[sane-devel] meaning of 1200x2400 etc. explained

m. allan noah kitno455 at gmail.com
Fri Feb 15 00:57:15 UTC 2008


so you want the backend to scan at 1200x2400 but return 2400x2400
data? it would have to interpolate 50% of the data in the scan with
'made up' data. or do you want to scan at 1200x2400 and return
1200x1200, in which case you have scanned twice as much data as you
need.

allan

On 2/14/08, Gernot Hassenpflug <aikishugyo at gmail.com> wrote:
> On Fri, Feb 15, 2008 at 8:44 AM, m. allan noah <kitno455 at gmail.com> wrote:
>  > you friend would be correct IF sane used a file format which allowed
>  >  the storage of the dpi, or exposed this data through some external
>  >  means. but, as i said before, sane does not. so, you are going to see
>  >  a stretched image.
>  >
>  >  allan
>
>
> Obviously I am not quite getting this, but how I understand it is this:
>
>  1. maximum horizontal resolution (physical spacing of CCDs etc) = 1200dpi
>  2. maximum fineness of vertical linear motion = 2400dpi
>
>  Now, when the application does scanning, I think the following happens:
>
>  a. set DPI (this is the horizontal scan dpi). I don't care
>  particularly what SANE does with this number, but in any case it is
>  the one the user cares about.
>  b. the scanner then uses a linear movement rate of two times this,
>  probably to avoid artifacts and to be able to give better
>  representation of diagonal lines.
>  c. The scanner interpolates or averages the vertical down by a factor of two.
>  d. The user gets back square pixels at the DPI setting he chose.
>
>  Is this radically different in practice? From your post I do not
>  understand how SANE influences in detail the scanner's vertical
>  motion, for example.
>
>  Regards, Gernot
>
>
>  >  On 2/14/08, Gernot Hassenpflug <aikishugyo at gmail.com> wrote:
>  >  > I cannot find the thread where this was discussed before, but here the
>  >  >  answer I received from a friend of mine, hopefully it will be useful
>  >  >  to clear up the confusion surrounding the resolution of scanners!
>  >  >
>  >  >  - The lower figure (1200) is what is called True DPI. This is the more
>  >  >  important value. It stands for the density of the aligned sensors (the
>  >  >  more you have the better quality the scan output. This figure
>  >  >  represents the horizontal scanned resolution.
>  >  >
>  >  >  - The higher figure (2400) stands for the vertical scanned resolution
>  >  >  and it is the steps the sensors are moving (vertically) in an inch.
>  >  >
>  >  >  -> You confuse the out put which is square inch and therefore always
>  >  >  has the same horizontal and vertical figures.
>  >  >
>  >  >  I hope that helps,
>  >  >   Gernot Hassenpflug
>  >  >
>  >  >
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>  >
>


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