Re: TIFF vs JEPG

Don wrote:
On Wed, 12 Apr 2006 13:50:35 -0400, "Alan Meyer" <ameyer2@xxxxxxxxx>
wrote:

If you blow up the images to the point where you can see
individual pixels, you will see that some of the pixels are
different in the two images. But even then, unless you know
in advance, you won't be able to tell which was the original,

That's just patently false!

At 100% magnification (i.e. 1:1) even a JPG image at lowest
compression (i.e. highest quality) stands out like a sore thumb when
compared to the original.

I downloaded the two images that Raphael referenced in his reply
then magnified them 16 times (i.e., 4004 pixels per dimension instead
of the 1001 pixels of the original image.)

I'm not sure I can tell them apart.

Then I magnified them almost 100 times i.e., almost 10 times
as many pixels per dimension. Now, if I looked closely, I could see
that some individual pixels had different colors. But I still couldn't
see JPEG artifact squares, and still wasn't sure which image was
which without looking at the file names.

If you find it hard to see simply overlay the images and flip between
them. The JPG one will have quite easily identifiable 8x8 pixel blocks
which is how JPG compression works.

Try it with the two images Raphael provided.

JPG uses 8-bit precision simply because that's all today's monitors
can display. However, in not too distant future monitors will expand
this dynamic range and then a 16-bit (or higher) dynamic range will
become essential.

Leaving aside the monitors, how much precision can the
human eye distinguish? I suspect the best eyes can only do
around 10 bits, though I'm not at all sure about that.

Furthermore, human perception is non-linear. There are some
ranges in which we are more sensitive than others. I believe
that some JPEG compression algorithms know that and take
advantage of it to produce images that are really very close to
the maximum human perception.

However I'm not an expert on this. Someone who is should post the
facts.

That's a subjective preference and everyone is, of course, entitled to
do that. But they should be aware of the conseqences. If they are,
then, more power to them! Enjoy!

However, projecting this *personal* preference and drawing false
conclusions is just factually incorrect!

Your observation about personal preference is perfectly valid.
There may indeed be some people for whom the subtle
differences between TIFF and good JPEG is detectable
and objectionable.

By all means, archive your images as JPGs but don't mislead others
into thinking they are not losing massive amounts of data if they do
so. They may still decide to use JPGs, and that's fine too, because in
that case they made an educated choice and did not base their decision
on factually wrong (mis)information.

I agree with part of that too. And I agree that people should do
their own tests and draw their own conclusions.

I'll go further and say that whether you are losing "massive"
amounts of data is also a subjective conclusion. There's no
doubt that a computer will find a signficant difference between
TIFF and good JPEG. But whether that's "massive" from a
human point of view is not obvious to me.

Finally, I want to defend my point that good scanning is more important
than saving TIFFs.

The quality of the scanner, the decisions made by the scanning
software, the adjustments for color and contrast, the cleaning of
the image and the glass plate - all have a bigger effect on final
results than TIFF vs. good JPEG.

But, as you say, people need to do their own tests to confirm
or disconfirm this for themselves.

Alan

.

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