[arghwhat]

The usual mechanism is to mark widgets that changed dirty, accumulate the bounding boxes of such dirty areas, take the next swapchain buffer and get its invalid regions, iterate through the widget tree and render anything that intersects with the bounding box or invalid regions, and submit the buffer + the dirty areas to the display server/driver.

And yeah, having a render step depend on the output of a previous non-trivial render step is Bad™.

[aeonfox]

I was under the impression that for GPU accelerated GUIs, all windows are rendered to a render target. It might be that windows underneath have gone to sleep and aren't updating, but they would have their last state rendered to a texture. This permits things like roll-over previews and layered effects to have a more trivial overhead.

Software renderers typically do the optimisation you're suggesting to reduce on memory and CPU consumption, and this was a bigger deal back in the day when they were the only option. I think some VNC-like protocols benefit from this kind of lazy rendering, but the actual VNC protocol just diffs the entire frame.

On the GPU, the penalty for uploading textures via the bus negate the benefit, and the memory and processing burden is minimal relative to AAA games which are pushing trillions of pixel computations, and using GBs of compressed textures. GPUs are built more like signal processors and have quite large bus sizes, with memory arranged to make adjacent pixels more local to each other. Their very nature makes the kinds of graphics demands of a 2D GUI very negligible.

[arghwhat]

> I was under the impression that for GPU accelerated GUIs, all windows are rendered to a render target.

Each window renders to one or more buffers that they submit to the display server, which will then be either software or hardware composited ("software" here referring to using the GPU to render a single output buffers vs. having the GPU scanout hardware stitch the final image together from all the source buffers directly).

Note that in the iPhone cases, the glass blur is mostly an internal widget rendered by the app, what is emitted to the display server/hardware is opaque.

> It might be that windows underneath have gone to sleep and aren't updating,

The problem with blur is when content underneath does update, it requires the blur to also update, and rendering of it cannot start until the content underneath completed rendering.

> Software renderers typically do the optimisation you're suggesting to reduce on memory and CPU consumption,

I am solely speaking about GPU-accelerated rendering, where this optimization is critical for power efficiency. It's also required to propagate all the way down to the actual scanout hardware.

It also applies to CPU rendering (and gpu-accelerated rendering still CPU renders many assets), but that's not what we're talking about here.

> I think some VNC-like protocols benefit from this kind of lazy rendering, but the actual VNC protocol just diffs the entire frame.

Most modern, non-VNC remote desktop protocols use h264 video encoding. Damage is still propagated all the way through so that the client knows which areas changed.

The frames are not "diffed" except by the h264 encoder on the server side, which may or may not be using damage as input. The client has priority for optimization here.

> Their very nature makes the kinds of graphics demands of a 2D GUI very negligible.

An iPhone 16 Pro Max at 120 fps is sending 13.5 Gb/s to the display, and the internal memory requirements are much higher. This is expensive.

Not rendering a texture and being able to pass it off to scanout hardware so that the render units can stay off is the difference between a laptop giving you a ~5 hour battery life and a 15-20+ hour battery life.

The GPU could texture your socks off, but you're paying a tax every microsecond your GPU's render units are active, which matter when you're battery powered or thermally constrained. This is why display servers and GUI toolkits go through lengths to not render anything.

[aeonfox]

> Note that in the iPhone cases, the glass blur is mostly an internal widget rendered by the app, what is emitted to the display server/hardware is opaque.

This sounds wild to me, so I'm just going to ask. Do you work on these kind of optimisations for a modern OS? If so, just ignore my ponderings and I'll just accept what you're saying here.

I honestly couldn't imagine this kind of compositing not happening completely on the GPU or requiring any back and forth between the CPU and GPU. That is, the windowing system creates a display list, and that display list is dispatched to the GPU along with any assets it requires (icons, font etc.). I'd also imagine this is the same as how the browser renders.

As for optimisations, if the display list is the same for a particular render target (e.g., window, widget, subsection, or entire screen), there's no reason to rerender it. There's no reason to even rebuild the display list for an application that is asleep or backgrounded. Tile-based culling and selective update of the screen buffer^ can also happen at the GPU level. Though hierarchical culling at the CPU level would be trivial and low-cost.

This is not my wheelhouse, so perhaps I'm missing something crucial here.

