At 02:25 AM 5/13/2005, Andrea wrote:...... because JPG throws away information each time it is saved, if I open an image, then save as something1.JPG, open something1.JPG, save as something2.JPG, open something2.JPG, save as something3.JPG, open something3.JPG, save as something4.JPG, open something4.JPG, by then it should look pretty horrible. And I haven't done any editing at all.
Have you ever tried this? Actually, if you decompress and recompress an image at the exactly same size and high quality ratio each time, very little further degradation will occur. If you make small local adjustments there will be very little change in other areas of the image and the area of the new adjustment will change only to the extent of that adjustment.
Oops! Dave's right. I've just tried this, twice. And I'm sort-of right too.
Here's the gory details (followed by my summary):
I followed the steps I outlined above. I took a recent photo of my sister's face, 2008x3008 pixels. The first time with The GIMP, using the following settings, for good-quality results: Optimize: ON, Progressive: ON, Force baseline JPEG: ON, Quality: 0.85 (default), Smoothing: 0.00, Subsampling: 2x2,1x1,1x1, DCT method: floating-point (chose this one for best quality). Then I viewed a sample (of my sister's left eye) at 200% zoom of the initial image, and the "something4.JPG". Pretty well identical. Hmm.
I tried this process again, with the photo software from one of the Big Two photography companies. This program allows for limited editing, and doing a "save as" of the edited image, as BMP, JPG or TIFF. There is a quality slider for JPG which I set for maximum quality. I again repeated the steps above. This time there was a definite cast to the skin tones, and evidence of noise.
The size in bytes, of the files was of interest. Using The GIMP, the sizes were (in order):
491,124 lou6805_1g.jpg -- 1st iteration
491,360 lou6805_2g.jpg
491,294 lou6805_3g.jpg
491,282 lou6805_4g.jpg -- 4th iteration
Using the entry-level software:
1,211,834 lou6805_1b2.jpg
1,415,546 lou6805_2b2.jpg
1,569,710 lou6805_3b2.jpg
1,702,807 lou6805_4b2.jpg
And here are the file sizes from the other two formats available from the entry-level software:
18,120,396 lou6805_4b2.tif
18,120,246 lou6805_4b2.bmp
Summary:
It looks like if the JPGs are created _carefully_, then the loss of information is minimised. With a blunt instrument, the results do suffer.
&i (:
