Hi Philip,

Philip Rhoades wrote:
> - When saving as JPG with 85% quality am I losing information?

JPG utilizes lossy compression, which means you'll loose information
every time you save as JPG, even at 100% quality setting.

That value does not specify the percentage of information stored
in the JPG. It is just a number which allows to choose a trade-off
between subjective image quality and file size.

In consequence, the workflow recommendation is to routinely save as XCF
and only create a JPG when the (finished) work leaves your system.

> - How can saving as JPG with 100% quality increase information (file size)?

As said above, it is wrong to assume you were saving
100% of the 85% of the original image's information here.

In practice, one has to look at the compression artifacts to
be able to adjust for minimum file size at acceptable quality.
There's no way to just rely on the numbers.

What happens in detail:

when opening the JPG it gets decompressed to 2048x1536 RGB pixels of 3 bytes
each, a whopping total of 9437184 bytes of RAM. (This holds true for any
color JPG of 2048x1536 size, regardless of file size).

Now when saving this image as JPG, it's these 9437184 byte of image that
get compressed, regardless from where this data originated.

Compressing an image of 9437184 bytes at 100% gives a larger file
size  than compressing the same image at 85%. There's no memory of
previously used compression rates.

And regardless of file size, each new JPG compression step adds new artifacts
to the image, degrading quality.

> - Why is PNG so inefficient?

PNG offers lossless compression and isn't designed for use with photos.
It excels at graphic data which e.g. has uniform color areas.


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