# Performance Comparison of H.264 and H.265 Encoders in a 4K FPV Drone Piloting System
## 2 Related Work  
An overview of H.264 and H.265 video coding standards and their core differences has been given in [3], chapter III. In addition to H.264 and H.265, ref. [4] has also discussed the novel H.266 video coding standard. Encoder comparisons typically revolve around on-demand adaptive streaming, which is a service based on a server-client architecture where multiple video streams (differing in resolution and/or bitrate) of the same content are stored at the server side. This kind of comparison focuses only on the compression performance, meaning that parameters such as encoding time and power consumption are not considered. [PERSON] et al. [5] have shown how significant bitrate saving for the same quality can be achieved by using the x265 instead of x264 encoder. They compared x264, x265, and libvpx codecs by measuring six objective quality metrics: PSNR, PSNR(MSE), SSIM, MS-SSIM (Multiscale SSIM), VIF (Visual Information Fidelity), and VMAF. This resulted in more than half a million bitrate-quality curves. The same result has been obtained in [6], where it was revealed that x265 (along with aomenc and libvpx) achieved a substantial compression efficiency improvement over x264, according to PSNR and VMAF metrics. Objective quality metric scores can deviate from subjective quality opinions, especially as not all objective metrics are able to recognize different types of distortions (e.g., flicker, frame drops, interlacing, and compression artifacts). Moreover, scene complexity also affects objective metric scores. For this reason, it is desirable to use as many different metrics as possible when determining video quality. A detailed analysis of objective quality metric performance, performed on UHD high-motion sports footage (LIVE database), is given in [7]. Not only objective quality metrics have shown that x265 is superior to x264. A description of subjective evaluation methods for video sequences, according to ITU-T P.910, has been presented in [8]. The ACR (Absolute Category Rating; which was chosen for evaluation), DCR (Degradation Category Rating), and PM (Pair Comparison Method) methods were implemented, and a group of 89 students evaluated the influence of video codecs on three scenes encoded with 1080, 720, 480, and 360 px resolutions. Again, libx265 resulted in overall higher median and modus scores when compared to libx264. According to similar analyses, when computational complexity is not of the essence, it is safe to say that x265 outperforms x264 in practically every aspect. Quality assessment differs when it comes to real-time encoding systems. [PERSON] et al. [9] have claimed that each newly developed codec outperforms its predecessor, in terms of coding efficiency, with a consequent increase in computational complexity. It is crucial to find a good trade-off between the encoded video bitrate and encoding time; for example, if a system is designed to operate under good network conditions with expected fast transmission speeds, the bitrate is not crucial, as there is a chance that the encoding time will consume more time than the transmission itself. In an FPV drone piloting system, a drone is limited by its battery capacity. Therefore, energy consumption is also an important factor. The authors of [10] have evaluated H.264, H.265, AVC, and VP9 compression quality for a live streaming environment by encoding eight popular video games. Their results showed that H.265 provided the best compression efficiency but was 2.6 times slower than H.264. A comparison of H.264 and H.265 encoders for use in VANETs (Vechicular Ad Hoc Networks) has been detailed in [11]. The comparison was carried out on four video sequences of 352 \(\times\) 288 px resolution. According to the authors, H.265 had an advantage of 49% bitrate savings when compared to H.264. Previous research has shown that real-time encoding can be achieved by parallelizing the encoder up to its parallelization limits, which are dependent on the encoder implementation [12].