Kas PNG-vorming pole kaotatud, kuna sellel on kompresseerimisparameeter?

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Kas PNG-vorming pole kaotatud, kuna sellel on kompresseerimisparameeter?
Kas PNG-vorming pole kaotatud, kuna sellel on kompresseerimisparameeter?

Video: Kas PNG-vorming pole kaotatud, kuna sellel on kompresseerimisparameeter?

Video: Kas PNG-vorming pole kaotatud, kuna sellel on kompresseerimisparameeter?
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Anonim
PNG-vorming peaks olema kaotatud formaadis, kuid kui salvestate pildi PNG-failina, palutakse teil valida pakkimise tase. Kas see tähendab, et PNG-vorming ei ole tegelikult kaotatud? Tänane SuperUseri Q & A postitus aitab selgitada uudishimulise lugeja segadust.
PNG-vorming peaks olema kaotatud formaadis, kuid kui salvestate pildi PNG-failina, palutakse teil valida pakkimise tase. Kas see tähendab, et PNG-vorming ei ole tegelikult kaotatud? Tänane SuperUseri Q & A postitus aitab selgitada uudishimulise lugeja segadust.

Tänane küsimuste ja vastuste seanss jõuab meile viisakalt SuperUserini - Stack Exchange'i ja kogukonnapõhiste Q & A veebisaitide rühmitusse.

Küsimus

SuperUseri lugeja pkout soovib teada, kas valitud tihendustasemele mõjutab PNG-pildi kvaliteeti.

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?

Kas pildi kvaliteedi erinevus sõltub valitud kompressioonitasemest?

Vastus

SuperUseri toetajad LordNeckbeard ja jjlin on meile vastuseks. Esiteks üles, LordNeckbeard:

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

Image
Image
  • 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

Image
Image

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

Järgneb Jjlini vastus:

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 smaller 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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