# Keyframe Insertion: Enabling Low-Latency Random Access and Packet Loss Repair
## 6 Discussion  
This paper applied the untested concept of keyframe insertion to the H.265/HEVC standard. Additionally, it extensively analyzed the impact on bitrate and quality to evaluate the technique's strengths and limitations, and it evaluated which configurations have the best performance. This is the first work to thoroughly demonstrate that inserting a keyframe from a companion stream into a normal stream has a limited effect on the bitrate and quality. For example, in our experiments, when using the same QP in the normal and companion stream, the inserted keyframe is approximately 7 to 30 times larger than the inter-frame that it replaces. Thereafter, all remaining frames are received as normal. Contrary to our intuition, the introduced errors generally do not drift uncontrollably and only result in a slight decrease in overall quality. The highest VMAF score differences stay well below a 6-point difference for practical QP values, which is claimed to be the just noticeable [34]. On average, the quality decrease is much smaller, and hence less perceptible. Although these low VMAF scores are frame averages and there may be local perceptible artifacts in some worst cases, these artifacts are not annoying, in our opinion. In general, the impact on the frame size and quality is lowest when inserting a keyframe from CSNS, i.e., when encoding the keyframes of the companion stream from the reconstructed normal stream. Additionally, using low QP values, i.e., a high-quality companion stream, results in a better performance than when using high QP values. Only when relatively few frames are remaining in the GOP of the normal stream, or when using very high QPs, it may be more beneficial to insert a keyframe of CSsrc rather than a keyframe of CSNS. Using the proposed method and the experimental results, design decisions about codec settings in the H.265/HEVC standard can be made more straightforward. That is, the normal stream takes the responsibility of compression efficiency while the companion stream covers random access and error resilience. As such, the service of high-performing end users does not suffer from the few low-performing devices or users that switch channels. Definitely in the context of low-latency video distribution over error-prone networks, this functionality is very beneficial. Future work should address the usage of TMVP. Using TMVP in combination with keyframe insertion results in explicit blocking artifacts, and thus has been disabled in the proposed method. A solution could be to additionally encode an inter-frame after every keyframe in the companion stream, which has the same motion vectors as the corresponding frame in the normal stream. Furthermore, the proposed work could be extended by introducing predicted frames in the companion stream referencing keyframes from the normal stream. In this way, packet loss problems could be resolved with more efficient predicted frames rather than less efficient keyframes. It must be noted that such technique could only resolve packet loss errors and not accommodate random access. Finally, future work may investigate the effect on other codecs, such as the older H.264/AVC and the recently standardized H.266/VVC. It would be interesting to know the influence of the codec on the applicability of keyframe insertion. Conceptualization, G.V.W., J.V. and P.L. ; Formal analysis, G.V.W. and H.M.; Funding acquisition, J.V. and P.L. ; Investigation, G.V.W. and H.M.; Methodology, G.V.W., H.M. and P.L. ; Project administration, P.L. ; Software, G.V.W. and H.M.; Supervision, P.L. ; Validation, G.V.W. and H.M.; Writing--original draft, G.V.W. and H.M.; Writing--review & editing, J.V. and P.L. All authors took part in the discussion of the work described in this paper. All authors have read and agreed to the published version of the manuscript.