vo_gpu: split 'alpha' into two options

[occivink]

Currently it covers two somewhat orthogonal concepts:

• whether to create a framebuffer with alpha component
• how to blend video with alpha information

This commit creates a new flag 'fbo-with-alpha' which now controls
the first part, while 'alpha' now only covers the second.

Previously it would only be possible to make the window transparent
with 'alpha=yes', which prevented blending of the image with tiles or
the background.
This split allow using different blending strategies while keeping the
window transparent.

One downside is that only specifying 'alpha=yes' no longer makes sense
without specifying 'fbo-with-alpha' as well.

Fixes #9615
If this seems ok conceptually, I can adjust the docs and document the new flag.
Tested on wayland with wayfire (wlroots-based) and weston.

[haasn]

I think I would prefer naming this option something like "--window-transparent", to go with the other "--window-" flags.

[occivink]

Updated the option name to window-transparent and moved it to options.c. I've also noticed that toggling window-transparent (or previously, switching alpha to/from yes) at runtime doesn't work, only the initial status of the option seem to matter. I'm not sure if this is meant to be a supported usecase, but I thought I'd mention it.

[Dudemanguy]

It's because you added the option value to mp_vo_opts instead of gl_video_opts.

[occivink]

Ok indeed. Does the option belong there? I added it to vo_opts since that's where others window- options were defined.

[Dudemanguy]

Setting the want_alpha flag is only ever going to work with gpu or gpu-next so I say it would belong with gl_video_opts and not generic mp_vo_opts.

[occivink]

I put the option in gl_video_opts, added a warning for alpha=yes without window-transparent, and updated the documentation accordingly

[occivink]

Added an interface-changes.rst entry (and also ping).

[haasn]

The sentence about --fbo-format should go into the documentation of --alpha=yes, since it's relevant only for alpha blending within the pipeline, rather than display onto the screen.

[occivink]

Like this? I'll admit that I don't full understand the workings of fbo-format, even after trying its different values out.

[occivink]

ping

[musicfunfan]

So if i understand correctly this PR will let users like me to have a video with alpha channel display properly.
What i mean is. I have video of mine that has transparent elements. My expectation from mpv-player is to play them with transparency. For example when i have a transparent element on the video i want to see what ever is behind the player. I want the mpv window to be transparent when the video is transparent. Only on the places that the video is transparent.

Now i can not do that on my system.
Os archlinux with X11 and picom.

If this PR can do that. I would like that very much.

If a way to do that already exist please let me know.

Sorry if this comment is not relevant i just searching for a way solve my problem and i find this PR.

[occivink]

What you're trying to achieve should already be possible, all you have to do is pass --alpha=yes.
This PR is only relevant if you don't want the window to be fully transparent, but instead blended with the mpv background (as specified by --background).

Regarding, this PR, I've noticed that it does not work with vo=gpu-next. Based on my understanding of the code, something in libplacebo would have to be changed. I had a look into it, but was not able to get it to work.

[musicfunfan]

When i try to do the --alpha=yes thing the transparent stuff just turn to black.... Not transparent. :(

[occivink]

Ah yes, it could be that on X11 + mesa this doesn't work, see also the comments in #9862. Can you try on a wayland compositor? Anyway this is not really on-topic for this PR, maybe ask on IRC or create a new issue.

[Dudemanguy]

Superseded by #13493.