Re: Downsampling images in pdfTeX

["George N. White III" <aa056@chebucto.ns.ca>]

On Tue, 27 Jul 2004, Mats Broberg wrote:

>> From: ntg-context-bounces@ntg.nl
>> [mailto:ntg-context-bounces@ntg.nl] On Behalf Of George N. White III
>
>> As a general principle, it makes no sense for pdftex to
>> provide image manipulation capabilities.  Such capabilities
>> are useful to a much wider
>> audience than the users of pdftex, so there are lots of tools
>> to do image resampling and format conversions.  All that
>> pdftex should do is support inclusion of pdf.  The limited
>> support for including png images is a convenience, but if you
>> are being careful you would want to make pdf images.
>>
>> --
>> George N. White III  <aa056@chebucto.ns.ca>
>>    Head of St. Margarets Bay, Nova Scotia, Canada
>
> Having checked the pdfTeX documentation, doesn't the internal parameter
> \pdfcompresslevel deal with this? The documentation says:
>
> "compress level This integer parameter specifies the level of text and
> in-line graphics compression. pdfTEX uses zip compression as provided by
> zlib. A value of 0 means no compression, 1 means fastest, 9 means best,
> 2..8 means something in between. Just set this value to 9, unless there
> is a good reason to do otherwise - 0 is great for testing macros that
> use \pdfliteral."

The compression this parameter controls is quite different.  Without 
compression (e.g., \pdfcompresslevel=0) a PDF file consists of almost 
readable text. Even images can be stored in an ASCII encoding.  Setting a 
non-zero value for \pdfcompresslevel applies a lossless compression 
algorithm to objects in the pdf file.

For images that will be displayed only at low resolution it may be useful 
to downsample the original image to reduce the size.  For example you 
might have a 2 inch by 2 inch image scanned at 400 dpi.  This image would 
have 800x800 pixels.  For screen display you might prefer to have a 
200x200 pixel image (or 100 dpi for 2 inches).  Downsampling refers 
to the process of reducing an 800x800 pixel image to 200x200 pixels.
ways to reduce image size (lossy compression, colorspace changes,
even converting certain images to line art).

Some tools to generate PDF include methods to downsample images.
In particular, people who have been using tex-->dvipsone-->distiller 
and are now using just pdftex encounter problems with much larger 
pdf file sizes and excessive load times until they resize the input 
images.

--
George N. White III  <aa056@chebucto.ns.ca>
   Head of St. Margarets Bay, Nova Scotia, Canada