58 votes

Does simply opening and closing a JPEG file decrease image quality?

This is based on a misunderstanding. Loss of quality happens only during the compression that is done when an image is saved as JPEG. But it doesn't matter whether it was edited or not. So: you will (...
35 votes

What factors cause or prevent "generational loss" when JPEGs are recompressed multiple times?

Almost all image quality losses occur the first time an image is compressed as JPEG. Regardless of how many times a JPEG is recompressed with the same settings, generational losses are limited to ...
  • 26.4k
23 votes

Does simply opening and closing a JPEG file decrease image quality?

Absolutely not. You need to edit the file and re-save it as a JPEG in order to compound the effects of image compression. Just viewing it has no effect at all — if it did, all of the JPEGs on the web ...
  • 1,262
21 votes

What factors cause or prevent "generational loss" when JPEGs are recompressed multiple times?

Recompression loss is real, especially when working with higher levels of JPEG compression. In theory, if you re-save a JPEG files with the exact same parameters and have aligned your crop to 8×8 ...
  • 140k
21 votes
Accepted

Google Photos high quality backup -- how does Google achieve great compression and am I losing some data?

Well, for one thing, all photos larger than 16 megapixels are resized to 16 MP — so, for many cameras today, that's an immediate, obvious drop in resolution. But all other photos are compressed too, ...
  • 140k
18 votes
Accepted

Is there a lossy compressed file format for 16-bit dynamic range images?

It sounds like what you're looking for is JPEG2000. It has a range of options including a 16-bit lossy compression and better compression ratios than JPEG. It hasn't been as widely adopted as hoped (...
  • 9,440
16 votes
Accepted

Is it possible to find out what compression ratio was used for a particular JPEG?

No, you can not and it does not make sense to do so, since there is no ubiquitous definition of the JPEG compression level. The actual result when saving a JPEG with compression level 60 in one ...
  • 2,853
11 votes
Accepted

Why are JPEG images from camera huge and the same images exported from Vistaprint software 10 times smaller?

Without going into a detailed explanation of JPEG, it is (in general) a lossy format: if you save a file as JPEG and then load it again, some of the pixels will have changed. However, unless you know ...
  • 1,467
9 votes

Why doesn't PNG show more detail than JPEG in a converted NASA image?

For photographic images and when a not too high level of compression is used, the loss of quality in the JPEG format is negligible and invisible. You'll pretty much only be able to notice it by ...
9 votes

Is it possible to find out what compression ratio was used for a particular JPEG?

You can, sort of. ImageMagicks' identify command can show a estiamte identify -verbose image.jpeg will produce (a lot of) information about the image. One of the lines will be something like: ...
8 votes

Why do modern DSLRs support JPEG but not lossless formats other than RAW?

Wouldn't it be useful to have a 24-bit RGB format (taking advantae of the camera's automatic processing modes)? Not really. Raw files are actually very space efficient, since they only store one ...
  • 6,800
8 votes

Is there a lossy compressed file format for 16-bit dynamic range images?

JPEG2000, and you may also want to look at OpenEXR because it is supported by video hardware.
  • 2,001
8 votes

Is it not ideal to save image as PNG for printing?

Yes, PNG is theoretically better than JPEG in preserving the ultimate image quality, but in practice this is the kind of exactness we don't really see, especially in print, where the physical ...
  • 4,845
7 votes

Why do modern DSLRs support JPEG but not lossless formats other than RAW?

The JPEG format is very good for final output, filesizes are small and with the highest quality settings artifacts are pretty much invisible. It's only if you start editing a JPEG that you will see ...
  • 117k
7 votes

Does simply opening and closing a JPEG file decrease image quality?

JPEG compression can be described as having two distinct phases: first a lossy phase, then a lossless phase. Understanding the difference between them is important to this question. This isn't so much ...
7 votes
Accepted

What are the anomalies in this picture of earth?

You're looking at JPEG artifacts. The JPEG compression scheme divides an image up into 8x8 pixel blocks and rebuilds each block using a collection of 2D waves as building blocks: You can faithfully ...
  • 117k
7 votes
Accepted

Should I resize photos before or after image optimization?

The "ImageOptim" tool pulls together a bunch of other things, and in the case of JPEG files, the relevant thing is the MozJPEG optimizing encoder. If you use this encoder and then resize and save with ...
  • 140k
7 votes
Accepted

why won't my photos compress past a certain size?

I do not know at which resolution you scanned your image, but i'm willing to bet that it is way larger than needed or useful for web viewing. The first thing you should do is resize the image down ...
  • 6,800
6 votes

How can I tell exactly what changed between two images?

You can use Imagemagick command line tools. composite imagesrc1.jpg imagesrc2.jpg -compose difference diffs.jpg will provide the absolute value of differences for each RGB channel.
  • 61
6 votes

How does the usual jpeg 0-100 quality settings translate into Photoshop's 0-12 quality setting?

There is not a direct correspondance between standard JPEG quality 1-100 and Photoshop 0-12. The only thing they have in common is that to a bigger number usually corresponds to better quality. ...
  • 666
6 votes

Why do modern DSLRs support JPEG but not lossless formats other than RAW?

Some do — for example, most or all Pentax models and higher-end Nikons support TIFF (which, as Raheel Khan notes in a comment above, is better for metadata than PNG). So, there you go. If this is ...
  • 140k
6 votes
Accepted

Does writing EXIF tags influence jpeg quality?

There are tags that affect how the image can be interpreted by software (think about orientation), but no, the image bits should not be touched. Now, lore says that there are (bad) editors that open ...
  • 1,124
6 votes

Compress a few small areas of an image in higher quality

You can do this with an experimental version of the jpegtran utility with the "drop" option. Get it from http://jpegclub.org/jpegtran/. Here's an example (using a sample image from http://www....
  • 140k
6 votes
Accepted

Quality of JPEG directly from RAW+JPEG versus JPEG embedded in RAW of RAW+JPEG?

To concentrate on what you should do, I suggest you stick to the raw+jpeg. Worst case you need another card and storage is cheap. Do you actually ever get close to filling all your cards? If not, you ...
  • 3,612
6 votes

Google Photos high quality backup -- how does Google achieve great compression and am I losing some data?

They are basically throwing processing power at the problem, using very compute intensive approaches that tries out various ways to compress the image within the JPG rules. Processing is something ...
  • 383
6 votes
Accepted

Watermarking processing proof

There are photoshop plugins around like imatag or digimarc, which offer this as a professional service. The software encrypts a bit of information in a bit pattern and then hides this pattern in your ...
  • 3,393
6 votes
Accepted

Can jpg be converted to progressive jpg without quality loss?

To convert a baseline JPEG to a progressive JPEG in a lossless manner you can use jpegtran: jpegtran works by rearranging the compressed data (DCT coefficients), without ever fully decoding the image....
6 votes

Image properties for social media?

None of 1-4 matter. All the major social networks (definitely including Facebook, Twitter and Instagram) re-encode any uploaded photos with their own settings, optimised for their use case (reducing ...
  • 19.9k
5 votes

What is the real world impact of the lossy compression used for NEF RAW files in some Nikon cameras?

The particular lossy compression used here is nothing to worry about. Here's why. Camera sensors are more or less linear devices, if you double the amount of light you double the signal produced by ...
  • 117k
5 votes
Accepted

How does the usual jpeg 0-100 quality settings translate into Photoshop's 0-12 quality setting?

In the Save As function the 0-12 quality scale is used, but in the Save For Web function a 0-100 scale is used. That 0-100 scale is probably close to the 1-99 scale specified in the standards. I ...
  • 20.8k

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