In this paper, we first propose the concept of strong idempotent codec based on idempotent codec. The idempotence of codec refers to the stability of codec to re-compression. Similarly, we define the strong idempotence of codec as the stability of codec to multiple quality re-compression, which is an important feature of codec in the context of cloud transcoding. We provide a detailed example of strong idempotent codec with known source distribution. Further, we formalize a testing protocol to evaluate the strong idempotence of image codec. And finally, we evaluate the strong idempotence of current image codecs, including traditional codec and recent neural codec. Experimental results show that current image codecs are not stable to multi-round re-compression with different qualities, even if they are close to idempotent.
翻译：在本文中，我们首先提出了基于幂等编解码器的强幂等编解码器的概念。编解码器的幂等性指的是编解码器对于重新压缩的稳定性。类似地，我们定义了强幂等编解码器的概念，它指的是编解码器对于多次质量重新压缩的稳定性，这是云转码场景下编解码器的重要特性。我们提供了一个基于已知源分布的强幂等编解码器的详细示例。此外，我们制定了一个测试协议来评估图像编解码器的强幂等性。最后，我们评估了当前图像编解码器的强幂等性，包括传统编解码器和最近的神经编解码器。实验结果表明，当前的图像编解码器，即使它们与幂等编解码器接近，也无法稳定地处理多轮不同质量的重新压缩。
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