[magick-users] identify - wrong resolution

[Anthony Thyssen]

On Tue, 30 Jun 2009 15:03:23 -0400
John Culleton <john at wexfordpress.com> wrote:

| On Monday 29 June 2009 09:13:16 pm Anthony Thyssen wrote:
| > On Mon, 29 Jun 2009 16:37:28 -0400
| >
| > John Culleton <john at wexfordpress.com> wrote:
| > | On Monday 29 June 2009 12:00:30 am Anthony Thyssen wrote:
| > | > On Sun, 28 Jun 2009 17:35:57 +0200
| > | >
| > | > Noel da Costa <noel at arc2.co.uk> wrote:
| > | > | Hi,
| > | > |
| > | > | Thanks...  what I did was open it into Photoshop, which
| > | > | reports the resolution as 300dpi.
| > | > | Perhaps Gimp would do the same?
| > | >
| > | > BOTH would be faking it!!!
| > | > Just as IM does.
| > | >
| > | >
| > | > All of them are raster image processors.
| > |
| > | Therefore in Gimp one can increase or decrease the dpi figure
| > | and run the dimensions up or down as you go.  The converse is
| > | not true for some reason.
| >
| > What do you mean the converse is not true.  Of course it is true.
| >
| In IMageMagick perhaps. In Gimp no. The resolution remains constant 
| if you adjust the  dimensions, meaning that it is doing some 
| downsampling  or other conversion under the covers. It is just a 
| Gimp peculiarity.   The basic rule, that size is inversely 
| proportional to density holds.  But to adjust size without 
| converting the raster image I have to 
| adjust the density.  For example I can scan a photo at 600dpi, and 
| then decrease the dpi to get a bigger image at 300 dpi. 

That is called -resample

   convert photo_at_600dpi.png  -resample 300  photo_at_300dpi.png

IM will do the resize and density adjustment so these two images
still remain the same size in real world terms, though pixel size
is halved.

See Im examples
   http://www.imagemagick.org/Usage/resize/#resample


  Anthony Thyssen ( System Programmer )    <A.Thyssen at griffith.edu.au>
 -----------------------------------------------------------------------------
      I don't make the rules... I just impliment them!
                                           -- Programmers Law of Policy
 -----------------------------------------------------------------------------
     Anthony's Home is his Castle     http://www.cit.gu.edu.au/~anthony/