Introduce RenderOptions.RequiresFullOpacityHandling

[Gillibald]

What does the pull request do?

  <Grid
    RenderOptions.RequiresFullOpacityHandling="True"
    Height="200" 
    HorizontalAlignment="Center" 
    VerticalAlignment="Center" 
    Background="Silver">
    <StackPanel Margin="10" Orientation="Horizontal"
      HorizontalAlignment="Center" VerticalAlignment="Top">
      <Ellipse Margin="0,0,-20,0" Width="80" Height="80" Fill="Green" />
      <Ellipse Margin="-20,0,0,0" Width="80" Height="80" Fill="Blue" />
    </StackPanel>
    <StackPanel Margin="10" Orientation="Horizontal"
      HorizontalAlignment="Center" VerticalAlignment="Bottom" Opacity="0.5">
      <Ellipse Margin="0,0,-20,0" Width="80" Height="80" Fill="Green" />
      <Ellipse Margin="-20,0,0,0" Width="80" Height="80" Fill="Blue" />
    </StackPanel>
  </Grid>

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[timunie]

@Gillibald I guess there are issues that would be solved? e.g.: #9227

[Gillibald]

Yes, it gives control over layered opacity handling per visual so the performance impact isn't that high anymore. It still applies to the whole sub-scene so if you define this on the TopLevel it behaves the same as if you enable it globally via the UserOpacitySaveLayer option.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0038506-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[kekekeks]

We need to push the _currentOpacity value and set it to 1. Otherwise rendering will be broken with the following visual tree structure:

- Border №1 - opacity: 0.8
  - Border №2 - opacity: 0.8, requires full opacity: true
     - Border №3 - opacity: 0.8, requires full opacity: true

In this case Border №3 would get rendered with 0.64 opacity into a layer created by Border №2, while it should be rendered with 0.8.

[kekekeks]

Another problem I think we have is the first two borders: we don't take the accumulated _currentOpacity from Border №1 when rendering the layer from Border №2.

[kekekeks]

So in this branch we still need to use the opacity stack, but instead of _currentOpacity *= opacity; we need _currentOpacity = 1

[billhenn]

Just curious, once this attached property gets added, would .With(new SkiaOptions { UseOpacitySaveLayer = true }); still need to be used or will this new RenderOptions property work independently of the other setting? From the last post's code it looks like either one will activate the feature, correct?

[Gillibald]

Just curious, once this attached property gets added, would .With(new SkiaOptions { UseOpacitySaveLayer = true }); still need to be used or will this new RenderOptions property work independently of the other setting? From the last post's code it looks like either one will activate the feature, correct?

Correct. With the small difference that the Skia option applies globally.

[billhenn]

That's great, it will be much nicer to have this render option available to only affect small subtrees where needed.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0038568-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]