"Glenn" schrieb im Newsbeitrag
...
http://www.warbirdz.net/largepic.php?ID=12327

The above link, when you view it, does it look identical to the image
below.
reason why I ask is on my monitor, the link above is obviously compressed
but the image below
is the same image, just not uploaded onto my website. The image i post
below, looks good and the compression is nowhere near as evident.

Yet it is the same image.
Is this glaringly obvious to you guys as well.

Having downloaded both pics, I ran a quick and dirty per-pixel comparison of
both using Adobe Photoshop.
The result can be seen in the attached file.
Dark areas (preferably true black) mark pixels that are absolutely
identical, bright (preferably true white) areas mark pixels that are 100%
different (directly contrary, eg black and white).
Shades of grey thereby indicate how much similar or different each pixel of
both images is.

As you can see, the image is mostly black, hence indicating little
difference in both versions.
There is, however, a slight one and it is indeed visible, though it requires
both a good monitor as well as good eyes and expertise to spot it (I'll
therefore claim I posses all of them ).

Anyway, since both images already have a difference in size (byte-wise), and
are also using a lossy compression (JPEG), one can definately conclude they
WILL be different, no matter what.
The true question is rather how much and does it matter

The comparison image is hence saved in the lossless, albeit big, PNG format
to not falsify any of the differences.
I hereby also want to apologize for the somewhat big size.

If you brighten up the image (or better yet, enhance contrast and/or gamma),
you will see the difference even better.
I just didn't do that because it could qualify as cheating (making it stick
out more than it actually does).

As a sidenote:
As you can see, the white spots are arranged in a quadratic shape, so-called
macro blocks, each 8x8 pixels in dimension.
That is because of the JPEG compression that will subdivide each image to
such 8x8 pxs blocks and compress each independantly.
This is also the cause for bad image quality on higher compression rating on
high-contrast areas in the picture.
A wavelet compression, such as featured by JPEG2000 will harvest much better
results with such complicated image material.
This is also the reason why JPEG isn't very well-suited for screenshots or
text images, btw.


PS: Since some of you wrote about the DPI differencies:
That does not matter at all when viewing the image on your computer screen.
DPI are only involved in analogue-digital or digital-analogue conversion
(i.e. scanning the picture or printing it).
If you are using IrfanView (a free and great image viewer for Windows,
http://www.irfanview.net ), you can freely adjust the DPI of an image, with
no results at all to the display on your screen.
Size (as on the screen) is only determined by the pixel dimensions.
Of course, it depends on the size and resolution of your minotr, too, which
is why you cannot ultimately say "This image is 15*10cm large!" - it may be,
but only for you on your current screen.
Once again me meet the DPI, this time, of the monitor.
The smaller the actual dimensions of it are and the more pixels it can
display (= higer resoltuion), the higher its DPI will be, and the smaller
(if meausered by a ruler put on the screen) the image will appear.
Also, when scanning and printing, using the same DPI setting for both (e.g.
300 DPI, which should be good enough for ordinary prints on normal paper),
the copy image will appear just as large as it was in reality.
Print with twice the DPI of the scan and it will be half as large (per
dimension, hence 1/4th in area) or half the amount for the vice versa.



Sincerely hoping to have cleared up more confusion than caused,

Martin