png格式是無損的，因為它有一個壓縮引數？

PNG格式應該是無損格式，但是當您將影象儲存為PNG檔案時，會要求您選擇壓縮級別。這是否意味著PNG格式實際上並不是無損的？今天的超級使用者問答有助於澄清好奇讀者的困惑。...

PNG格式應該是無損格式，但是當您將影象儲存為PNG檔案時，會要求您選擇壓縮級別。這是否意味著PNG格式實際上並不是無損的？今天的超級使用者問答有助於澄清好奇讀者的困惑。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

問題

超級使用者讀者pkout想知道PNG影象的質量是否受所選壓縮級別的影響：

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?

根據您選擇的壓縮級別，影象質量是否有差異？

答案

超級使用者貢獻者LordNeckbeard和jjlin為我們提供了答案。首先，領主：

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.

接著是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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