VIDEO: Fix reverse playback in AVIDecoder

video/avi_decoder.cpp

@@ -129,6 +129,23 @@ bool AVIDecoder::isSeekable() const {
 	return isVideoLoaded() && !_indexEntries.empty();
 }
 
+const Graphics::Surface *AVIDecoder::decodeNextFrame() {
+	// When playing in reverse, we need to seek to the correct prior frame
+	AVIVideoTrack *track = nullptr;
+	bool isReversed = false;
+	for (int idx = _videoTracks.size() - 1; idx >= 0; --idx) {
+		track = static_cast<AVIVideoTrack *>(_videoTracks[idx].track);
+		isReversed |= track->isReversed();
+	}
+
+	if (isReversed) {
+		Audio::Timestamp time = track->getFrameTime(getCurFrame());
+		seekIntern(time);
+	}
+
+	return VideoDecoder::decodeNextFrame();
+}
+
 const Graphics::Surface *AVIDecoder::decodeNextTransparency() {
 	if (!_transparencyTrack.track)
 		return nullptr;
@@ -544,11 +561,6 @@ void AVIDecoder::handleNextPacket(TrackStatus &status) {
 	if (!isReversed) {
 		// Start us off in this position next time
 		status.chunkSearchOffset = _fileStream->pos();
-	} else {
-		// Seek to the prior frame
-		assert(videoTrack);
-		Audio::Timestamp time = videoTrack->getFrameTime(getCurFrame());
-		seekIntern(time);
 	}
 }
 

video/avi_decoder.h

@@ -75,6 +75,26 @@ public:
 	bool isRewindable() const { return true; }
 	bool isSeekable() const;
 
+	/**
+	 * Decode the next frame into a surface and return the latter.
+	 *
+	 * A subclass may override this, but must still call this function. As an
+	 * example, a subclass may do this to apply some global video scale to
+	 * individual track's frame.
+	 *
+	 * Note that this will call readNextPacket() internally first before calling
+	 * the next video track's decodeNextFrame() function.
+	 *
+	 * @return a surface containing the decoded frame, or 0
+	 * @note Ownership of the returned surface stays with the VideoDecoder,
+	 *       hence the caller must *not* free it.
+	 * @note this may return 0, in which case the last frame should be kept on screen
+	 */
+	virtual const Graphics::Surface *decodeNextFrame();
+
+	/**
+	 * Decodes the next transparency track frame
+	 */
 	const Graphics::Surface *decodeNextTransparency();
 protected:
 	// VideoDecoder API