Derive NDI FourCC from snapshot pixel format instead of OS check

Rather than inferring the pixel channel order from the OS platform,
read it directly from the Bitmap.Format of the compositor snapshot.
The backend decides the layout (BGRA on D3D, RGBA on Metal) and
Format reflects this, so the FourCC now tracks any future backend
changes automatically without manual platform guards.

Adds FourCC to PendingFrame so the send thread uses the value
recorded at capture time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

EmoTracker/Extensions/NDI/NdiSendContainer.cs

@@ -1,6 +1,7 @@
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Media.Imaging;
+using Avalonia.Platform;
 using Avalonia.Rendering.Composition;
 using Avalonia.Threading;
 using NewTek;      // NDIlib + nested types
@@ -202,6 +203,7 @@ private struct PendingFrame
             public int FrameRateNum, FrameRateDen;
             public int Scale;
             public bool DropShadow;
+            public NDIlib.FourCC_type_e FourCC;
         }
 
         // -------------------------------------------------------------------------
@@ -434,6 +436,14 @@ public async Task TriggerCaptureAsync()
                 if (snapshot == null || _disposed)
                     return;
 
+                // Derive the NDI pixel format from the snapshot's reported format rather
+                // than inferring it from the OS.  The compositor backend decides channel
+                // order (BGRA on Windows/D3D, RGBA on macOS/Metal), and reading it here
+                // handles any future backend changes automatically.
+                NDIlib.FourCC_type_e ndiFormat = snapshot.Format == PixelFormat.Rgba8888
+                    ? NDIlib.FourCC_type_e.FourCC_type_RGBA
+                    : NDIlib.FourCC_type_e.FourCC_type_BGRA;
+
                 int width  = snapshot.PixelSize.Width;
                 int height = snapshot.PixelSize.Height;
                 int stride     = width * 4;
@@ -488,6 +498,7 @@ public async Task TriggerCaptureAsync()
                     FrameRateDen = NdiFrameRateDenominator,
                     Scale        = Math.Max(NdiIntegerScale, 1),
                     DropShadow   = NdiDropShadow,
+                    FourCC       = ndiFormat,
                 });
 
                 LogCaptureHeartbeat(width, height);
@@ -575,12 +586,10 @@ private void ProcessOneFrame()
                 {
                     xres                 = scaledWidth,
                     yres                 = scaledHeight,
-                    // Avalonia's Metal backend (macOS/Linux) produces RGBA pixels;
-                    // the Windows D3D/Skia backend produces BGRA.  Tell NDI which
-                    // layout the bytes are actually in so colours render correctly.
-                    FourCC               = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
-                                             ? NDIlib.FourCC_type_e.FourCC_type_BGRA
-                                             : NDIlib.FourCC_type_e.FourCC_type_RGBA,
+                    // FourCC is derived from the snapshot's reported pixel format at
+                    // capture time — see TriggerCaptureAsync — so it matches whatever
+                    // the compositor backend actually produced.
+                    FourCC               = frame.FourCC,
                     frame_rate_N         = frame.FrameRateNum,
                     frame_rate_D         = frame.FrameRateDen,
                     picture_aspect_ratio = (float)frame.Width / frame.Height,