perf(windows): skip the webcam frame copy when its sequence has not changed

copyLatestFrame now takes the caller's last seen sequence and returns false without copying when nothing new arrived. Both capture backends share the rule through snapshotWebcamFrame, covered by webcam_snapshot_test.

Author: EtienneLescot

electron/native/wgc-capture/CMakeLists.txt

@@ -157,3 +157,16 @@ target_compile_definitions(frame_visibility_test PRIVATE
 )
 
 target_compile_options(frame_visibility_test PRIVATE /EHsc /W4 /utf-8)
+
+add_executable(webcam_snapshot_test
+  src/dshow_webcam_capture.h
+  src/webcam_snapshot_test.cpp
+)
+
+target_compile_definitions(webcam_snapshot_test PRIVATE
+  NOMINMAX
+  WIN32_LEAN_AND_MEAN
+  _WIN32_WINNT=0x0A00
+)
+
+target_compile_options(webcam_snapshot_test PRIVATE /EHsc /W4 /utf-8)

electron/native/wgc-capture/src/dshow_webcam_capture.cpp

@@ -576,17 +576,10 @@ void DirectShowWebcamCapture::storeFrame(const BYTE* buffer, long length) {
     latestFrameSequence_ += 1;
 }
 
-bool DirectShowWebcamCapture::copyLatestFrame(WebcamFrameSnapshot& destination) {
+bool DirectShowWebcamCapture::copyLatestFrame(WebcamFrameSnapshot& destination, uint64_t lastSeenSequence) {
     std::scoped_lock lock(frameMutex_);
-    if (latestFrame_.empty() || width_ <= 0 || height_ <= 0) {
-        return false;
-    }
-
-    destination.data = latestFrame_;
-    destination.width = width_;
-    destination.height = height_;
-    destination.sequence = latestFrameSequence_;
-    return true;
+    return snapshotWebcamFrame(
+        latestFrame_, width_, height_, latestFrameSequence_, lastSeenSequence, destination);
 }
 
 int DirectShowWebcamCapture::width() const {

electron/native/wgc-capture/src/dshow_webcam_capture.h

@@ -16,6 +16,33 @@ struct WebcamFrameSnapshot {
     uint64_t sequence = 0;
 };
 
+/**
+ * Copies the latest camera frame into `destination`, unless the caller already has it.
+ *
+ * The video writer asks on every screen tick, 60 times a second, while a camera
+ * delivers 30: copying first and comparing sequences after cost a full frame
+ * copied for nothing on every other tick. Returns false, leaving `destination`
+ * untouched, when there is no frame yet or its sequence is `lastSeenSequence`.
+ * Both capture backends route `copyLatestFrame` through this.
+ */
+inline bool snapshotWebcamFrame(
+    const std::vector<BYTE>& frame,
+    int width,
+    int height,
+    uint64_t sequence,
+    uint64_t lastSeenSequence,
+    WebcamFrameSnapshot& destination) {
+    if (frame.empty() || width <= 0 || height <= 0 || sequence == lastSeenSequence) {
+        return false;
+    }
+
+    destination.data = frame;
+    destination.width = width;
+    destination.height = height;
+    destination.sequence = sequence;
+    return true;
+}
+
 class DirectShowWebcamCapture {
 public:
     DirectShowWebcamCapture() = default;
@@ -33,7 +60,7 @@ class DirectShowWebcamCapture {
         int requestedFps);
     bool start();
     void stop();
-    bool copyLatestFrame(WebcamFrameSnapshot& destination);
+    bool copyLatestFrame(WebcamFrameSnapshot& destination, uint64_t lastSeenSequence);
 
     int width() const;
     int height() const;

electron/native/wgc-capture/src/main.cpp

@@ -1175,8 +1175,7 @@ int wmain(int argc, wchar_t* argv[]) {
                 }
                 if (webcamActive) {
                     WebcamFrameSnapshot candidateWebcamFrame;
-                    if (webcamCapture.copyLatestFrame(candidateWebcamFrame) &&
-                        candidateWebcamFrame.sequence != latestWebcamSequence &&
+                    if (webcamCapture.copyLatestFrame(candidateWebcamFrame, latestWebcamSequence) &&
                         hasVisibleWebcamContent(candidateWebcamFrame.data, webcamCapture.deliversNv12())) {
                         latestWebcamFrame = std::move(candidateWebcamFrame.data);
                         latestWebcamWidth = candidateWebcamFrame.width;
@@ -1470,7 +1469,7 @@ int wmain(int argc, wchar_t* argv[]) {
         const auto webcamDeadline = std::chrono::steady_clock::now() + std::chrono::seconds(3);
         while (std::chrono::steady_clock::now() < webcamDeadline && !hasVisibleWebcamFrame) {
             WebcamFrameSnapshot candidateWebcamFrame;
-            if (webcamCapture.copyLatestFrame(candidateWebcamFrame) &&
+            if (webcamCapture.copyLatestFrame(candidateWebcamFrame, latestWebcamSequence) &&
                 hasVisibleWebcamContent(candidateWebcamFrame.data, webcamCapture.deliversNv12())) {
                 latestWebcamFrame = std::move(candidateWebcamFrame.data);
                 latestWebcamWidth = candidateWebcamFrame.width;

electron/native/wgc-capture/src/webcam_capture.cpp

@@ -621,20 +621,13 @@ void WebcamCapture::captureLoop() {
     CoUninitialize();
 }
 
