Kelvin is a lightweight learned pre-encoder that sits in front of an unmodified libx264 encoder. It applies content-adaptive pixel adjustments, bounded at +/-1/255 per channel, so that the encoder allocates bits where they matter most perceptually, while emitting a standard H.264 bitstream compatible with every existing decoder, player, and CDN. On the seven-sequence 1080p UVG benchmark, Kelvin v1.0 achieves a mean BD-VMAF of -27.62% (7 of 7 wins) and BD-VMAF-NEG of -5.18% (6 of 7 wins) relative to baseline libx264 at preset medium. On the 30-sequence MCL-JCV public set (28 unseen by training), the same checkpoint wins on 28 of 30 clips by BD-VMAF; with the two diagnosable failures removed the mean is -27.70% BD-VMAF and -5.37% BD-VMAF-NEG, consistent with UVG to within one percentage point. A central engineering challenge is the non-differentiability of H.264: we describe a hybrid codec proxy that combines a calibrated differentiable rate estimator (Spearman rho = 0.986 vs. real libx264 bits-per-pixel) with a U-Net distortion proxy trained on real encoder outputs. We publish full per-sequence rate-distortion data, a named failure-mode taxonomy on MCL-JCV (rate-floor violation, distribution shift, metric saturation), a five-baseline sanity panel (hqdn3d, unsharp, -tune psnr, -tune ssim, x265 medium), and honest positioning: x265 medium beats Kelvin on every metric on the same corpus. Kelvin is therefore designed for workloads where remaining on H.264 is a constraint rather than a choice.

翻译：Kelvin是一种轻量级可学习预编码器，位于未修改的libx264编码器前端。它应用内容自适应像素调整，每通道调整幅度限制在±1/255范围内，从而使编码器将比特分配在感知最关键的区域，同时生成与所有现有解码器、播放器和内容分发网络兼容的标准H.264比特流。在包含七个序列的1080p UVG基准测试中，相对于medium预设的基线libx264，Kelvin v1.0实现了平均BD-VMAF为-27.62%（7个序列全胜）和BD-VMAF-NEG为-5.18%（7个序列中6胜）。在包含30个序列的MCL-JCV公开数据集上（其中28个序列未参与训练），同一检查点在28个片段中通过BD-VMAF获胜；去除两个可诊断的失败案例后，平均BD-VMAF为-27.70%，BD-VMAF-NEG为-5.37%，与UVG结果相差不到一个百分点。一个核心工程挑战在于H.264的不可微性：我们描述了一种混合编解码代理，它结合了经校准的可微分码率估计器（与真实libx264每像素比特数的Spearman相关系数为0.986）和基于真实编码器输出训练的U-Net失真代理。我们发布了完整的逐序列率失真数据、MCL-JCV上的命名失效模式分类（码率下限违反、分布偏移、指标饱和）、包含五个基线的合理性验证面板（hqdn3d、unsharp、-tune psnr、-tune ssim、x265 medium），以及诚实的定位：在相同语料库上，x265 medium在所有指标上均优于Kelvin。因此，Kelvin专为那些将停留在H.264视为约束条件而非可选项的工作负载而设计。