重新编码JPEG图像是幂等操作吗？ [英] Is re-encoding JPEG images an idempotent operation?
问题描述
我知道JPEG压缩是有损的。
我有2个问题：
I am aware that JPEG compression is lossy. I have 2 questions:
给出一个操作T：
1.拍摄JPEG-80图像
2.将其解码为字节缓冲区
3.将给定的字节缓冲区编码为JPEG-80
Given an operation T:
1. Take a JPEG-80 image
2. Decode it to a byte buffer
3. Encode given byte buffer as JPEG-80
从视觉上讲，这是幂等运算质量？
还是在我重复T时图像质量会继续下降？
JPEG-XR编解码器是否也一样？
Is T an idempotent operation in terms of visual quality? Or will the quality of the image keep degrading as I repeat T? Does the same hold true for the JPEG-XR codec?
谢谢！
编辑：
既然存在冲突的答案，那么您可以提供引用将非常有用！
Edit: Since there have been conflicting answers, it would be great if you could provide references!
推荐答案
不能保证，但是可能会发生。尤其是，如果您重复执行编码->解码->编码->解码过程足够多次，它最终将落在一个固定点上，并不再进一步降低质量（只要您坚持使用相同质量和相同编码器即可）。
It's not guaranteed, but it may happen. Especially if you repeat the encode -> decode -> encode -> decode process enough times, it will eventually settle on a fixpoint and stop losing quality further (as long as you stick to the same quality and same encoder).
JPEG编码分几个步骤完成：
JPEG encoding is done in several steps:
- RGB到YUV的转换
- DCT（更改为频域）
- 量化（丢弃DCT的比特）
- 无损压缩
- RGB to YUV conversion
- DCT (change into frequency domain)
- Quantization (throwing away bits of the DCT)
- Lossless compression
和解码的过程相同。
第1步和第2步存在舍入误差（尤其是在使用整数数学的速度优化编码器中），因此对于幂等重新编码，您需要幸运地获得编码和解码舍入误差很小或彼此抵消。
Steps 1 and 2 have rounding errors (especially in speed-optimized encoders using integer math), so for idempotent re-encoding you need to be lucky to get encoding and decoding rounding errors to be small or cancel each other out.
第3步是主要的有损步骤，实际上是幂等的。如果您解码的像素转换为足够相似的DCT，它将再次量化为相同的数据！
The step 3, which is the major lossy step, is actually idempotent. If your decoded pixels convert to similar-enough DCT it will quantize to the same data again!
JPEG XR也使用YUV，因此可能会遇到一些舍入误差，但是OTOH代替DCT，它使用可以计算而不会舍入误差的不同变换，因此与其他格式相比，往返JPEG-XR应该更容易。
JPEG XR also uses YUV, so it may suffer some rounding errors, but OTOH instead of DCT it uses a different transform that can be computed without rounding errors, so it should be easier to round-trip JPEG-XR than other formats.
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