-bool WebcamCapture::copyLatestFrame(WebcamFrameSnapshot& destination) {
+bool WebcamCapture::copyLatestFrame(WebcamFrameSnapshot& destination, uint64_t lastSeenSequence) {
     if (usingDirectShow_) {
-        return directShowCapture_.copyLatestFrame(destination);
+        return directShowCapture_.copyLatestFrame(destination, lastSeenSequence);
     }
     std::scoped_lock lock(frameMutex_);
-    if (latestFrame_.empty() || width_ <= 0 || height_ <= 0) {
-        return false;
-    }
-
-    destination.data = latestFrame_;
-    destination.width = width_;
-    destination.height = height_;
-    destination.sequence = latestFrameSequence_;
-    return true;
+    return snapshotWebcamFrame(
+        latestFrame_, width_, height_, latestFrameSequence_, lastSeenSequence, destination);
 }
 
 bool WebcamCapture::deliversNv12() const {

electron/native/wgc-capture/src/webcam_capture.h

@@ -32,7 +32,7 @@ class WebcamCapture {
         bool preferNv12);
     bool start();
     void stop();
-    bool copyLatestFrame(WebcamFrameSnapshot& destination);
+    bool copyLatestFrame(WebcamFrameSnapshot& destination, uint64_t lastSeenSequence);
 
     int width() const;
     int height() const;

electron/native/wgc-capture/src/webcam_snapshot_test.cpp

@@ -0,0 +1,46 @@
+#include "dshow_webcam_capture.h"
+
+#include <cstdio>
+
+namespace {
+
+int failures = 0;
+
+void expect(const char* label, bool condition) {
+    if (condition) {
+        return;
+    }
+    std::printf("FAIL %s\n", label);
+    ++failures;
+}
+
+}  // namespace
+
+int main() {
+    const std::vector<BYTE> frame(4 * 2 * 2, 0x80);
+
+    // A new sequence is copied, with the size and sequence it was stored with.
+    WebcamFrameSnapshot fresh;
+    expect("new sequence is copied", snapshotWebcamFrame(frame, 2, 2, 7, 6, fresh));
+    expect("new sequence carries its data", fresh.data == frame);
+    expect("new sequence carries its size", fresh.width == 2 && fresh.height == 2);
+    expect("new sequence carries its number", fresh.sequence == 7);
+
+    // The regression this file exists for: the writer polls at the screen's
+    // rate, twice the camera's, and a frame it already holds must cost nothing.
+    WebcamFrameSnapshot unchanged;
+    expect("unchanged sequence reports nothing new", !snapshotWebcamFrame(frame, 2, 2, 7, 7, unchanged));
+    expect("unchanged sequence copies nothing", unchanged.data.empty() && unchanged.sequence == 0);
+
+    // No frame stored yet: nothing to hand out, whatever the caller last saw.
+    WebcamFrameSnapshot empty;
+    expect("empty frame is not copied", !snapshotWebcamFrame({}, 2, 2, 1, 0, empty));
+    expect("unknown size is not copied", !snapshotWebcamFrame(frame, 0, 2, 1, 0, empty));
+
+    if (failures > 0) {
+        std::printf("%d webcam snapshot check(s) failed\n", failures);
+        return 1;
+    }
+    std::printf("webcam snapshot checks passed\n");
+    return 0;
+}

scripts/build-windows-wgc-helper.mjs

@@ -125,3 +125,12 @@ if (!fs.existsSync(frameVisibilityTestPath)) {
 // reads every black frame as content.
 await run(frameVisibilityTestPath, [], { cwd: BUILD_DIR });
 console.log(`Passed ${frameVisibilityTestPath}`);
+
+const webcamSnapshotTestPath = path.join(BUILD_DIR, "webcam_snapshot_test.exe");
+if (!fs.existsSync(webcamSnapshotTestPath)) {
+	throw new Error(`WGC helper build completed but ${webcamSnapshotTestPath} was not found.`);
+}
+// Guards the per-tick webcam poll. The writer asks at the screen's rate, twice
+// the camera's, so a frame it already holds must not be copied again.
+await run(webcamSnapshotTestPath, [], { cwd: BUILD_DIR });
+console.log(`Passed ${webcamSnapshotTestPath}`);