PNG格式应该是无损格式，但是当您将图像保存为PNG文件时，会要求您选择压缩级别。这是否意味着PNG格式实际上并不是无损的？今天的超级用户问答有助于澄清好奇读者的困惑。

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问题

超级用户读者pkout想知道PNG图像的质量是否受所选压缩级别的影响：

As I understand it, PNG files use lossless compression. However, when I am using an image editor such as Gimp and try to save an image as a PNG file, it asks for a compression level ranging between 0 and 9.

If it has a compression parameter that affects the visual precision of the compressed image, how is PNG lossless then? Can someone please explain this to me? Do I get lossless behavior only when I set the compression level to 9?

根据您选择的压缩级别，图像质量是否有差异？

答案

超级用户贡献者LordNeckbeard和jjlin为我们提供了答案。首先，领主：

PNG is Compressed, but Lossless

The compression level is a trade-off between file size and encoding/decoding speed. To overly generalize, even non-image formats such as FLAC have similar concepts.

Different Compression Levels, Same Decoded Output

Although the file sizes are different due the the different compression levels, the actual decoded output will be identical. You can compare the MD5 hashes of the decoded outputs with ffmpeg using the MD5 muxer. This is best shown with some examples.

Create PNG Files

• By default, ffmpeg will use -compression_level 100 for PNG output.
• A quick, sloppy test showed that 100 (the highest compression level) took roughly three times longer to encode and five times longer to decode than 0 (the lowest compression level) in this example.

Compare File Size

Decode the PNG Files and Show MD5 Hashes

Since both hashes are the same, you can be assured that the decoded outputs (the uncompressed raw files) are exactly the same.

接着是jjlin的回答：

PNG is lossless. GIMP is most likely not using the best choice of wording in this case.

Think of it as quality of compression or level of compression. With lower compression, you get a bigger file, but it takes less time to produce, whereas with higher compression, you get a **aller file that takes longer to produce.

Typically you get diminishing returns, i.e. not as much decrease in size compared to the increase in time it takes when going up to the highest compression levels, but it is up to you.

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