Edit "JPEG" then "Save As" is destructive ????

Question

Having searched around I can't find an answer to a simple question. If I open a jpeg and edit it "with or without" layers, then flatten (In the case of layers) the image and select the "Save As" command and rename the file or select "As A Copy", will my original jpeg suffer from and adverse effects if I don't save the original file ???. I keep hearing about destructive editing and layers etc. I have tried editing the same photo three times without saving the original and the file size is the same, suggesting that its non destructive, but would like a definitive answer. Thanks in advance

Answer 1

Noel Carboni wrote:
Generally speaking, saving as a JPEG causes a lossy compression of the data.  So as a rule, any save of a JPEG file results in degradation.   
If you talk about Photoshop only, then yes. But "generally speaking": no, this is not universally true.
There are a number of modifications that CAN be done losslessly on a JPEG file (i.e. without decompressing and recompressing the data, thereby preventing a reduction of image quality due to generation loss):
rotating (lossless)
flipping (lossless)
cropping (lossless)
canvas enlarging, i.e. adding a frame (lossless)
grayscaling (lossless except for the [intended] loss of chroma information, i.e. the operation does not add jpeg compression artefacts and thus is equal to converting a color image to a grayscale image in lossless format, such as png)
editing (lossless re-saving of entire jpg image except modified MCU blocks. This comes in handy e.g. for red-eye reduction!)
MCU block rotating, flipping and moving (lossless)
Many programs are capable of lossless rotating and flipping of jpeg files, whereas the other lossless operations are less frequently found.
Photoshop is NOT capable of any of the above lossless operations.
modifications which do not change image pixels:
optimizing the Huffman tables of a JPEG file to increase compression (lossless)
converting between progressive and non-progressive JPEG format (lossless)
Note: Lossless rotating and flipping requires the image size to be a multiple of 1 MCU block (i.e. 8x8 for a jpeg image without chroma subsampling, 16×8 for 4:2:2 subsampling and 16×16 for 4:2:0) as the top and left edge of a JPEG image must lie on a block boundary. When a losslessly cropped JPEG image's bottom or right side does not coincide with a block boundary, then the rest of the data from the partially used blocks will still be present in the cropped file and can be recovered.
lossless editing (e.g. adding of text):
1) original JPEG image
2) edited + losslessly resaved JPEG image
3) comparison
white: identical pixels
black: different pixels
lossy editing in Photoshop (e.g. adding fo text):
1) original JPEG image
2) edited + saved in Photoshop
(JPEG, max. quality 12)
3) comparison
white: identical pixels
black: different pixels