[robski]

Whilst I was in the mood for pixel peeping I thought I would investigate the other favourite question about Jpeg.
If I re-save a Jpeg will it degrade the image ? The question stems from those who only work in the Jpeg format when Post Processing.
If you scan forums where this question has been asked the reply will be yes it will discard information each time because of the lossy compression. But nobody has a feel as to what extent the image will degrade.

So I've taken a 760 x 760 pixel (577600 pixels) Tiff and saved it as Jpeg. I ran my program to compare the two images in the same manor as in previous Postings. I then opened and re-save the file. I repeated this operation 10 times on each occasion making no adjustment to the image. I ran the program again to compare the Tiff with the Jpeg file that was processed 10 times. I carried out this exercise at Max Quality and a Quality setting of 10. The results are below.

Compare Tiff with Jpeg 12 Max quality setting saved once
  450319 Pixels are identical to the original Tiff
  127267 Pixels error by less than 1 percent
      14 Pixels with  1 to 2  percent error
       0 Pixels with  2 to 5  percent error
       0 Pixels with  5 to 10 percent error
       0 Pixels with 10 to 15 percent error
       0 Pixels error by more than 15 percent

Compare original Tiff with Jpeg 12 Max quality setting saved 10 times
  362346 Pixels are identical to the original Tiff
  209950 Pixels error by less than 1 percent
    5303 Pixels with  1 to 2  percent error
       1 Pixels with  2 to 5  percent error
       0 Pixels with  5 to 10 percent error
       0 Pixels with 10 to 15 percent error
       0 Pixels error by more than 15 percent

Compare original Tiff with Jpeg 10 quality setting saved  once
  211658 Pixels are identical to the original Tiff
  294235 Pixels error by less than 1 percent
   69615 Pixels with  1 to 2  percent error
    2092 Pixels with  2 to 5  percent error
       0 Pixels with  5 to 10 percent error
       0 Pixels with 10 to 15 percent error
       0 Pixels error by more than 15 percent

Compare original Tiff with Jpeg 10 quality setting saved 10 times
  209957 Pixels are identical to the original Tiff
  295319 Pixels error by less than 1 percent
   70189 Pixels with  1 to 2  percent error
    2135 Pixels with  2 to 5  percent error
       0 Pixels with  5 to 10 percent error
       0 Pixels with 10 to 15 percent error
       0 Pixels error by more than 15 percent