>"Terry and Tracey" <foxcroft@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>> Also, he said jpeg is a mathmatical representation of the image, not the
>> image itself. And it is the same quality as the original. So with no
>> compression selected, jpeg will be about 1/8 of a tiff file. Increase
>> compression, and the trade off with image quality happens quick. Resave, and
>> it recomputes and quality drops.
Well, a jpeg file is not really any more of a mathematical representation
of the image than the original file is. It's still just pixel
intensity/color
information that's had some processing done on it so the "unimportant"
bits
can be thrown out. The usual idea of a mathematical representation of an
image
is a so-called vector image, where instead of saying what color each dot
should
be, the image is drawn with a series of mathematical commands like:
Start at point (x1,y1).
Draw a puce-colored straight line 1 unit wide to point (x2,y2)...
By definition, if the JPEG is 1/8 the size of the TIFF (which may be
smaller
than the uncompressed image), then the JPEG has been compressed. The
amount
of loss associated with this compression might not be noticable, however.
And yes, multiple compressions with lossy JPEG compression further reduce
quality, which is why the conversion to JPEG should be the last step.
Vaughan Bromfield <vaughan.bromfield@xxxxxxxxxx> wrote:
>Crap. Get better friends. ;-)
Well, maybe just better informed ones :)
>All Jpeg images are compressed to a greater or lesser extent. Jpeg
>compression is lossy meaning that image data is discarded in the process
>of converting the image size. The trick with Jpeg is that human eye is
>unlikely to notice the missing data. But data has gone and there is no
>way to get it back.
>
>I haven't read the Jpeg spec, but was it originally intended for
>low-resolution work (ie, computer screen)?
The file format and image compression scheme are not always the same
thing. As I think one other person pointed out, the JPEG spec does
provide a lossless compression scheme, based on LZW if I remember right,
which I might not. However, I don't think I've ever seen a program
that even implements the lossless scheme (full version of Photoshop
maybe?)
much less an image in that format. The compression scheme virtually
always encountered in JFIF files (what everyone calls "JPEG files") is,
as others have said, always lossy to some extent, and that extent
is controllable.
It's been a while since I've read the JPEG spec, or programmed with the
JPEG library, but I don't think it's tied to any specific resolution.
It actually works better on high-resolution images, where the artifacts
caused by quantizing the discrete-cosine transform (the lossy part) are
less noticable. High-resolution images also tend to have smoother
transitions between pixel intensity extremes, where the artifacts are
most pronounced.
>Would it be a generalisation to say that comsumer digital cameras store
>Jpeg images, while pro-quality produce Tiff or other non-compressed formats?
As a warning, the TIFF specification does allow for images to be
compressed
with the lossy JPEG compression scheme. TIFF is one of the more
complicated
image formats out there, and very few programs actually implement all of
the
options for writing TIFF files, so again I've never encountered a JPEG
compressed TIFF file, but the possibility exists.
Mark Marr-Lyon
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