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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.
Yes, other Mac AV models did the same thing. I remember doing this on the 6100/60AV that we had.
Are you the one who created the techno-anthem "pump up the jam"? Sweet!
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.
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. :)
> 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?

Those are called regions.
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 made extensive use of this, when I found it by accident, in my Winamp skins and GUI programs!
I found this also by accident when I was kid and I used it as putting video as wallpaper for my desktop. I thought it was pretty cool.
It definitely was pretty cool.
"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.

A green stripe on the right/bottom is usually due to a different issue: interpolation errors in chroma planes when decoding YCbCr video. The chroma planes use a biased encoding where 0 (no color) is mapped to 128. A sloppy YCbCr to RGB conversion without proper clamping can interpolate against 0 at edges, which is interpreted as max negative chroma red/blue -- which combined together produces green. This can happen either due to an incorrectly padded texture or failing to handle the special final sample case for 4:2:2 chroma.

This issue can happen with overlays, but also non-overlay GPU drawing or CPU conversion routines.

Video rendering can still be done with overlays, but it's a little more substantial, involving separate planes with the locations configurable on the graphics card. Look up MPO, Multi-Plane Overlay.

Your green stripe is likely because of the classic combination of unclamped bilinear filtering and a texture that's larger than the output region being used as the drawing surface for the video.

They still use "overlays" - just they're a lot more featureful in modern implementations than "Replace all of one colour with another surface" - so they tend not to have the same limitations and quirks.

MS started exposing some capabilities using MPO in the windows 8 era [0], and they've pretty much always had pretty comprehensive composition pipes in hardware for mobile platforms due to the power/bandwidth limitations meaning compositing the display can be a significant fraction of the total device's performance.

I suspect green (or other block colour) artifacts on video edges are due to bugs/mismatches in specification with the hardware video decoder and how the app displays it, and the bugs that often fall out of that.

Most video compression requires pretty large blocks, normally from 16x16 up 64x64 depending on format, and that may not align with the actual video size (e.g. 1080 does not divide by either). But often implementations still need that extra surface, as things like motion vectors may still refer to data in the "invisible" portion. And it has to be filled with something. It's then real easy to fall into bugs where small numeric errors in things like blending, or even just mix-ups between the different "sizes" that surface has, to cause issues at the edges.

I suspect the other comment about using ANGLE/dx9/dx11-on-12 may be effective as it /also/ causes the hardware video decoder not to be used, which is often a source of "unexpected" differences or limitations that commonly cause errors like that.

[0] https://learn.microsoft.com/en-us/windows-hardware/drivers/d...

I did some Googling on your behalf as I remember having something like that but can't reproduce it right now:

https://old.reddit.com/r/OLED_Gaming/comments/1kovgdx/green_...

I'd make sure your drivers are up to date before fiddling with Chrome flags though.

This is simply wrong. Videos are still rendering in overlays in windows in Chrome right now. There are many reasons why overlays are still used.
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.
That quirky effect is a known quirk of early DirectDraw/WinQuake on Windows 95/98. When a game grabbed the primary surface in certain modes, parts of the desktop GDI (like the Start button/taskbar) could briefly be overdrawn or show the game’s backbuffer due to how DirectDraw managed the primary surface and cooperative levels in fullscreen and windowed modes on those systems.
Iirc you can also set that "green screen" as wallpaper and have video as desktop background!
Or you could set the WinAmp AVS visualizer to render to overlay, and have it as your desktop background.
Yeah, I used to have a few "live wallpaper" type videos I'd use this way. Around the time AVC-ish algorithms were democratized by DivX. IIRC the player I used had #0000A0 as its overlay color... may have even been the DivX branded player.

...This is the oldest I've ever felt, unsure of my own memories regarding something I have to consult historical records about...

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.
Right, because we have alpha channels now
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
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.

As far as I can tell, it's still the case if the video is DRM-ed. Then any screenshots of it will be black square, because the OS can't "see" the video, it's sent directly to the monitor, similar as in the article.
that's literally what the article is about
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?

yeh, I remember it being the darkest red too, maybe depended on bith depth ?
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 realized this when I tried to capture the test pattern used by a PowerDVD diagnostics application.
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
In the earlier days of graphics on Linux this was a common right of passage. As far back as 2003 I can remember running an i915 graphics driver, which Intel has always had pretty good open drivers. When you would open a video in VLC and take a screenshot of the desktop, you would be left with a hole as it would grab the overlay compositing color (blue, green, etc.). However, if X11 was doing a video thing it would be filling these areas with whatever video is happening.
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.
Around 2005 digital cameras were commonplace. Mobile phones also had cameras by then, even if not very good ones by today's standards. Maybe you're talking about an earlier time?
I had my first digital camera in 2003 and my first videocall-capable phone in 2004 (Nec e313). I was a kid
> And typing text from scratch was a tedious job

At least by typing them the typer might learning something. :)

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

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

I remember this personally!

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