Xaml.Media.Imaging.SurfaceImageSource variable margins

[tom-huntington]

Edit I realized the problem only reproduces if I have the Image in a ScrollViewer (i.e. the left margin is always 2px otherwise) and so don't care about this anymore.

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Describe the bug

Tutorial on SurfaceImageSource for context.

ISurfaceImageSourceNative::BeginDraw returns a surface that is bigger than the pixelWidth and pixelHeight you parse to the SurfaceImageSource constructor.

The problem is that the left margin actually varies between different runs of the program. The top margin is always 0px. The right and bottom margins change in proportion with the hight and width.

If I translate my rendering with a left offset of 4px. I will get only my clear colour displayed some of the time. Other times I will get a pixel or two of SurfaceImageSource's clear colour 😢.

I can get the actual size from IDXGISurface::GetDesc but that doesn't help.

Maybe being a pixel or two off is not the end of the world.

Steps to reproduce the bug

<ScrollViewer HorizontalScrollMode="Enabled" HorizontalScrollBarVisibility="Visible">
    <Image Margin="960,780,0,0" Stretch="None" x:Name="SentenceBySentenceImage"/>
</ScrollViewer>

NuGet package version

WinUI 3 - Windows App SDK 1.5.4: 1.5.240607001

[DarranRowe]

There is no here of whether or not the offset parameter is being used correctly. It is also true that the documentation for this parameter isn't good in the reference. The Windows.UI.Xaml version only describes it as "Receives the point (x,y) offset of the surface that will be drawn into." where the Microsoft.UI.Xaml version doesn't even document anything.
Looking at the DirectComposition documentation for IDCompositionSurface::BeginDraw and the Windows.UI.Composition ICompositionDrawingSurfaceInterop::BeginDraw provide some extra context for the parameter with the same name. That is, you need to offset your drawing rectangle by the x and y values given in the offset parameter.

[tom-huntington]

Yes I forgot about the [out] POINT *updateOffset parameter.

Thanks!