scanning resolution

Analoui, Mostafa manaloui at IUPUI.EDU
Mon Apr 10 15:39:45 PDT 2000


Dear Grace:

Here is a brief note regarding your questions. DPI, PPI, LPI are units for
expressing resolution of an imaging system, commonly used for printing and
scanning. Assuming that the system has square pixels, all these units refer
to the same concept, i.e. density of basic patterns.

The pixel shape in a flat bed scanner is a function of two parameters:
1. Density of CCD receptors in the linear array. This is the most limiting
factor and (once designed) cannot be changed.
2. Precision by which the step motor can be controlled. This precsion is
commonly independent of CCD resolution and that is why a scanner can have
different resolution in X and Y directions.

If one sets the X and Y resolutions of the scanner at the same number, s/he
should get approximately square pixles. Then, DPI, LPI, and PPI would refer
to the same thing (divide it by two to get  line pairs per inch).

Regarding I (Input) and O (Output) resolution, I am going to take a "guess",
since it seems your scanner/software uses this option.  If the scanner were
to scan a "binary" printed image, it is important to know at whatt
resolution the image was printed (to prevent  alisaing and Moire pattern).
It seems your software is providing two features:

1. Scan at I resolution and convert it to O resolution
2. Descreening

Resolution conversion (through interpolation) and descreening are needed for
scanning "binary" images and converting them to the gray scale images.

Since you are scanning intra-oral films (which are not binary),
1. I and O resolutions must be the same (there might be an option in your
software to tell the scanner that you are not scanning binary or halftone
images).
2. Turn off any descreening and/or interpolation option in your software.

The above two steps will prevent interpolation/scaling of the images.

I hope that helps.

Mostafa,



-----Original Message-----
From: GRACE G PETRIKOWSKI [mailto:ORADLIST at LISTSERV.UCLA.EDU]
Sent: Monday, April 10, 2000 1:05 PM
To: ORADLIST at LISTSERV.UCLA.EDU; grace.petrikowski at UTORONTO.CA
Subject: scanning resolution


Hi All,

This message is a request for help and is a follow-up on a
message I posted a while back on scanning parameters.  The question
this time has to do with scanning resolution units.

I am scanning radiographic films using a flatbed scanner and am
confused regarding various ways of expressing resolution.  Choices
given are:  dpi (dots per inch), ppi (pixels per inch) and lpi (lines
per inch).  My impression is that dpi and ppi are essentially the
same thing and can be used to express input as well as output
resolution but that lpi refers to output resolution in terms of the
printing process.

My questions are:
1.
If I am scanning something (eg. a panoramic view)
at 150 dpi as the INPUT resolution without changing the scaling
factor, will the OUTPUT resolution be higher, lower or the same?

2.
Conversely, if I am scanning the same radiograph at 150 dpi as the
OUTPUT resolution without changing the scaling factor, what will the
actual input resolution be?

3.
How does lpi fit into all of this?  Obviously a line is different
from a dot but if you have 150 lpi, would that be the same as 150 dpi
if you count one entire line as being the same as one dot?  (ie. one
dot = one line but all the lines = 150 dpi going from the top to the
bottom of the page).

For our purposes, since we want to make quantitative measurements
from the radiograph, we want to make sure our input resolution is at
a certain setting but it seems that some scanning software only
allows you to set an output resolution.  Any advice/comments on the
above questions would be much appreciated.

Thanks!

Grace

----------------------
Dr. C. GRACE PETRIKOWSKI
Oral and Maxillofacial Radiologist
Faculty of Dentistry, University of Toronto
124 Edward Street
Toronto, ON Canada  M5G 1G6
grace.petrikowski at utoronto.ca



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