Why does a JPEG file lose quality
Does simply opening and closing a JPEG file degrade the image quality?
JPEG compression can be described as having two distinct phases: first a lossy phase, then a lossless phase. For this question, it is important to understand the difference between them. This is not so much because it helps understand what is going on, but because it helps understand where the most common mistakes are coming from.
Lossy compression occurs only on, when the file is saved . This is the part that leads to loss of quality. Closing the file is not enough, however, to trigger lossy compression: you need to save it . Some editors may refuse to save unedited JPEG files to avoid accidentally triggering lossless compression. However, I am not sure if this is actually done by editors or not.
Lossless compression also occurs only on, when the file is saved . The main difference is that even if the file was closed without saving, it doesn't matter because it is lossless. JPEG uses both techniques together.
Lossless decompression occurs whenever the file is opened, but not at any other time . Not when it is closed and not even when it is saved. As with lossless decompression, it wouldn't matter if it happened during these times either because it is lossless.
"Loss of decompression" never occurs.Something like that does not exist . This may not be possible because the data that was discarded during the lossy compression phase no longer exists. If you could somehow reconstruct it, you would have a lossless compression algorithm, not a lossy one. I only mention the concept because after mentioning two types of compression it would look strange if I mentioned just one type of decompression without explaining why.
Note that saving the file will trigger both types of compression . There's not much getting around it unless you know the image hasn't been edited, but it doesn't make much sense to save it either. Also note that just closing the file without saving does not trigger either phase , not even "safe" lossless compression. Because of this, opening and closing the file cannot affect the quality of the image .
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