Clyde wrote:
No it isn’t just a communication problem. I believe that you are wrong. However, I’m not the greatest expert on this.
When you execute the Save command in Photoshop, it saves the current state of the open file to a file on disk. It does that the same every time you save. It makes no difference whether you keep working or not. Photoshop has no concept of a "session". That is just your human time being projected.
So if you open a JPEG file, edit it, and just do Save, it will compress the file and save it as a JPEG file. It rewrites it over the older file that you opened. You can NOT save a JPEG file without doing some degree of compression. That degree may be very small at levels 10 – 12, but it will always change the the bits of what you had just edited.
Therefore, opening, editing, saving, editing, saving, editing, saving, editing, saving without doing a Save As into a lossless file format will give you JPEG compression every single time you Save. Photoshop doesn’t know any other way of doing it.
Keep in mind that the level of destruction is a debate that flairs up here many times. It may or may not compress it to a point that destroys the image. It will depend greatly on how much and what kind of editing you do AND the level of JPEG compression that you use. Some of this can be accumulative and really screw up your picture. Some of it may never been seen by the human eye and may not matter. The problem is that you don’t and can’t know when you are saving as JPEG over and over. Once you do know, it will be too late.
The file in memory has no compression applied to it. When a JPEG is opened the file is decompressed into individual pixels. Whenever the file is saved, a copy of the pixels is compressed and written to the disk. The actual pixels in memory are not changed.
Degradation only happens when the file is opened again because that is the only point that the compressed pixels are accessed.
This is not hard to test:
1) Load a file. Any file will do but a small one will be better.
2) look carefully for compression artefacts. If it already has
visible artefacts, it’s not a good file for this test.
3) Save the file as Test1.jpg To accentuate any error choose a high compression (low quality), say level 3.
4) Save again as Test2.jpg Use the same compression as the first file.
5) Grab a brush and paint something (anything) in one corner.
6) Save the file (Ctrl+S)
7) Paint in the same corner again. This is necessary to make PS recognise a change so it will save. Keep all painting in the same corner.
8) Repeat steps 6 and 7 as many times as you like.
9) Close the file.
10) Open Test1.jpg and Test2.jpg.
On examination, you will notice that apart from near the area you were painting, the 2 files are close to identical.
-Mike