Reallocate Shared Video Frame Buffer Without Copying

Util/FFmpeg/FFmpegNativeEncoder.cs

@@ -824,16 +824,7 @@ namespace ADOFAIRenderer.Util.FFmpeg
                     if (_pSrcFrame == null || _pDstFrame == null || _pPacket == null)
                         throw new Exception("Failed to allocate frame or packet structure.");
 
-                    // Set format and size of destination frame, then allocate actual pixel buffer.
-                    // av_frame_get_buffer: Allocates memory matching format, width, height and
-                    // sets data[]/linesize[] pointers. Second argument 0 means default alignment.
-                    _pDstFrame->format = (int)_dstPixFmt;
-                    _pDstFrame->width = _settings.Width;
-                    _pDstFrame->height = _settings.Height;
-                    _pDstFrame->color_range = AVColorRange.AVCOL_RANGE_JPEG;
-
-                    if (ffmpeg.av_frame_get_buffer(_pDstFrame, 0) < 0)
-                        throw new Exception($"Failed to allocate {_dstPixFmt} frame buffer.");
+                    AllocateDstFrameBuffer();
 
                     // sws_getContext: Initialize software scaler (colorspace converter).
                     // Input: RGBA (Unity capture format, 4 bytes/pixel)
@@ -1201,12 +1192,50 @@ namespace ADOFAIRenderer.Util.FFmpeg
             }
         }
 
+        /// <summary>
+        /// Sets the format, size and color range of the destination frame (_pDstFrame) and allocates a new pixel buffer for it.
+        /// av_frame_get_buffer allocates memory matching format, width and height and sets the data[]/linesize[] pointers.
+        /// Second argument 0 means default alignment.
+        /// Reference: https://ffmpeg.org/doxygen/trunk/group__lavu__frame.html (av_frame_get_buffer)
+        /// </summary>
+        private void AllocateDstFrameBuffer()
+        {
+            _pDstFrame->format = (int)_dstPixFmt;
+            _pDstFrame->width = _settings.Width;
+            _pDstFrame->height = _settings.Height;
+            _pDstFrame->color_range = AVColorRange.AVCOL_RANGE_JPEG;
+
+            var ret = ffmpeg.av_frame_get_buffer(_pDstFrame, 0);
+            if (ret < 0)
+                throw new Exception($"Failed to allocate {_dstPixFmt} frame buffer — {GetFFmpegErrorString(ret)} (Code: {ret})");
+        }
+
+        /// <summary>
+        /// Makes the pixel buffer of the destination frame (_pDstFrame) writable before the next frame is written into it.
+        ///
+        /// After avcodec_send_frame, the encoder may keep a reference to the buffer of the submitted frame
+        /// until that frame is encoded (e.g. QSV in system memory input mode). The buffer is then shared and not writable
+        /// (av_frame_is_writable returns 0).
+        /// In that case the frame releases its reference (av_frame_unref) and a new buffer is allocated.
+        /// The encoder keeps the previous buffer until it no longer needs it. The previous contents are not copied,
+        /// because every plane is overwritten by the next frame.
+        ///
+        /// Reference: https://ffmpeg.org/doxygen/trunk/group__lavu__frame.html (av_frame_is_writable, av_frame_unref)
+        /// </summary>
+        private void PrepareDstFrameForWrite()
+        {
+            if (ffmpeg.av_frame_is_writable(_pDstFrame) != 0) return;
+
+            ffmpeg.av_frame_unref(_pDstFrame);
+            AllocateDstFrameBuffer();
+        }
+
         /// <summary>
         /// Encodes a frame by directly copying already pre-converted NV12 or YUV420P frame data.
         /// </summary>
         private void WritePreConvertedFrame(byte* buffer)
         {
-            ffmpeg.av_frame_make_writable(_pDstFrame);
+            PrepareDstFrameForWrite();
 
             var width = _settings.Width;
             var height = _settings.Height;
@@ -1320,20 +1349,17 @@ namespace ADOFAIRenderer.Util.FFmpeg
         /// Core logic for actual frame encoding.
         ///
         /// Processing flow:
-        ///   1. av_frame_make_writable: Check if destination frame buffer is writable
+        ///   1. PrepareDstFrameForWrite: Makes the destination frame buffer writable
         ///      (Allocates a new buffer if encoder is still referencing previous frame's buffer)
         ///   2. sws_scale: RGBA → YUV420P/NV12 colorspace conversion
         ///   3. (QSV hw_frames mode) av_hwframe_transfer_data: Upload NV12 → QSV hardware surface
         ///   4. Pass to encoder after setting PTS
         ///
-        /// Reference: https://ffmpeg.org/doxygen/trunk/group__lavu__frame.html (av_frame_make_writable)
         /// Reference: https://ffmpeg.org/doxygen/trunk/group__libsws.html (sws_scale)
         /// </summary>
         private void WriteFrame()
         {
-            // Ensure writable state since encoder might still be referencing previous frame's buffer.
-            // Internally allocates a new buffer if referenced.
-            ffmpeg.av_frame_make_writable(_pDstFrame);
+            PrepareDstFrameForWrite();
 
             // ── Colorspace conversion: RGBA → YUV420P or NV12 ──
             // sws_scale parameters explanation: