Adaptive live video streaming applications use a fixed predefined configuration for the bitrate ladder with constant framerate and encoding presets in a session. However, selecting optimized framerates and presets for every bitrate ladder representation can enhance perceptual quality, improve computational resource allocation, and thus, the streaming energy efficiency. In particular, low framerates for low-bitrate representations reduce compression artifacts and decrease encoding energy consumption. In addition, an optimized preset may lead to improved compression efficiency. To this light, this paper proposes a Content-adaptive Variable Framerate (CVFR) encoding scheme, which offers two modes of operation: ecological (ECO) and high-quality (HQ). CVFR-ECO optimizes for the highest encoding energy savings by predicting the optimized framerate for each representation in the bitrate ladder. CVFR-HQ takes it further by predicting each representation's optimized framerate-encoding preset pair using low-complexity discrete cosine transform energy-based spatial and temporal features for compression efficiency and sustainable storage. We demonstrate the advantage of CVFR using the x264 open-source video encoder. The results show that CVFR-ECO yields an average PSNR and VMAF increase of 0.02 dB and 2.50 points, respectively, for the same bitrate, compared to the fastest preset highest framerate encoding. CVFR-ECO also yields an average encoding and storage energy consumption reduction of 34.54% and 76.24%, considering a just noticeable difference (JND) of six VMAF points. In comparison, CVFR-HQ yields an average increase in PSNR and VMAF of 2.43 dB and 10.14 points, respectively, for the same bitrate. Finally, CVFR-HQ resulted in an average reduction in storage energy consumption of 83.18%, considering a JND of six VMAF points.

翻译：自适应实时视频流应用在会话中使用固定的预定义配置，包括恒定帧率和编码预置的码率阶梯。然而，为每个码率阶梯表示选择优化的帧率和编码预置，可以增强感知质量、改善计算资源分配，进而提升流媒体的能效。具体而言，低码率表示采用低帧率可减少压缩伪影并降低编码能耗。此外，优化编码预置可能提升压缩效率。基于此，本文提出一种内容自适应可变帧率（CVFR）编码方案，提供两种运行模式：生态模式（ECO）和高品质模式（HQ）。CVFR-ECO通过预测码率阶梯中每个表示的优化帧率，实现最高的编码能耗节省；CVFR-HQ则进一步利用基于低复杂度离散余弦变换能量的空间与时间特征，预测每个表示的优化帧率-编码预置对，以提升压缩效率并实现可持续存储。我们利用x264开源视频编码器验证CVFR的优势。结果表明，相较于采用最快编码预置且帧率最高的编码方案，在相同码率下，CVFR-ECO的平均PSNR和VMAF分别提升0.02 dB和2.50分；考虑6个VMAF分值的恰可察觉差异（JND）时，CVFR-ECO的平均编码能耗和存储能耗分别降低34.54%和76.24%。相比之下，在相同码率下，CVFR-HQ的平均PSNR和VMAF分别提升2.43 dB和10.14分；考虑6个VMAF分值的JND时，CVFR-HQ的平均存储能耗降低83.18%。