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Bypassing composition is a key feature in Wayland. The protocols are all written with zero-copy in mind. The X11 tricks are already there, forwarding one client

Bypassing composition is a key feature in Wayland. The protocols are all written with zero-copy in mind. The X11 tricks are already there, forwarding one clients buffers straight to hardware, while the end-game with libliftoff is offloading multiple subsurfaces directly to individual planes at once. The compositor is the entire display server in Wayland, and is also the component responsible for bypassing composition when possible.

fulafel 3y ago
In my meager understanding which I'm happy to be corrected about: In a windowed scenario (vs fullscreen), in both the X direct-rendering and Wayland scenarios the application provides a (possibly gpu backed) surface that the compositor uses as a texture when forming the full screen video output. In a full-screen case AFAIK it's possible to skip the compositing step with X11, and maybe with Wayland too. "Zero copy" seems a bit ambiguous term in graphics because there's the kind of copying where whole screen or window sized buffers are being copied about, and then there are compositing operations where various kinds of surfaces are inputs to the final rendering, where also pixels are copied around possibly several times in shaders but there aren't necessary extra texture sized intermediate buffers involved.
- arghwhat 3y ago
  > In a full-screen case AFAIK it's possible to skip the compositing step with X11, and maybe with Wayland too. This is the trivial optimization all Wayland compositors do. The neater trick is to do this for non-fullscreen content - and even just parts of windows - using overlay planes. Some compositors have their own logic for this, but libliftoff aims to generalize it. Zero-copy is not really ambiguous, but to clarify: Wayland protocols are designed to maximize the cases where a buffer rendered by a client can be presented directly by the display hardware as-is (scanned out), without any intermediate operations on the content. Note "maximize" - the content must be compatible with hardware capabilities. Wayland provides hints to stay within capabilities, but a client might pick a render buffer modifier/format that cannot be scanned out by the display hardware. GPUs have a lot of random limitations.

fulafel 3y ago

In my meager understanding which I'm happy to be corrected about: In a windowed scenario (vs fullscreen), in both the X direct-rendering and Wayland scenarios the application provides a (possibly gpu backed) surface that the compositor uses as a texture when forming the full screen video output. In a full-screen case AFAIK it's possible to skip the compositing step with X11, and maybe with Wayland too. "Zero copy" seems a bit ambiguous term in graphics because there's the kind of copying where whole screen or window sized buffers are being copied about, and then there are compositing operations where various kinds of surfaces are inputs to the final rendering, where also pixels are copied around possibly several times in shaders but there aren't necessary extra texture sized intermediate buffers involved.

arghwhat 3y ago
> In a full-screen case AFAIK it's possible to skip the compositing step with X11, and maybe with Wayland too. This is the trivial optimization all Wayland compositors do. The neater trick is to do this for non-fullscreen content - and even just parts of windows - using overlay planes. Some compositors have their own logic for this, but libliftoff aims to generalize it. Zero-copy is not really ambiguous, but to clarify: Wayland protocols are designed to maximize the cases where a buffer rendered by a client can be presented directly by the display hardware as-is (scanned out), without any intermediate operations on the content. Note "maximize" - the content must be compatible with hardware capabilities. Wayland provides hints to stay within capabilities, but a client might pick a render buffer modifier/format that cannot be scanned out by the display hardware. GPUs have a lot of random limitations.