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------- Additional comments from [EMAIL PROTECTED] Sat Mar 15 22:03:44 +0000
2008 -------
Unfortunately, there are many misunderstandings regarding image resolution and
DPI/LPI/PPI/(SPI).
I therefore need to clarify some aspects. Also, many things do NOT look like
they are (or, are NOT perceived as they are). Hopefully, some aspects will be
clarified after reading this post.
I will attach later on some screenshots from Adobe Illustrator to emphasise
these points.
1.) DPI and LPI are printer measures:
- DPI is the maximum number of dots a printer can access per inch
-- DPI points are just printer ink present or absent
-- a DPI dot can NOT show various nuances of a colour
- LPI is lines per inch, lines made up of the smaller dots from DPI
to create the various colours we see in an image
-- an LPI dot can be made of many dozen DPI dots
-- more DPI dots will more accurately recreate the original colour,
so smaller LPI is not necessarily worse
[LPI is the printer equivalent of the monitor pixel, of what we see.]
2.) Images are stored as pixels (lets ignore for the beginning vector graphics
for the sake of clarity). However, pixels have NO dimension.
- a 100 x 50 px image could be 10 x 5 inch, or 2 x 1 inch, or 50 x 25 inch
3.) There is NO relationship between pixels and DPI/LPI
- pixels just tell us what information is stored (actual information in
digital pictures)
- therefore, changing the DPI won't modify the quality of a non-vector image,
just the size of the printed output
[we will see a benefit only for vector images - arguably, this is
why this feature will impact Draw export as we work with vector images]
CRUCIAL VARIABLE
================
LPI is the important measure, but it is little known by ordinary people. Most
importantly, it is usually not provided with the printer documentation (see
later).
I therefore would have preferred initially to specify the DPI for the target
output. It is known for a specific printer and people have heard of it (though
many will misunderstand it).
There are however some problems with DPI that made me revise my initial opinion:
- newer printers have stated DPIs of 1,200 and higher dots per inch values
- this higher DPIs don't scale linearly with LPI values
- actually, a 2,400 DPI office/home user printer will fare only slightly
better than a 600 DPI ordinary printer (2,400 DPI is in this respect
completely misleading)
Even professional printing requires at most a picture scaled for 200-300 LPI
output.
A.) PRAGMTIC APPROACH
=====================
This is why I favour a pragmatic approach:
- Let user choose between 3 predefined OPTIONS:
-- screen: 72 PPI (or LPI; for screens, PPI is the more accurate term)
-- medium: 150 LPI (pixel equivalent, or PPI)
-- high: 300 LPI (pixel equivalent, or PPI)
This is exactly what Adobe Illustrator Dialog "Document Raster Effects Settings"
offers, too (see the first screenshot).
B.) ADVANCED APPROACH
=====================
The advanced approach will display both the LPI and DPI options for that
particular printer. This is also available in Adobe Illustrator, see the 2nd
screenshot. Unfortunately, the translation of DPI to LPI:
- isn't straightforward,
- is printer dependent (and also on page type)
- I do not know reliable informations for such translations
- Adobe offers so called PPD-files containing such information for
a particular printer (but I do not know if such information is opensource)
EXAMPLE OF PPD FILE-INFO:
Modified PPD (postscript printer description) file for Agfa Avantra 25
year 2002, postscript level 2).
Defined paper formats:
* A0 to A10,
* B0 to B10,
* USLetter and USLegal,
* user defined formats.
Defined resolutions:
* 600 dpi – 40 lpi,
* 846 dpi – 85 lpi,
* 900 dpi – 60 lpi,
* 1016 dpi – 70 lpi, 80 lpi,
* 1200 dpi – 65 lpi, 75 lpi, 85 lpi, 100 lpi, 110 lpi, 120 lpi,
133 lpi, 140 lpi, 150 lpi,
* 1270 dpi – 65 lpi, 75 lpi, 80 lpi, 90 lpi, 100 lpi,
* 1693 dpi – 80 lpi, 100 lpi, 133 lpi,
* 1800 dpi – 112 lpi, 133 lpi, 150 lpi,
* 2032 dpi – 85 lpi, 110 lpi, 133 lpi,
* 2400 dpi – 85 lpi, 100 lpi, 110 lpi, 120 lpi, 133 lpi, 140 lpi,
150 lpi, 160 lpi, 175 lpi, 200 lpi,
* 2540 dpi – 100 lpi, 110 lpi, 120 lpi, 133 lpi, 138 lpi, 150 lpi,
175 lpi,
* 3200 dpi – 200 lpi,
* 3386 dpi – 100 lpi, 120 lpi, 133 lpi, 175 lpi, 200 lpi,
* 3600 dpi – 133 lpi, 140 lpi, 150 lpi, 175 lpi, 200 lpi, 225 lpi,
250 lpi, 300 lpi,
* 4800 dpi – 300 lpi.
>From the previous example, see that even a 4,800 DPI printer outputs actually
only 300 LPI. So, e.g. for a 10 x 5 inch image, the resolution suffices to be
3,000 x 1,500 pixels.
Hope this removes some of the misunderstanding and also will foster the
implementation of a simple but powerful approach to this issue. This
implementation should also guide unsuspecting users into following the best
practice available. The pragmatic approach would just do this.
[See: http://www.schulz-kirchner.de/dwl/Adist4.ppd.txt for another PPD-file.]
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