> I think at least the last bit is incorrect: DRM protected video is still often decoded by a hardware decoder built into GPU.

I misspoke, sorry — the last paragraph is meant to say that the CPU "decrypts", not "decodes", the video (inside the Intel SGX enclave.)

> I am also in doubt about the claim that there's never a single VRAM texture that contains the entire desktop

Some GPUs do work this way internally, but my phrasing was specific — even when such a VRAM texture buffer exists, it's not addressible outside the CPU itself. You can't get a handle to it. (And even if you could, it'd just be another HW-accel draw context — and so would be DRM-tainted if-and-when any buffer being copied onto it is considered tainted.)

But why wouldn't you want a GPU work this way? What's the advantage of not having some kind of "final screen buffer"?

It mostly comes down to a quirk of history. Movies are 24FPS, while computer CRTs were mostly designed for 60Hz refresh rates (with no support for 24Hz modes.) And when people see "hardware-accelerated video playback" as a feature on the box of a video card, one of the primary things that people expect that to mean, is that you should be able to play back your 24FPS movie on the 60Hz display. In windowed mode. With the desktop being refreshed behind the 24Hz movie. Smoothly.

If videos were mostly 30FPS, this would be easy, because 30Hz divides 60Hz; each video frame could just be copied to the 60Hz "final screen buffer" twice with the same content. But 24Hz doesn't cleanly divide 60Hz — you'd need each video frame to appear for 2.5 "final screen buffers." (And just doing that naively — that's called 2-2-3 rendering — is very stuttery.)

If you're doing this by trying to render out a "final screen buffer" VRAM texture in lockstep with your display frequency, all in advance of sending that texture down the pipe to the display, then for each frame, you need to complete that rendering pass with enough time left to communicate it to the display. But what happens when it takes a really long time to send the signal down the wire, leaving you only a little time each frame to actually render everything? In fact, what happens when the display is analogue — like a CRT — where the timing of sending the data down the wire is directly related to the timing of drawing?

Well, certainly, you can pipeline this final rendering — i.e. make everything show up one frame behind, so that you can work on each frame for an extra entire frame-time. But gamers won't like you messing with their draw-contexts that way. And gamers are the core audience for your product.

And also, that still doesn't let you render the video in windowed mode "smoothly." It just means that you have now have slightly less than one full frame-time, each frame, to do the concurrent work of decoding... 2.5 (or some even worse multiple) frames of video, and then doing something with them to squeeze it down to one frame.

And also, how should an active hw-accel video-decode draw context react to the OS changing the display resolution to one that uses a different frequency? Or worse — though not invented at the time — to a Variable Refresh Rate monitor, that can be arbitrarily sent frames using FreeSync?

What the GPU vendors decided to do to make this work, was to require one of two things — either:

1. that the algorithm of the video codec directly support decoding of a fractional frame position; or

2. that the creator of the draw-context pass it a VRAM buffer with enough space for not just one, but several (usually specifically three) full extracted frames. This buffer gets used as a ring buffer, with each new frame becoming a source texture; and these frames are then interpolated together (with some GPUs offering fancy motion-interpolations, and others just doing a linear interpolation) according to the fractional time-between-frames that the timing for the center of the dest draw context lands on.

In either case, the goal was to make the video-decode process something that could be demand-driven — where at any point the GPU could say "I'm at the point where I'm drawing the video; give me what the video should look like now" — and get a result that cleanly maps to the frame the video lands on.

The whole toplevel render pass of early GPUs was designed to operate this way, with final VRAM buffers being "copied" not back into VRAM, but directly into the ring-buffer of the display protocol encoder; and with any final hw-accel passes — like interpolations — generating their output directly into that same display-protocol-encoder ring-buffer. (Or, in the modern day, executing the final compute shader that outputs video tiles only one line-of-tiles at a time, on a vastly sub-frame frequency, as signals like "scanlines matching tiles T50..T80 coming up, get them rendered so they can be read from" are sent by the DPE.)

This decoupled and demand-driven final copy to the DPE, has some interesting properties; it means that you can actually have two or more display-protocol encoders, rendering the same screen, or different parts of the screen — without that implying the need for a lowest-common-multiple frequency. In fact, GPUs are not only happy with multiple displays plugged into them at once — they're happy with those displays even when they have no supported refresh rates in common!

(This is in contrast to the old 2D video cards, which did operate on a single global base frame-raster clock. This meant that 2D video cards were always either single-headed, or only supported a set of monitors all running at the same refresh rate. And if anything wanted to draw to the screen at a different rate, it had to be full-screen, with all the heads switching to that rate.)