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

[Gernot Hassenpflug]

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
>  >
>  >
>  >  --
>  >  sane-devel mailing list: sane-devel at lists.alioth.debian.org
>  >  http://lists.alioth.debian.org/mailman/listinfo/sane-devel
>  >  Unsubscribe: Send mail with subject "unsubscribe your_password"
>  >              to sane-devel-request at lists.alioth.debian.org
>  >
>
>
>  --
>  "The truth is an offense, but not a sin"
>