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Old 05-21-2008, 09:52 PM
Hendrik Boom
 
Default Image compression

On Wed, 21 May 2008 21:04:41 +0100, michael wrote:

> On Wed, 2008-05-21 at 20:02 +0000, Hendrik Boom wrote:
>> I've noticed that some 40K byte jpeg files are very good, as good as
>> ones ten times the size, and that others are awful. The question
>> naturally arises about the proper way to further compress the large
>> images to save disk space.
>>
>> What image compression programs have people found to give good results?
>>
>> And to what extent is good image quality the result of the algorithms
>> that display the image, adjusting image size to screen size and
>> resolution and the like?
>
> isn't it image dependent? It is for GIFs and so I thought for JPEGs
> too...

Of course it is to some extent. Your going to have a really hard time
compressing random pixels, say, whatever you do. If there are a variety
of tools that work in different circumstances, I'd like to hear about
them.

If any of it works better producing PNGs, or GIFs, or even XBMs
(unlikely) I'd like to hear about that, too.

-- hendrik


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Old 05-22-2008, 03:50 AM
"Mumia W.."
 
Default Image compression

On 05/21/2008 03:02 PM, Hendrik Boom wrote:
I've noticed that some 40K byte jpeg files are very good, as good as ones
ten times the size, and that others are awful. The question naturally
arises about the proper way to further compress the large images to save
disk space.


What image compression programs have people found to give good results?

And to what extent is good image quality the result of the algorithms
that display the image, adjusting image size to screen size and
resolution and the like?


-- hendrik




First, you can't recompress JPEGs without degrading them permanently, so
don't do that unless you don't care about the data.


Second, I've heard that progressive JPEG compression can outperform
"normal" JPEG compression, so you're probably witnessing the advantage
of progressive JPEGs. The program cjpeg (part of libjpeg-progs in
Debian) can create progressive JPEGs.


HTH


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Old 05-22-2008, 02:23 PM
Hendrik Boom
 
Default Image compression

On Wed, 21 May 2008 22:50:56 -0500, Mumia W.. wrote:

> On 05/21/2008 03:02 PM, Hendrik Boom wrote:
>> I've noticed that some 40K byte jpeg files are very good, as good as
>> ones ten times the size, and that others are awful. The question
>> naturally arises about the proper way to further compress the large
>> images to save disk space.
>>
>> What image compression programs have people found to give good results?
>>
>> And to what extent is good image quality the result of the algorithms
>> that display the image, adjusting image size to screen size and
>> resolution and the like?
>>
>> -- hendrik
>>
>>
>>
> First, you can't recompress JPEGs without degrading them permanently, so
> don't do that unless you don't care about the data.

I'm aware of this. It's a damage/disk-space/processing-time tradeoff.

> Second, I've heard that progressive JPEG compression can outperform
> "normal" JPEG compression, so you're probably witnessing the advantage
> of progressive JPEGs. The program cjpeg (part of libjpeg-progs in
> Debian) can create progressive JPEGs.

Thanks. I'll try it out sometime.

I google for "progressive jpeg" and find http://www.faqs.org/faqs/jpeg-
faq/part2/section-15.html, which tells me that there's also jpegtrans,
which losslessly converts between baseline and progressive JPEGs. So if
progressive really outperforms baseline, It may have to do with the
orograms that implement it rather than any inherent capability of the two
file formats. Unless there's a lot of useless overhead in the baseline
format, but I can't see compression experts designing useless overhead.

>
> HTH



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