What program is best at JPEG compression?

External Since: Jul 24, 2007 Posts: 18

Bill Tuthill added these comments in the current discussion du
jour ...

> What's the difference between PSP's 2x1 1x1 1x1 and PSP's 1x2
> 1x1 1x1 encoding? I used the former, not the latter. PSP
> also has a bunch of weird encodings not encountered in other
> software.
>
My experience with PSP 8 some time ago and 9 more recently is that
1x2 creates larger files but higher quality due to less artifacts
of various types, one insidious one looking for all the world like
noise but it isn't. 2x2, OTOH, produces a much smaller file size;
you need to visually judge the difference in quality to see which
you like better. I only use 2x2 when I am desperate for smaller
size and upping the JPEG compression number is also producing
unacceptable defects. It is a big juggling act and some
experimentation may be necessary for you.

--
HP, aka Jerry

External Since: Jul 27, 2007 Posts: 13

On Jul 27, 2:53 pm, "HEMI-Powered" <n....RemoveThis@none.en> wrote:
> Martin Brown added these comments in the current discussion du
> jour ...
>
> >> Bill, I don't want to hit this at all hard, but I could swear
> >> that during the beta test for PSP 8, Jasc's Chief Scientist,
> >> the same guy, Kris Zaklika, that wrote the math for DCNR,
> >> said that PSP 8 could decide in a "smart" way which block
> >> size to use. I could be
>
> > Either way that statement is incredibily funny.
>
> Why is that "incredibly funny", Martin? I get the feeling that
> you are calling me a fool.

No. But I am laughing at the claim that PSPro v8 could decide in a
"smart" way which block size to use when it had one of the most broken
JPEG implementations ever seen in a commercial released product.

> > PSP 8 has a demonstrably very broken chroma subsampling
> > implementation in its JPEG codec. It is a testament to just
> > how robust JPEG is that more people have not noticed this
> > flaw.
>
> Come again? I do not know either the math or the implementation
> but NEVER had any major problems with PSP 8 during the years from
> its release until PSP 9 was released,

You were unaware of the problem. So perhaps you don't consider colour
errors a problem. Or you never use chroma subsampling AFAIK the not
subsampled 1x1 1x1 1x1 variant works OK in PSPro 8. NB Photoshop
defaults to not subsampled for the top half of its highest quality
settings and for some images there is a big step in quality there.

> > The simplest line art test to show its failings on chroma
> > encoding is to encode a pure red and pure blue 16x16 pokerdot
> > test pattern in each of the possible phases save as JPEG
> > default 2x2 subsampled at maximum quality and then reload it.
> > Half of the newly decoded image not even a vaguely
> > satisfactory approximation to the original (with gross
> > residual colour errors - completely the wrong colour bright
> > red instead of purple). 2x1 subsampling gets an even more
> > incorrect result with alternating blue and red stripes in
> > addition.
>
> I don't do line art, don't do vectors, don't do text, don't do
> layers, I ONLY do raster graphics bitmap photo/scan editing. So,
> pathological examples such as ANY JPEG implementation mangling
> line art or text does not surprise me, as that is hardly what
> it's authors had in mind. I keep in mind that the "P" in JPEG
> means "Photographer". So, I will hardly dispute your conclusions
> but do not see how that is relevant to JPEG's real purpose and
> usage, other than line art people also want mimimal size images
> as well. But, I would think that to maintain maximum quality, you
> would want to use a lossless format or even save only to your
> favorite editors proprietary format, e.g., pspsimage.

The line art example is constructed to demonstrate a specific
implementation fault in the starkest possible form. However, the
problem is still present in every colour JPEG image made with chroma
subsampling. The damage is less, but there is still unwarranted and
irrecoverable damage to the stored JPEG image coefficients.

Incidentally for some line art JPEG at highest quality not subsampled
can sometimes be the smallest file size. JPEG was designed for
photographic images, but it is quite capable of storing line art with
the right settings (although usually PNG or GIF will be more compact).

> For my car pictures, 2x2 subsampling turns them into one big
> artifact, I never touch the stuff, so never suffer from its
> limitions or damage.

Chroma subsampling tends to hit sharp red or blue colour transitions
disproportionately. And they are rather common colours for cars. But
unless you zoom in to pixel level it should be virtually invisible.
Some of the damage you are seeing could well be from a flawed JPEG
implementation. Check that the problem persists with IrfanView or
GIMP.

Regards,
Martin Brown