Saving a JPG at "12" image quality. What does this do and why is the file size larger?

Hi

To understand what the control does and why data is still lost during the process regardless of its setting, it is worth understanding the steps in jpeg compression /decompression.

1. Colour transformation - the RGB image data is converted to Luminance and colour components.

2. The colour components are sampled at a lesser definition than the luminosity. This in itself loses some of the image data so is lossy.

3. The image data is split into blocks for processing using an algorithm known as the Discrete Cosine Transform which(this is not inherently lossy) but allows the next step to treat the high frequency components differently to the low frequency components.

4. The data is now quantized using algorithms that treat high frequency information differently to low but the aim throughout is to reduce the amount of data passed to the next step. This is controlled by the quality setting and the 12 setting gives more steps than lower settings. This stage is lossy and the biggest opportunity to reduce the image data in a lossy way.

5. Re-ordering and encoding using Huffman -Coding which compresses the data from the previous step but is recoverable.

Decompression reverses the steps, but of course the lossy steps cannot be reversed completely accurately so there is data lost at every compress/decompress, which is why the general advise is not to open (decompress) and re-save(compress) a jpeg file as losses are cumulative.

There is some comparative data here on the quantisation settings for different packages including Photoshop

https://www.impulseadventure.com/photo/jpeg-quantization.html

Dave

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@dnemeth01 wrote:

 When I open a JPG file and save it, I get a option to save in 1 thru 12 quality.

When you open a jpeg and save it, don't resave it as a jpeg. Choose a format that is not lossy such as tiff or psd.

~ Jane

The highest quality setting will inflict the least damage to the original data. But there will always be some.

https://en.wikipedia.org/wiki/JPEG 

It could be argued that saving a jpeg at quality 12 defeats the whole purpose of jpeg, which is to save bandwidth and storage space. It will be destructive in any case, so you might as well maximize payoff without losing too much quality. Around 8 is often a good compromise.

Don't resave a jpeg if it can be avoided! It will deteriorate with every new save. Use PSD or TIFF as working formats.

It should be said that content plays a part. Some types of images show the effects of the jpeg compression more easily than others, so this should be determined case by case.