[OT] Re: anchoring end points to objects

[Ross Burton <r burton 180sw com>]

On 23 May 2001 21:36:12 -0600, D. Stimits wrote:
> James Henstridge wrote:
> > 
> > On Wed, 23 May 2001, D. Stimits wrote:
> > 
> > > Despite being named "portable", PNG is not as well supported for many
> > > readers/viewers as is jpeg. Jpeg quality is also not necessarily worse,
> > > it has a controllable "lossy" compression, which can be turned off
> > 
> > Maybe JPEG 2000 supports lossless compression (bug is supported by even
> > less programs than PNG), but the normal JPEGs are always lossless (quality
> > factor 100 does not mean lossless -- just big files).  Jpeg usually forms
> > artifacts along edges, which doesn't look very good for most diagrams.
> 
> jpeg compression is always "lossy" as far as I know, but the amount of
> loss decreases as compression does. It is possible to turn off
> compression (large files) and not have the loss. The artifacts though
> are not a result of jpeg itself, but how it is rendered. Put in
> anti-aliasing, and it'll look as good as anything else.

Sorry for being so off-topic... but last year I did a computer graphics
course at uni, and JPEG was one of the subjects.  I think what I'm
saying here is right, but it was a dull course... :-)

JPEG has two modes: loosy and lossless.  Almost everyone uses the lossy
mode, but some programs allow the "100% quality" setting to use the
losseless algorithm.  Actually this "quality" setting is an abstraction
- there are two independant variables and several options in the
algorithm - there is a plugin for Photoshop which lets the user get to
the hard-code JPEG algorithm.

The artifacts in JPEG are part of the algorithm, not the rendering.  The
image is split into sections (typically 8x8 - look at a low quality JPEG
close up one day) and the colour levels are compared to a series of
patterns - gradients from left to right, blobs in the corners etc etc.
The image segment is said to be formed out of a sum of these patterns in
various levels.  This is hard to explain but quite simple really -
although it does emphasis why JPEG images are very bad a representing
line drawings.  Also, red is bad in simple JPEG encoders - Most encoders
quantise the red channel and average the levels.

Sorry again for the OT post!

Ross
-- 
Ross Burton                     Software Engineer
OneEighty Software Ltd          Tel: +44 20 8263 2332
The Lansdowne Building          Fax: +44 20 8263 6314
2 Lansdowne Road                r.burton@180sw.com
Croydon, Surrey CR9 2ER, UK     http://www.180sw.com./
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