Screencast: Clarify which PipeWire features are mandatory

[columbarius]

Some weeks after implementing screenrecording via the portal API the zoom client broke again with the wlroots implementation. It showed that it treats every buffer without cropping information as invalid (emersion/xdg-desktop-portal-wlr#226).
Since PipeWire has a very broad scope and doesn't force the usage of specific meta-data, we should document which features a portal has to use and which are optional.

My proposal would be

From spa/buffer/meta.h:

• SPA_META_Header: mandatory
  Describes essential buffer header metadata such as flags (buffer corrupt: mandatory) and timestamps (optional).
• SPA_META_VideoCrop: optional
  Describes a region to which the buffer should be cropped
• SPA_META_VideoDamage: optional
  Array of regions describing what part of the buffer was damaged wrt. to the previous one
• SPA_META_Cursor: mandatory with cursor mode Metadata
  Metadata to describe the position and appearance of a pointing device.

CC: @GeorgesStavracas @aleixpol @grulja

[aleixpol]

I guess you meant Screenshare?

Specifying which as necessary, that's a good idea. Specifying the optional ones is a bit weird, since it would mean that there would be some that are not permitted (?), but it can be useful for documentation otherwise.

As for us, we're supporting most of these already (except for SPA_META_VideoCrop, that is).

[columbarius]

I guess you meant Screenshare?

Yeah, I've worked on screenshot things before and it was late ^^'.

Specifying which as necessary, that's a good idea. Specifying the optional ones is a bit weird, since it would mean that there would be some that are not permitted (?), but it can be useful for documentation otherwise.

I used optional as the opposite of mandatory meaning that a client can handle that, but it should not expect the impl to always use it (especially if it is not necessary, like setting the cropping region to the whole buffer). Maybe there is a technical more specific word pair for this.

As for us, we're supporting most of these already (except for SPA_META_VideoCrop, that is).

Didn't kwin use SPA_META_VideoCrop for window sharing at some time?

[aleixpol]

Maybe there is a technical more specific word pair for this.

Or maybe we can link to pipewire documentation where they describe these and just say "and $these are mandayory";

Didn't kwin use SPA_META_VideoCrop for window sharing at some time?

No, I don't think so.