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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 ...
41
This is most likely caused by entropy coding, which is the final lossless stage of JPEG compression, after the image data has been quantized to reduce its size.
When a JPEG image is losslessly rotated, this final lossless encoding layer must be undone, the unpacked DCT coefficients shuffled around, and then the shuffled coefficients need to be entropy coded ...
37
Although Philip's answer is the best way to go, it is possible to do what you want entirely within the sphere of JPEG.
JPEG works by breaking your image up into blocks called Minimum Coding Units (MCUs), typically 16×16 each, and compressing them separately. You can see this in images when you crank the compression level up very high. At more ...
36
In addition to the points Alex S made, you need to consider why they want RAW. There are several possible reasons:
Bit depth as Alex S said.
JPG suffers from compression artefacts which RAW doesn't. Blown up to exhibtion size these can jump out and ruin a print.
Having the RAW file is often used as a proxy for having taken the photo, as RAWs aren't ...
35
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 ...
35
In my opinion, none of the other answers addresses the obvious misconception in the question:
There is no use in converting a JPG (comparatively low quality) to a RAW file (high quality), because you do not gain anything.
The reason why people shoot in RAW is that, as others have stated, RAW captures all the sensor data and saves it in a file. JPGs have ...
34
In short: there is no reason to care about this value and it its presence does not make your file larger.
identify -verbose filename(s)
only displays the quality if the image uses the standard quantization matrix. You can use nonstandard matrix (-define jpeg:q-table=...) to make this value disappear.
The quality will be, however still displayed when the (...
32
Beyond the very obvious memory card requirement differences between RAW and JPEG images as noted in the question:
JPEGs are compressed and typically have much smaller file sizes. For example a RAW file from a Nikon D800 can be 50MB and the JPEG may be a fraction at 10MB. This benefits not only memory card capacity but also editing workflow speed, archival ...
30
The size of files compressed with JPEG vary depending on the complexity of the image. Trying the control the file sizes the way you describe will result in highly variable perceived image quality.
Consider the following options instead:
The good-enough approach. Use a quality setting that you find acceptable, like 75. Compare the size of the result with ...
29
RAW is not (or minimally) processed image data from camera sensor.
JPEG is processed image data.
Typically, raw-files from modern cameras have 12-14-bit per pixel which means up to 16384 values (for more details see Michael Clark's comment). JPEG can have only 256 luminance values per RGB channel. This means that jpeg contains much less data than a ...
26
I went ahead and repeated the experiment to see if I could figure out what's going on.
Procedure
I generated a random 256-by-256 pixel RGB image using the "Solid Noise" filter in GIMP (Filters > Render > Clouds > Solid Noise...) using the default settings (shown below):
And the result:
Then I saved the image as a JPEG using the default settings:
Then I ...
25
The JPEG compression algorithm works like this:
Instead of being saved as three R, G and B planes, your image is decomposed into 3 planes, one that carries luminosity, and two that carry color (aka chroma) information.
The next step is called "chroma sub-sampling": the two chroma planes can then be scaled down by a factor of two in one dimension (...
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.
21
The point to remember here is that you lose quality when saving the photo into a lossy compression format. So long as you save the photo in a lossless format (PSD, TIFF, etc) after adding the border, you won't lose any more data than you've already lost by saving the photo as a JPEG in the first place.
21
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
Short Answer
No, decoding is not guaranteed to always be the same. However, the differences are guaranteed to be very, very small.
ISO Specifications
The International Organization for Standardization (ISO) specifications for JPEG has the following specifications for decoders (emphasis mine):
A decoder shall
a) with appropriate accuracy, convert ...
20
EDIT: This answer was posted before I knew that the files had increased in size by around 9 KiB (9055 bytes for the 256×256 image, 9612 KiB for the 512×512 image).
In all likelihood, when you first rotated the image, Windows Picture Viewer did one (or both) of the following things:
Added an EXIF tag that was not in the original JPEG image (perhaps the ...
19
If you are talking about JPEG files, then the utility jpeginfo is exactly what you're looking for. It can check files for different types of JPEG errors and corruption and either return an error code (the most useful thing for scripting), or just delete files with errors.
I use this as part of my initial file transfer, to make sure everything copied okay ...
19
In the absence of real raw files, the JPG is your "raw". Most image editors, including Lightroom, can open or import JPGs. You may choose to save in another format while editing, but do not lose or destroy the original JPGs. Also take care not to save over the original files.
It is possible to convert JPGs to DNG. But it's usually used to test and develop ...
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 ...
18
If this image were RAW, the color would still be there. But since it is JPEG, I'm afraid not. The fact that the image is in RGB format does not help, because I'd you look, you will find that in fact for each pixel, each of these values is set to the same thing: (0,0,0), (37,37,37), (221,221,221), or whatever. That is, they're all gray levels, just ...
18
No. This is a wrong approach.
File size in pixels, yes, has something to do with the final weight, but it is not the only factor.
Make a test. Take a completely white file of the same 2400x600px, and save it as JPG.
Now take a photo of a forest (same 2400x600px) with lots of details and save it. This file will be larger using the same compression settings....
17
Going from JPG (which is a lossy format) to any lossless one (like PNG) does not.
Going from any format to a lossy one, yes, including JPG to JPG. It could be too little to notice, and using the same compression ratio loses a lot less on the second saving than on the first one, but yes, it is cumulative.
But beware... Some image formats store more ...
17
JPEG is an image format with lossy compression.
This means, every time you save a modified JPEG image, its quality gets slightly reduced because the algorithms used for encoding and compressing the information (namely DCT - discrete cosine transform) do not work pixel-perfect but only store the most important (depending on the quality/compression level) ...
16
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 ...
14
Use the "Export..." option on the File menu. .xcf is GIMP's native format, similar to how Photoshop uses .psd files, and stores all the information (layers and the like) GIMP needs to edit files in the future.
It's possibly worth noting this is a change from GIMP 2.6 and earlier, where the "Save..." menu option could be used to save to formats other than ....
14
To simplify a bit, JPEG image compression splits the image into different frequency components and stores them separately. Depending on the compression settings, it also stores the higher frequency components with less precision, since any round-off errors in those components are less visible to the human eye. After rounding off the data, it then applies a ...
13
It's all to do with the profile applied to the raw files and guesses other software makes as to what that profile ought to be...
RAW is not an 'absolute' format in terms of the image displayed, it's raw data to which an 'opinion' of what the image ought to look like can be manipulated from. It will already contain several view options added by the camera, ...
12
You can use cross-platform open source software jhead, with the options
jhead -autorot *.jpg
Which losslessly rotates the images based on the embedded rotation flag (and removes that flag, of course.
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