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diff --git a/src/apps/cam/frame_sink.cpp b/src/apps/cam/frame_sink.cpp
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+++ b/src/apps/cam/frame_sink.cpp
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+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (C) 2021, Ideas on Board Oy
+ *
+ * frame_sink.cpp - Base Frame Sink Class
+ */
+
+#include "frame_sink.h"
+
+/**
+ * \class FrameSink
+ * \brief Abstract class to model a consumer of frames
+ *
+ * The FrameSink class models the consumer that processes frames after a request
+ * completes. It receives requests through processRequest(), and processes them
+ * synchronously or asynchronously. This allows frame sinks to hold onto frames
+ * for an extended period of time, for instance to display them until a new
+ * frame arrives.
+ *
+ * A frame sink processes whole requests, and is solely responsible for deciding
+ * how to handle different frame buffers in case multiple streams are captured.
+ */
+
+FrameSink::~FrameSink()
+{
+}
+
+int FrameSink::configure([[maybe_unused]] const libcamera::CameraConfiguration &config)
+{
+	return 0;
+}
+
+void FrameSink::mapBuffer([[maybe_unused]] libcamera::FrameBuffer *buffer)
+{
+}
+
+int FrameSink::start()
+{
+	return 0;
+}
+
+int FrameSink::stop()
+{
+	return 0;
+}
+
+/**
+ * \fn FrameSink::processRequest()
+ * \param[in] request The request
+ *
+ * This function is called to instruct the sink to process a request. The sink
+ * may process the request synchronously or queue it for asynchronous
+ * processing.
+ *
+ * When the request is processed synchronously, this function shall return true.
+ * The \a request shall not be accessed by the FrameSink after the function
+ * returns.
+ *
+ * When the request is processed asynchronously, the FrameSink temporarily takes
+ * ownership of the \a request. The function shall return false, and the
+ * FrameSink shall emit the requestProcessed signal when the request processing
+ * completes. If the stop() function is called before the request processing
+ * completes, it shall release the request synchronously.
+ *
+ * \return True if the request has been processed synchronously, false if
+ * processing has been queued
+ */