Webcam grab: SharedReadOnly mode, VideoPreview/VideoRecord source fallback, name the blocking app in the error (NVIDIA Broadcast finding)

Services/MediaCaptureFrameSource.cs

@@ -43,14 +43,21 @@ public sealed class MediaCaptureFrameSource : ICameraFrameSource
                 VideoDeviceId = _deviceId,
                 StreamingCaptureMode = StreamingCaptureMode.Video,
                 MemoryPreference = MediaCaptureMemoryPreference.Cpu,
+                SharingMode = MediaCaptureSharingMode.SharedReadOnly,
             };
             await capture.InitializeAsync(settings);
 
-            var colorSource = capture.FrameSources
+            var colorSource = capture.FrameSources.Values
-                .FirstOrDefault(pair => pair.Value.Info.MediaStreamType == MediaStreamType.VideoPreview)
+                .OrderBy(s => s.Info.MediaStreamType == MediaStreamType.VideoPreview ? 0 : 1)
-                .Value;
+                .FirstOrDefault(s => s.Info.MediaStreamType is MediaStreamType.VideoPreview or MediaStreamType.VideoRecord);
             if (colorSource == null)
-                throw new InvalidOperationException($"No video preview source on camera '{_deviceId}'.");
+            {
+                var kinds = string.Join(", ", capture.FrameSources.Values
+                    .Select(s => s.Info.MediaStreamType).Distinct());
+                throw new InvalidOperationException(
+                    $"Camera '{_deviceId}' exposes no video source (found: {kinds}). " +
+                    "It may be locked by another app (e.g. NVIDIA Broadcast).");
+            }
 
             reader = await capture.CreateFrameReaderAsync(colorSource, MediaEncodingSubtypes.Bgra8);
             reader.FrameArrived += OnFrameArrived;

ai.md

@@ -84,10 +84,19 @@ C# / WPF (.NET 8) following MVVM:
 - **Seam-first:** everything above the WinRT layer speaks only `VideoFrame` (normalized tightly-packed
   BGRA8) + `CameraDeviceInfo`/`ICameraEnumerator`/`ICameraFrameSource` interfaces. Tests inject fakes;
   screen capture and background removal later feed the same seam.
-- **CPU-first:** `MediaCaptureInitializationSettings { MemoryPreference = Cpu, StreamingCaptureMode = Video }`,
+- **CPU-first:** `MediaCaptureInitializationSettings { MemoryPreference = Cpu, StreamingCaptureMode = Video,
-  frames pulled via `CreateFrameReaderAsync(colorSource, MediaEncodingSubtypes.Bgra8)` — the pipeline does
+  SharingMode = SharedReadOnly }`, frames pulled via
-  any format conversion, so every `FrameArrived` yields a ready BGRA8 `SoftwareBitmap` (bytes read via
+  `CreateFrameReaderAsync(colorSource, MediaEncodingSubtypes.Bgra8)` — the pipeline does any format
+  conversion, so every `FrameArrived` yields a ready BGRA8 `SoftwareBitmap` (bytes read via
   `WindowsRuntimeMarshal.TryGetDataUnsafe`, not marshalled copies).
+- **Source pick, not first hit:** the frame reader is bound to the first source that is `VideoPreview`
+  (preferred) or `VideoRecord`, not blindly the first preview source. If a camera exposes neither, the
+  failure names the device and the stream types it *does* expose. `SharedReadOnly` lets the capture
+  coexist with other apps that share the camera.
+- **Known failure: NVIDIA Broadcast** — it opens the physical webcam exclusively, so `InitializeAsync`
+  fails with "camera in use" (or, if init slips through, the device exposes no preview source). Fix:
+  quit Broadcast while streaming, or pick its virtual "NVIDIA Broadcast" device from the picker and the
+  app captures the processed feed. This is a real-device finding (Logitech `VID_046D&PID_082D`).
 - **TFM:** `net8.0-windows10.0.19041.0` (app + tests) pulls the WinRT projection from the SDK reference
   packs — no NuGet package, no capability manifest (unpackaged desktop app works; the Windows privacy
   camera toggle still applies). `EnableWindowsTargeting` keeps WSL builds working.