty chat window in EQ that color and where it overlapped the card's application window behind the video would render. Was super jank picture in picture, but it worked.

> These special surfaces were called “overlays” because they appeared to overlay the desktop.

I have some vague memory of programs whose windows had funky shapes (i.e. not rectangular) also using overlays of some kind. Maybe that's a different sort of overlay?

I realized this when I tried to capture the test pattern used by a PowerDVD diagnostics application.

This reminds me of the time when Quake started rendering inside the start button of the Windows 95 desktop (or maybe Win 98). I wish I could remember the details but it think it was something to do with alt-tab.

When I did the same experiments in win98, the magic color was not bright green but #010000, the darkest possible red.

Was this dependent on the OS? Or the driver?

This was always annoying, when you wanted to take an actual screenshot of a high resolution video and use it as a desktop background for example. I assumed it was connected to hardware/software acceleration or something.

I also remember that I couldn't make any screenshots of videos at all.

They were rendered in some kind of overlay, that the rest of the GUI didn't know anything about.

"Nowadays, video rendering is no longer done with overlays."

Darn, I thought this explained why, after upgrading my GPU, videos playing in Chrome have a thin green stripe on their right edge.

I made extensive use of this, when I found it by accident, in my Winamp skins and GUI programs!

Iirc you can also set that "green screen" as wallpaper and have video as desktop background!

This unlocked some memories. I remember on my system the chroma colour not being green but some very dark shade of grey that was almost black but not really black… something like #010101

This brings back memories of my old HP laptop with an Athlon 64 and a Radeon X200M. The crappy FGLRX driver only supported overlays (afair) and so when running something like Compiz it would transform the window with the green background but the video itself would stay in place and it would just stick parts of the video on top where it happened to overlap. I still remember being excited when the open source drivers finally gained support for r300 and could do proper textured video...

If you watch Twitch, you can see that all instances of the same emote in chat animate together. Then I tested this more generally in a web page, and the same thing happens - if the same gif is placed multiple times in a page, all instances of that gif will play in sync even if loaded at different times. I guess there's a similar idea in browsers then, where maybe there's only one memory representation of the gif across the page or the browser.

This was a nice trick to protect text from copying. For instance, student assignments. Students could still use digital camera on CRT display, but 20 years ago cameras were costly and students did not have them. And typing text from scratch was a tedious job. So online served assignments were not shared too fast.

I remember this personally!

I was always stumped as to what the hell was happening.

i had forgotten about this technique when i was at the excellent https://tnmoc.org recently, looking at their sgi irix exhibit featuring a webcam.

the latency of the camera feed on the crt screen was unbelievable even (especially?) by modern standards!

after a minute of pure wonder i remembered about overlays. still mighty impressive.

Good to see the first comment there corrects him, and that it's not actually green pixels; at least for the Intel and nVidia drivers I've used before, it appears to be more of a dark magenta. It could be configurable or hardcoded somewhere in the driver, but I don't think it's fixed in hardware.

The desktop compositors takes the graphics content of all the windows, including their composition visuals, and combines them to form a full desktop image that is sent to the monitor.

...at the cost of latency and efficiency.

Xine and TVTime worked the same with the overlay video output. If you tried to create a screenshot with X11, you would get a blue/greenish window instead of the video.

You had to use the builtin screenshot function from your video player/tv viewer.

The current MPlayer under OpenBSD 7.7 still has the overlay video output

        mplayer -vo xover

The irony is that in 2025, this answer is now wrong again. Starting with smartphones, scanout hardware supports multiple planes/overlays again that are composited on the fly by fixed function blocks. This bypasses having to power on the GPU and wasting memory bandwidth (a large amount of power use in a smartphone). No longer involves hacks with green pixels though.

I had a Matrox Millenium card with a breakout box for capturing RCA, S-Video, and Cable TV; I'd watch TV on my Windows 98 SE2 computer, which was the craziest thing back then, but I always felt like the green-screen like effect was some kind of mysterious bug that I'd better not mess with, or video capture would break. Windows 98 was barely working on a good day, so it felt like the computer was in the process of failing in a graceful and useful way, so I'd better not push my luck.

Every so often you could get a glimpse of the man behind the curtain, by dragging the window quickly or the drivers stuttering, which would momentarily reveal the green color (or whatever color it was) before the video card resumed doing its thing. Switching between full screen and windowed mode probably also revealed the magic, or starting a game that attempted to grab the video hardware context. And of course sometimes other graphical content would have the exact right shade of color, and have video-displaying pixels.

I've used that trick as far as Windows XP, playing videos inside 3D models in programs like SketchUp

I remember using overlay mode in Winamp AVS made it run much faster. Wonder why that was.