Transparent window support

[mitchmindtree]

It seems that since switching to Vulkan it's not as trivial to get a transparent window working!

Expected behaviour

You should be able to create a transparent window like this:

app.new_window()
    .with_transparency(true)
    .build()

Actual behaviour

The above results in a black screen. This is likely due to the default CompositeAlpha::Opaque value specified when building the window's swapchain. We can override this with the following:

app.new_window()
    .with_transparency(true)
    .swapchain_builder({
        window::SwapchainBuilder::default()
            .composite_alpha(vk::swapchain::CompositeAlpha::PostMultiplied)
    })
    .build()

However so far on my Linux machine with mesa intel vulkan driver the PostMultiplied value is not supported and results in a panic!:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: SwapchainCreation(UnsupportedCompositeAlpha)'

Possible solutions

• Investigate the state of support for the PostMultiplied composite value across Vulkan implementations in the wild.
• Check that the incompatibility is not due to some other specified swapchain parameter like ColorSpace or Format.
• Wait for the switch to rendy and address the issue then to avoid duplicate work.

[mitchmindtree]

Seems to also be the case on the following systems:

• Ubuntu 18.04 with an i7 and a GTX970 graphics card (I think with proprietary drivers).
• Win 10 with a GTX 1050 card.

[zperk13]

Having basically the same issue.
Nannou version: 0.18.1
Windows 10 with GTX 1080

I have this code in my mode(&App) function

app
  .new_window()
  .fullscreen()
  .transparent(true)
  .always_on_top(true)
  .view(view)
  .build()
  .unwrap();

and the window has a black background. Theere is no code in my view(...) function drawing a background

[danwilhelm]

@zperk13 I verified this does indeed happen on Windows 10 w/ GTX 1080. For now, just replace fullscreen() with maximized() and make sure you're clearing with a transparent alpha, e.g. draw.background().color(rgba(0.0, 0.0, 0.0, 0.0));.

Other than that, it seems very likely to be a winit issue. There are multiple reports of this here: rust-windowing/winit#2502 ("undecorated" means a window without title bar/etc).

[dirkjf]

Same issue on Linux with X11. Not sure if this is a winit issue, for creating a window with a transparent background with just winit is not a problem.