Re: gEDA-user: Titleblocks: unlocking and filling in.

[Magnus Danielson <cfmd@xxxxxxxxxxxx>]

From: Bert Douglas <bertd@xxxxxxxxxxxxx>
Subject: Re: gEDA-user: Titleblocks: unlocking and filling in.
Date: Sat, 18 Dec 2004 12:21:09 -0600
Message-ID: <41C47515.50901@xxxxxxxxxxxxx>

> Charles Lepple wrote:
> > On Sat, 18 Dec 2004 07:26:59 -0700, Kim Lux <lux@xxxxxxxxxxxxxxxxxxx> wrote:
> > 
> >>I guess it depends on your usage.  We send CAD files created by qcad as
> >>jpegs all the time and I've yet to have anyone complain.
> > 
> 
> JPEG is intended for use on "continuous tone" images.  That is -- 
> photographs.  The heart of jpeg is the DCT (discrete cosine transform). 
>   There is an assumption of smoothness over an 8x8 DCT cell.  Computer 
> generated line drawings violate the basic assumption of smoothness.  And 
> computer drawings tend to use just a few discrete colors, rather than 
> continuous tones, as in a photograph.

Indeed.

> Remember that JPEG stands for "joint photographic expert group", whereas 
> PNG stands for "portable network graphics".
> 
> Computer generated images generally compress better with LZW techniques, 
> as used in PNG.

Computer generates images should be using LZ (Lempel-Ziv) techniques, where
LZW (Lempel-Ziv-Welch) is a particular kind which has fallen out of fashion.
PNG EXPLICITLY does not use LZW, it uses good old LZ77 as being used by GZIP.

> JPEG is lossy and produces an approximation of the original photograph.
> PNG is lossless and preserves all details of the original graphic.

Indeed. For CAD stuff PNG is the choice.

BTW. TIFF is not a compression method, it is rather a unified format for a
set of very different compression methods.

Cheers,
Magnus