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

[J. Larry Hendry]

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.
:-)