Wpf: WebP with alpha image is broken

[neelabo]

• .NET Core Version: 3.0 Preview7
• Windows version: Windows 10 version 1903
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

Problem description:

WebP can now be displayed on Windows 10, but when trying to display transparent WebP with WPF, the transparent part becomes broken display.

However, BitmapImage using DecodePixelWidth different from the source is displayed correctly.

I submitted a similar report as an issue with the .NET Framework.

Minimal repro:

• The test WebP image is here.
• <Image Source="1_webp_a.webp"/> only.

[jairbubbles]

Indeed, same behavior on .NET Framework, the .webp version is broken while the .png version displays correctly.

This code:

<UniformGrid Rows="1">
     <Image Source="https://www.gstatic.com/webp/gallery3/1_webp_ll.webp"></Image>
     <Image Source="https://www.gstatic.com/webp/gallery3/1_webp_ll.png"></Image>
</UniformGrid>

Will produce:

(Windows version 1903 too)

[h82258652]

https://github.com/dotnet/wpf/blob/ac9d1b7a6b0ee7c44fd2875a1174b820b3940619/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Imaging/BitmapDecoder.cs
There is no webp decoder. So wpf use the UnknownBitmapDecoder and it can not handle the alpha in webp format.

[weltkante]

There is no webp decoder. So wpf use the UnknownBitmapDecoder

Thats a contradiction, I think you mean that there's no explicit C#/.NET class for it but (I assume) it just falls back to WIC and lets Windows resolve the decoder. All decoders are native implementations just some have explicit wrappers in .NET - but if you look at their implementation they all do the same as the UnknownBitmapDecoder and forward to base class

So the fact that there is no named class for it doesn't really mean anything.

[weltkante]

@grubioe /cc @vatsan-madhavan can we get that reevaluated? ~If this is a bug in WIC or (external to WPF) WEBP decoder it would be really annoying if its not forwarded to the right Windows team just because the WPF team planned it for "future"~

[edit] its a bug in WPF but it'd still be nice to reevaluate and fix in 5.0 timeframe instead of leaving it on "future"

[jairbubbles]

The thumbnail appears correctly on Windows explorer. I don't know if it ends up using the same decoder though.

[weltkante]

Unless someone beats me to it I'll test with native WIC when I get home to determine if this is a WPF bug or WIC bug, for some reason my Windows 10 / 1909 installation at work doesn't have the webp decoder (I get an exception when referencing the image in WPF) and Windows doesn't offer it as optional feature either. ~I'm pretty sure I've seen it yesterday on my home machine when I happened to scroll through optional features.~

[edit] apparently its not an optional feature, you can find it under "apps and features" as installed application but it isn't uninstallable. ~Still no idea how to install webp support on my work machine, would have assumed it gets installed automatically with the 1903 or 1909 update.~

[edit] apparently you can install it from the store

[weltkante]

The bug is in WPF, probably in native code which hasn't been open sourced yet. WIC can load the image just fine, test code to load the image via WIC attached.

In fact I also figured out what happens, if you call new BitmapImage(new Uri(...)).Format on the webp image it reports Bgr32 which is wrong. WPF simply ignores the alpha channel even though WIC reports that the image has an alpha channel. The webp image has color artifacts in transparent places so it looks weird when they aren't transparent. (Its probably an optimization of the webp encoding because color in transparent regions is irrelevant so it doesn't bother to encode it and just leaves it at whatever is convenient.)

example code to load image via WIC

private void LoadImage()
{
    // Referencing SharpDX.Direct2D1 nuget package
    using (var factory = new SharpDX.WIC.ImagingFactory2())
    using (var decoder = new SharpDX.WIC.BitmapDecoder(factory, "1_webp_a.webp", SharpDX.WIC.DecodeOptions.CacheOnLoad))
    {
        Debug.Assert(decoder.FrameCount == 1);
        using (var frame = decoder.GetFrame(0))
        {
            Debug.Assert(frame.PixelFormat == SharpDX.WIC.PixelFormat.Format32bppRGBA);
            var size = frame.Size;
            var stride = size.Width * 4;
            var pixels = new byte[stride * size.Height];
            frame.CopyPixels(pixels, stride);

            for (int iy = 0; iy < size.Height; iy++)
            {
                for (int ix = 0; ix < size.Width; ix++)
                {
                    // convert from RGBA to BGRA
                    Swap(ref pixels[iy * stride + ix * 4 + 0], ref pixels[iy * stride + ix * 4 + 2]);

                    // uncomment to reproduce the bug, WPF seems to ignore the alpha channel
                    // which you can simulate by setting alpha to 255 (opaque)
                    //pixels[iy * stride + ix * 4 + 3] = 255;
                }
            }

            var bitmap = new WriteableBitmap(size.Width, size.Height, 96, 96, PixelFormats.Bgra32, null);
            bitmap.Lock();
            try { bitmap.WritePixels(new Int32Rect(0, 0, size.Width, size.Height), pixels, stride, 0); }
            finally { bitmap.Unlock(); }
            ImageControl.Source = bitmap;
        }
    }
}

private static void Swap(ref byte lhs, ref byte rhs)
{
    var tmp = lhs;
    lhs = rhs;
    rhs = tmp;
}

[vatsan-madhavan]

@arpitmathur can we bring this one up in our bug triage again for discussion ?

More than just WebP, I’d like us to think about how we are going to support emerging media formats for which _native_ WIC support is lacking. We may want to find out how Office does this today, for e.g. to see if their approach is reusable for us.

{It could be that we wait till Jan to dig into this.}

[jairbubbles]

Would be nice to have native support for .gif. I usually use https://github.com/XamlAnimatedGif/WpfAnimatedGif for this.