Intense black flickering and artifacts on Windows

[luleyleo]

To check out what the invalidation example looks like on Windows for #872 and noticed intense flickering and rendering artifacts since partial invalidation (#817) has been merged.
Resetting druid to the last commit before the PR was merged gets rid of the flickering.

Does this happen on normal Windows? Or can someone else reproduce this for a VM?

[raphlinus]

Yes, this is broken for me too 😢

[luleyleo]

@jneem do you have any ideas what could cause this?

[jneem]

Sorry about that, I don't know the cause but I'll try to get a windows VM to check it out.

In the meantime, if you change handle.invalidate_rect in druid/src/window.rs to handle.invalidate, does it fix the flickering?

[luleyleo]

@jneem yes, that gets rid of the flickering
Do you have time right now to dig into this or should we disable invalidate_rect for now?

[jneem]

I can start looking into it as soon as the VM image finishes downloading. I have no idea how long it will take, though, so maybe disable it for now.

[jneem]

On windows, I'm having issues with rendering even prior to partial invalidation. If you look at my test branch here, I've reverted to an earlier commit and introduced the example druid-shell/examples/invalidate.rs. This example doesn't do partial invalidation (because it doesn't exist yet); all it does is draw a new rectangle (in a different position) every time it paints. I'm getting a lot of artifacts, including a sort of "shaking" where it looks like the canvas is moving between frames. I can get the artifacts to stop by either

• dragging the window (as long as I'm dragging it, it looks good), or
• changing the default PresentStrategy to Hwnd.

I tried to capture it, but the capture software I tried (OBS studio) doesn't capture it properly unless I change the PresentStrategy to Hwnd.

[xStrom]

I tested this on two physical computers with Windows.

Windows 7

Works just fine, probably because it uses PresentStrategy::Hwnd.

Windows 10

Has flicker problems, although there are no weird artifacts, black rects, or shaking. Instead the problem seems to be rather specific and reproducable.

1. I click on the right side to set the circle on the bottom
2. I click again to move the circle to the top
3. I go to the left side of the window with my mouse
   • When the mouse is over a button everything seems fine
   • When the mouse is over the background then on the right side it shows the older bottom circle

So when I just move the mouse over all the buttons, the right side flickers between the circle being at the top and bottom.

[jneem]

Thanks for checking this out! So I've spent some time reading the docs, and I think the issue is that IDXGISwapChain is using multiple buffers (possibly overlapping?) in the background, and it uses different buffers on different frames. That's why we sometimes see flickering and shaking (flipping between buffers that are partly black, or are overlapping), and it also explains your incorrect circle position (because one of the buffers has the circle in the wrong place).

It appears that IDXGISwapChain::Present1 has some parameters to pass information about invalid rects, and I guess I need to use them. I'll see if I can figure them out later today or tomorrow.

[raphlinus]

Yes, swapchains use multiple buffers, this is generally true in any high performance presentation framework, as using a single buffer entails copying. Also yes, IDXGISwapChain::Present1 is the right API to use for incremental presentation on Windows.