From e35e8b2da19a2a2d2eeb46a5efd20f8e8ef42e78 Mon Sep 17 00:00:00 2001
From: Etienne Lescot <etiennelescot@gmail.com>
Date: Sun, 19 Jul 2026 10:34:48 +0200
Subject: [PATCH] fix(recording): round window-capture dimensions up to even
 for WGC
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Fixes #60: recording a single window (as opposed to a full display) on
Windows produced a solid black video for the entire clip.

Root cause: GraphicsCaptureItem::Size() for a window capture item reports the
window's real client-area pixel dimensions verbatim, which are frequently odd
(arbitrary resize, DPI rounding) — unlike monitor/display capture items,
which are always even. H.264 encoding (and the RGB32->NV12 conversion
feeding it) requires even width/height. The frame pool and captureWidth()/
captureHeight() (which configures the encoder's input media type) both used
the raw, possibly-odd item size, so an odd-dimensioned window capture fed the
Media Foundation H.264 encoder a size it can't encode correctly, producing
black output.

Fix: round window capture dimensions up to the nearest even value once, in
WgcSession::createCaptureItem(HWND), and use that same rounded size (not the
raw item size) for both the Direct3D11CaptureFramePool buffer and
captureWidth()/captureHeight(), so every consumer of the session agrees on
one even-dimensioned size. Monitor/display capture is untouched (always even
in practice).

Verified: built wgc-capture.exe via CMake/Ninja (VS 18 Insiders x64), opened
a real Notepad window resized to an odd 789x595 client area, and drove the
helper directly against it (sourceType:"window", real HWND).
- Original (pre-fix) binary: 5.4KB output, ffmpeg blackdetect flags the
  entire clip as black (black_duration ~= full clip length).
- Fixed binary: 36.5KB output (consistent with real, compressible frame
  content vs. a solid-color clip), ffmpeg blackdetect reports zero black
  segments across the same clip.
- Sanity-checked normal display/monitor capture is unaffected (unchanged
  code path).
---
 .../native/wgc-capture/src/wgc_session.cpp    | 30 ++++++++++++++++---
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/electron/native/wgc-capture/src/wgc_session.cpp b/electron/native/wgc-capture/src/wgc_session.cpp
index e20096c4ea..89f0b55fe0 100644
--- a/electron/native/wgc-capture/src/wgc_session.cpp
+++ b/electron/native/wgc-capture/src/wgc_session.cpp
@@ -32,6 +32,28 @@ int64_t timeSpanToHns(wf::TimeSpan const& value) {
     return value.count();
 }
 
+// H.264 encoding (and the RGB32->NV12 conversion feeding it) requires even
+// frame dimensions. Monitor resolutions are always even in practice, so
+// CreateForMonitor items never hit this. Windows, however, frequently have
+// odd client-area dimensions (arbitrary drag-resize, DPI rounding), and
+// GraphicsCaptureItem::Size() reports the window's *actual* size verbatim.
+// If we requested a Direct3D11CaptureFramePool sized to that odd value while
+// the rest of the pipeline (main.cpp's bitrate calc, MFEncoder) rounds down
+// to even, the frame pool's real DXGI textures end up one pixel wider/taller
+// than the staging texture the encoder allocates. ID3D11DeviceContext::
+// CopyResource silently no-ops on a size mismatch (it only emits a debug-
+// layer warning), so the staging texture never receives pixel data and the
+// output is solid black for the entire recording -- or, if the mismatch
+// trips up the video MFT's input negotiation, SetInputMediaType fails
+// outright. Rounding up to the nearest even size here, and using that
+// rounded size (not the raw item size) for both the frame pool and
+// `captureWidth()`/`captureHeight()`, keeps every consumer of this session
+// looking at the exact same dimensions as the real captured texture.
+int roundUpToEven(int value) {
+    const int clamped = std::max(2, value);
+    return (clamped % 2 == 0) ? clamped : clamped + 1;
+}
+
 } // namespace
 
 WgcSession::~WgcSession() {
@@ -135,8 +157,8 @@ bool WgcSession::createCaptureItem(HWND window) {
 
     item_ = item;
     const auto size = item_.Size();
-    width_ = static_cast<int>(size.Width);
-    height_ = static_cast<int>(size.Height);
+    width_ = roundUpToEven(static_cast<int>(size.Width));
+    height_ = roundUpToEven(static_cast<int>(size.Height));
     return width_ > 0 && height_ > 0;
 }
 
@@ -199,7 +221,7 @@ bool WgcSession::initialize(HMONITOR monitor, int fps, bool captureCursor) {
         winrtDevice_,
         wgdx::DirectXPixelFormat::B8G8R8A8UIntNormalized,
         2,
-        item_.Size());
+        winrt::Windows::Graphics::SizeInt32{width_, height_});
     session_ = framePool_.CreateCaptureSession(item_);
 
     if (!applySessionOptions(captureCursor)) {
@@ -223,7 +245,7 @@ bool WgcSession::initialize(HWND window, int fps, bool captureCursor) {
         winrtDevice_,
         wgdx::DirectXPixelFormat::B8G8R8A8UIntNormalized,
         2,
-        item_.Size());
+        winrt::Windows::Graphics::SizeInt32{width_, height_});
     session_ = framePool_.CreateCaptureSession(item_);
 
     if (!applySessionOptions(captureCursor)) {