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JPEG compression options -- can anybody explain?

 
 
Beowulf
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      08-03-2004
When I save a file as jpeg, I understand basic compression and quality (I
might save as 90% quality or 10% compression, for example; of course I
save as TIFF or other lossless format for editing and prints). But there
are lots of other JPEG compression options I do not understand -- amount
of "smoothing" (0-100); checkboxes to activate "optimize" and
"progressive"; "subsampling" choices of: "2x2,1x1,1x1", "2x2,1x1,1x1
(4:2:2)", "1x1,1x1,1x1",; "DCT Method (speed/quality tradeoff)" choices
of: Integer, Fast Integer, Floating Point.

Can anybody explain these option for fine tuning the saving of a JPEG
image, or direct me to a website that might explain the best such settings
for JPEG image saving?


--
"It said it needed Windows98 or better installed, so I installed Linux."

 
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Gordon Richardson
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      08-03-2004
http://www.photo.net/learn/jpeg/

"Beowulf" <(E-Mail Removed)> wrote in message
news(E-Mail Removed)...
> When I save a file as jpeg, I understand basic compression and quality (I
> might save as 90% quality or 10% compression, for example; of course I
> save as TIFF or other lossless format for editing and prints). But there
> are lots of other JPEG compression options I do not understand -- amount
> of "smoothing" (0-100); checkboxes to activate "optimize" and
> "progressive"; "subsampling" choices of: "2x2,1x1,1x1", "2x2,1x1,1x1
> (4:2:2)", "1x1,1x1,1x1",; "DCT Method (speed/quality tradeoff)" choices
> of: Integer, Fast Integer, Floating Point.
>
> Can anybody explain these option for fine tuning the saving of a JPEG
> image, or direct me to a website that might explain the best such settings
> for JPEG image saving?
>
>
> --
> "It said it needed Windows98 or better installed, so I installed Linux."
>



 
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deimos
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Posts: n/a
 
      08-04-2004
Beowulf wrote:

> When I save a file as jpeg, I understand basic compression and quality (I
> might save as 90% quality or 10% compression, for example; of course I
> save as TIFF or other lossless format for editing and prints). But there
> are lots of other JPEG compression options I do not understand -- amount
> of "smoothing" (0-100); checkboxes to activate "optimize" and
> "progressive"; "subsampling" choices of: "2x2,1x1,1x1", "2x2,1x1,1x1
> (4:2:2)", "1x1,1x1,1x1",; "DCT Method (speed/quality tradeoff)" choices
> of: Integer, Fast Integer, Floating Point.
>
> Can anybody explain these option for fine tuning the saving of a JPEG
> image, or direct me to a website that might explain the best such settings
> for JPEG image saving?
>
>


Smoothing -- artificially blurs the whole photo such an amount. It can
help compressibility as JPEG works very well with smooth gradients of
color; sharp edges tend to work against it.

Optimize -- does a few reordering and newer optimizations to affect file
size without affecting quality.

Progressive -- creates a multi-resolution image that is "progressively"
downloaded and decoded as to allow a web browser with a slow connection
to preview part of the graphic. i.e. Goes from no detail and pixelated
to full resolution as it's downloaded.

Subsampling -- what blocksize it operates on. 1x1 will give you the
sharpest image, but may bloat the file size unnecessarily. Use only for
computer generated and rendered material. 4:2:2 should always be used
for photos. 2x2 is useful only when either recompressing a previous
JPEG or when there is little sharp detail at all (like thumbnails or bad
material).

DCT Method -- Accuracy of the DCT algorithem. Use Floating Point
always. Speed of the jpeg algorithem is generally a non-issue and you
shouldn't ever have to use an integer format unless you're creating a
grpahic for a integer processor only device. Effects image quality
also. Colors become blocky and inaccurate with lower accuracy.
 
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