57

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 (with some very specific exceptions, see comments) lose quality if you open an image in an image editor and re-save it, even if you didn't make any edits. But ...


40

To be frank, it is entirely anecdotal that a JPEG image should be exported at a certain compression level all the time. The amount of JPEG compression should really depend on the usage purpose for the JPEG, and the contents of the JPEG. The quality level one should choose when exporting an image to JPEG is highly dependent upon the kind of detail contained ...


34

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 rounding error. MCU boundaries remain are intact (8x8 blocks). Chroma subsampling is disabled. Constant DQT (same quality setting). However, rounding errors may ...


23

I think you're going about this the wrong way. If you have 2GB of images at 4-5MB each, that's somewhere between 400 and 500 images. That's way too many. Even your close friends probably don't want to wade through all of that. Instead, go through and pick out the very best 10%. Or 5% or even 1%. Take some care and write a meaningful caption for each one. An ...


23

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 would "wear out" completely in a day or two at most.


20

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 blocks, the degradation should be minimal. However, if you're using a high level of compression, you will see further loss, because the artifacts introduced by the ...


20

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, using an optimizing compression algorithm. I don't think Google comes out and says exactly what they do, but it's probably their open source Guetzli algorithm. ...


19

An example Using the current photo of the week image. This is the high-quality JPEG: re-saved in Gimp with JPEG quality 80 (low); please note the general loss of sharpness, "dots" around high-contrast edges, loss of detail in low-contrast areas: and re-saved in GIMP with JPEG quality 30 (very low); please note evident 8x8 blocks and severe loss of ...


18

The image will be recompressed. The two scenarios you describe are actually effectively the same, because the lossy part of the JPEG compression discards information which stays gone when the image is decompressed. (Hence, lossy.) That means that reapplying with the exact same parameters shouldn't do much, either in terms of further space saves or in terms ...


18

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 (for a host of reasons) and may have some patent issues that might make it difficult to use in certain situations but otherwise it fits your needs. Personally ...


13

The first two images both have embedded color profiles. The smaller one has Adobe RGB, and the larger one has "TIFF RGB", which happens to consume more space. My guess is you probably want these to be sRGB anyway, with no embedded color profile. In the second case, it's the details. The hand photograph has big areas of the same color, a lot of blur, and ...


13

All image processing packages should make this easy. I'll show you how to do it in Mathematica, if you have access to this system. Mathematica is a programming language, but it's really easy to do these kinds of manipulations, so if you have access to it (e.g. through a university site license), I recommend you give it a go! First, import the image: img =...


13

Uncompressed RGB files (3 values per pixel) will be larger than your raws, as the raws contain a monochrome bitmap (1 value per pixel), and usually a downscaled, aggressively compressed preview that takes a fraction of the size, 400k for Canon 10MP cameras, and 1M for Nikon D5100 (I know these numbers because I used to read them out of the raw files and ...


11

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 what to look for you may not notice the changes. Moreover, there is flexibility as to how much information is thrown away: the more you lose, the smaller the ...


10

You are correct that once you compress, there is no going back. The best way I would consider compression is to actually delete images which are low quality, uninteresting and near duplicates, rather than systematically reducing quality globally. This is probably not the question you were asking though :) The level of tolerable compression is mostly ...


10

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 software can differ significantly from what another software produces when set to level 60. If you use ImageMagick as suggested by Rolazaro Azeveires, it will indeed ...


9

Disks are still pretty cheap these days so there are advantages to JPEG files, even if they are as big as the original RAW, instant image preview, being able to display on computers without RAW software. The "quality" parameter determines the quantization matrix used to compress the data. Without going into too much detail this determines the degree to ...


9

Setting a minimum filesize whilst fixing the image width and height is silly. If you don't use compression then the filesize is determined by the image dimensions. And if you do use compression then the filesize is determined by the level of entropy (disorder) in the image. Some images have higher entropy because there is more going on, more detail etc. in ...


9

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 directly comparing individual pixels around sharp edges or in very smooth color gradients. This is why JPEG is so popular. If it always resulted in noticeable loss ...


8

As is often the case, "there is no free lunch". JPEG is the de-facto standard image format, and uses lossy compression. That means that to get smaller file sizes, you will lose image quality. The only question is how much you lose, and whether that loss is acceptable. That said, there are lossless compression techniques that are used by other formats (...


8

The question here really resolves around what you mean by "quality", followed closely by needing to understand what RAW really is and how it relates to other image formats. On the first TIFF is generally not compressed in a lossy way, so artifacts weren't introduced. By that measure, quality isn't degraded in the same way it might be with JPEG (and ...


8

There are a few tricks that can help, but you'll never get the same quality out of facebook as you would from a site that allows larger files without compressing them so aggressively. Here's a link to a facebook help page that describes some of the issues. Expand the section titled "How can I make sure that my photos display in the highest possible quality?"...


8

My solution is that I don't use Facebook to host my media files. When I post files to Facebook, I post them as links to my server. I can't use the gallery function of Facebook, but it does allow for me to have greater control over the quality of work I display through Facebook. Ultimately you get what you pay for, and Facebook is looking to use your ...


8

JPEG2000, and you may also want to look at OpenEXR because it is supported by video hardware.


8

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 properties of the paper and ink technology limits what can be achieved. For convenience, just stick with the universally accepted JPEG and be happy with smaller file ...


7

Two images with the same dimensions contain the same amount of data but not necessarily the same amount of information. A pure white image contains virtually no information and can be compressed into a very small space (it is sufficient to store only the height and width in order to fully recreate this image). Scenes with a lot of detail contain a lot of ...


7

JPEG is a lossy format and it has variable quality levels which result in larger or smaller file sizes. Whenever you save a file in a lossy format, additional detail is lost, so even though you saved the finished version as a higher quality JPEG, it is still actually lower quality than the original was, despite the growth in the file size. It is still ...


7

Yes. How else could it be displayed? The screen needs to show the actual pixels, not an abstracted mathematical representation of them. Perhaps more crucially, a JPEG needs to be converted to a bitmap to be edited, which is why re-saving an image in JPEG can cause artifacts and loss of detail even if you don't change anything.


7

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 because it helps understanding what's going on, but because it helps to understand where the common mistakes come from. Lossy compression happens only when ...


7

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 recreate any image by adding together a combination of wave images with the correct brightnesses. However JPEG is a lossy compression algorithm and so it throws ...


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