[sdiy] OK, one of you computer geeks, explain this..

[james]

acrobat has separate compression settings for grayscale vs. color 
images - it sounds like you have your pdf settings configured to 
compress color images in a different way than grayscale. bitmap images 
in pdf's are compressed using either jpeg or some kind of zip format.

if you're using distiller, you can change all of this easily - some 
apps with built-in pdf-making don't let you get as deep into the 
options, though.


On Feb 3, 2005, at 11:48 AM, Matt Simpson wrote:

> It has a whoooole lot to do w/file compression & graphic rendering.
> I *believe* PDF format utilizes some form of vector rendering, which
> is similar to what Flash uses (and maintains small file sizes).
> Bitmaps are raw pixel data - each pixel is represented by a unique
> value, and that value is the same bit-size each time.  PDFs take
> advantage of tons of facts about the document in question (for
> instance - are there wide swaths of a certain color?  if so that
> significantly decreases file size - same thing MPEG compression does)
> and adds its own algorithmic flair to compress the data
>
>
> On Sun, 30 Jan 2005 18:36:53 -0500, J. Larry Hendry 
> <jlarryh at iquest.net> wrote:
>> While this pertains to scanning and file structure, it is not off 
>> topic
>> because I am scanning synth schematics.  Due to the black and white 
>> scanning
>> I have done in the past, I pretty much understand how to minimize PDF 
>> file
>> size for a given resolution (through the BMP file).  However, I am now
>> working on documents that require color and grayscale. While some of 
>> the
>> document is color, about 2/3 is grayscale.  So, I thought I would 
>> save file
>> size by scanning those in gray instead of color.
>>
>> Here is what puzzles me. Let me throw some file sizes out there for 
>> one of
>> my large 11x17 pages to see if any of this makes sense.
>>
>> This page was scanned (paste a few together) in grayscale as a bitmap 
>> and
>> results in 13.2 MB file size.  I normally scan everything to Bitmap 
>> and then
>> convert to PDF as that has always given me the most efficient file 
>> size
>> (past B&W experience).  So, I convert the file to a PDF and the 
>> resulting
>> file is about 2.6 MB.  Well, I was hoping for better, but I thought 
>> that was
>> it.  Then, I started noticing some of the color pages were smaller 
>> once
>> converted to PDF.  I wondered why since the color BMP files were so 
>> much
>> larger.  So, as an experiment, I took my 13.2 MB grayscale page and
>> converted it to full 16 million colors file in my photo editor.  As
>> expected, the file size jumper up to 39.7 MB.  I'm still 
>> understanding OK up
>> to here.  But, now I take that color file and convert it to PDF.  The
>> resulting file is only 1.15 MB.
>>
>> So in summary,
>> gray file started 13.2 MB bitmap
>> converted to PDF @ 2.6 MB
>>
>> same gray file to full color = 39.7 MB
>> converted that color file to PDF @ 1.15 MB
>>
>> Of course, since the original source file was grayscale, even the 
>> color PDF
>> looks grayscale.  In fact I cannot tell the two images apart.  
>> However, the
>> one that was converted to the very large color file first ended up 
>> the MUCH
>> smaller file size when converted to PDF.
>>
>> If any of you who are considerable more geeked than I concerning 
>> computers
>> can 'splain this to me and Lucy, that would be great.  Maybe I will 
>> learn
>> something that will help me additionally control the file size on 
>> these
>> large, highly detailed scans.
>>
>> Larry Hendry
>> computer geek NOT.
>> :-)
>>
>>
>
>
> -- 
> ~mt
> sonic alchemist
> eat ! or ! be ! eaten
>