^ Edit: It does look like the Apple silicon GPUs do use tile-based deferred rendering.

https://developer.apple.com/documentation/metal/tailor-your-...

[arghwhat]

> Do you work on these kind of optimisations for a modern OS

I have worked on modern display servers and application interfaces, and as such also dealt with (but not written) a fair amount of client application render code, generally optimizing for power consumption and latency.

> I honestly couldn't imagine this kind of compositing not happening completely on the GPU or requiring any back and forth between the CPU and GPU.

Well the CPU is always involved, and even responsible for rendering certain assets that are not viable to render on the GPU - fonts in particular are usually rendered by the CPU, with occasional attempts at GPU rendering being made such as servo's pathfinder - but for simplicity let's talk about only widgets rendered by the GPU.

In most cases[^1], a window is rendering to a single "render target" (texture/buffer), handing off a flat and opaque buffer to the display server which the server will either try to display directly (direct scanout) or composite with other windows. In this context, the display server's main purpose is to have exclusive control over display hardware, and apart from compositing multiple windows together it is not involved in render processes.

The application itself when rendering will normally walk its widget tree, accumulate damage and cull widgets to create some form of a render list. Depending on the toolkit and graphics API in question, you'll ultimately end up submitting an amount of GPU work (e.g., a render pass) to render your window buffer (an IOSurface or DMA-BUF), and you then send the buffer to the display server one way or another. The window buffer will become ready later once the asynchronous render tasks complete, and the display server will wait on the relevant fences (on the CPU side, which is responsible for most GPU scheduling tasks) before starting any render task that would texture that buffer or before attempting scanout from the buffer.

The problem with blur is that you have a render task that depends on the full completion of all the prior render tasks, as its shader must read the output buffer[^2] as it completed rendering to an intermediate state. Additionally, other render steps depend on that render task as it has to be overlaid on top of the blur widget, and only after that completes is the buffer ready for the display server. That's a pipeline stall, and because it's on top of the primary content, it's holding up every frame from that app, and due to the blur operation itself an update that before only affected one tile now affects several.

Reading your own output is something you avoid like the plague, and blur is that. If you're used to web/network development, think of it like blocking network roundtrips.

... well this turned out to be a wall of text ...

---

^1: The more advanced case of hardware compositing is where you send a small number of buffers to the display server, e.g. a video buffer, an underlay with some background and an overlay with some controls, and have the display server configure hardware planes using those buffers such that the window is stitched together as the signal is about to be sent to the display. This is not the general case as planes are limited in count and capability, they cannot perform any effects other than basic transforms and blending, and scanout hardware is very picky about what it can use as input.)

^2: One could implement this instead by creating a different render list for just the area the blur needs to sample instead, in the hopes that this will render much faster and avoid waiting on completion of the primary buffer, but that would be an app specific optimization with a lot of limitations that may end up being much slower in many scenarios.

[aeonfox]

> and the display server will wait on the relevant fences (on the CPU side, which is responsible for most GPU scheduling tasks) before starting any render task that would texture that buffer or before attempting scanout from the buffer.

Given the fetching and compute performance per watt of modern GPUs I'm still surprised that the watts saving of reducing overdraw is anything but negligible, and certainly if you're talking about pipeline stalls, having pixel data shuttling over the bus between the GPU and CPU seems like a much bigger deal?

> ^2: One could implement this instead by creating a different render list for just the area the blur needs to sample instead, in the hopes that this will render much faster and avoid waiting on completion of the primary buffer, but that would be an app specific optimization with a lot of limitations that may end up being much slower in many scenarios.

It looks like Apple Silicon avoids the overdraw problem with TBDR, and the tile system would efficiently manage the dependency chain right back to the desktop background if needed. So if a browser is maximised over a bunch of other windows, only a portion of a portion of render targets are being sampled, with no intermediate CPU rendering.

To me, the flex by Apple here is that they can do this efficiently, because their rendering system is likely fully GPU and also resource efficient in a way that other typical display servers and GPUs can't be. For this to work on Linux or Windows, a complete refactoring of the display servers would be required, and it would only service GPUs that have tile-based deferred rendering, which seems to be nil outside of Apple's Silicon (and their older PowerVR chips).