## 2 HFR Processing and Challenges
This represents temporal resolution recovery or TRR. Any conversion is difficult and comes with a lot of challenges, especially in the temporal domain. HFR leads to higher video file size and bandwidth challenges. Since frame rate affects storage and the capacity of telecommunication channel, HFR quality is accompanied with compression. This inevitably introduces possible unwanted artifacts and undesirable components in motion picture result, where coding and compression solutions enable to decrease the video size by keeping the video quality high. Giving appropriate insight into such content is needed. HFR assessment goes with video compression. In a nutshell, the design of variety of coding standards and codecs is needed to achieve specific tasks. MPEG is dedicated to efficient coding and compression algorithms, where MPEGx and H26x standards have been popular over the years [26; 27; 28; 29; 30; 31]. Algorithms are becoming more complex, and advancements are being made to deal with new video technologies. In general, coding steps include block-oriented making intra- and interpretations, transformation and quantization, filtering and entropy coding. MPEG-2 has become popular over the years in practical applications like broadcasting, where MPEG-4 continues to be the leading choice for streaming implementations. H.264, or AVC standard (MPEG-4 Part 10), was introduced in 2003 by International Telecommunication Union--Telecommunication (ITU-T) Standardization Sector and Orga  
Figure 1: High frame rate (HFR) as high number of frames per second. nization for Standardization/International Electrotechnical Commission (ISO/IEC) [26; 27; 28]. It is a video compression standard based on block-oriented and motion-compensated coding supported by a wide range of devices and systems and is still one of the most widely accepted standards known for high compression efficiency and high-definition television implementation. AVC was followed by HEVC, introduced in 2013, also known as H.265 or MPEG-H Part 2 [29; 30]. HEVC, briefly speaking, enables compression of approximately half the size of AVC as a next-generation standard. It supports streaming and broadcasting with higher resolution, where HEVC is mostly used in action cameras and smart phones for HFR purposes. Also, there are many other available solutions for IP delivery, like VP9 and its successor AV1 [31; 32; 33]. Nevertheless, it should not be neglected that AVC is still in force for various implementations, but when it comes to HFR, it is expected to transfer to standards like HEVC [26]. HFR HEVC compressed video content effects have not been considered to a large extent, and this is relevant to practical implementations. For example, recently, in [32], HFR was analyzed from a perceptual quality point of view in the case of HEVC and VP9 by authors for full high-definition (HD) video sequences and five constant quality factor values, showing the better performance of HEVC for higher rates using standard metrics. Frame size trace sequences for AVC HD and 4k/UHD HEVC video can be seen in Figure 2 for a sequence taken from the publicly available UVG dataset described in [23]. Comparison after frame frequency alignment in Figure 2 shows different trace sequence behavior. Still, self-similarity-based analysis related to HFR HEVC has not been performed so far to the best of the author's knowledge. There are a lot of challenges related to HFR processing that need to be further investigated, such as effects due to compression and codec settings, effects due to differences in frame rates, no-reference HFR content characterization, HFR reproduction, editing and hardware utilization. Here, only some of these issues are tackled. Effects due to compression are considered in this paper for HEVC standard and TRR, but other standards like VP9 and content modifications can also be taken into account. No-reference characterization and quality estimation is of general interest for HFR processing. A relatively large amount of different raw HFR video content is needed in the research community.