Hello,

i want to change the size of a picture, both print size in mm AND the
resolution. How to do that? They seem to be coupled together.

Best wishes,
  Christoph

Christoph Conrad <nospam@spamgourmet.com>, you wrote on Wed, 06 Apr
2005 11:14:01 +0200:

>i want to change the size of a picture, both print size in mm AND the
>resolution. How to do that? They seem to be coupled together.

You cannot do that without a solid restructuring of the laws of
physics.

However, you can change the number of pixels in the image, using
regular scaling. 

Perhaps it would help if you told us some more about what you are
trying to achieve and why.

Currently I am entertaining one of two possible scenarios: 

1) Somebody told you the image needs to be so and so large, using a
resolution of X DPI. Tip: art people generally do not know what they
are talking about when they use the word DPI. They say insane things
like "the web is 72 DPI". Get your values straight from the horse's
mouth: from the printer's or the pre-press people. 

What's more, keep in mind that the DPI number usually is a minimum. If
you need to produce a 3 by 4 inch image at 150 DPI, anything over 150
DPI is good too. (Although images that take up too much disk space may
be a nuisance for other people downstream, because their compositing
tools lack the memory.)

2) You have an image that is too small to be printed at a reasonable
quality in a certain size. Increasing the number of pixels isn't
really going to help you: the image will probably only come out worse,
unless you use really smart scaling techniques. (For instance using
the http://www.logarithmic.net/pfh/resynthesizer plug-in.)

If this is the case, have you got an example of the image somewhere?
Perhaps there are simple ways to scale the image up without losing too
much qualities.

-- 
Deze geheele aarde door ijzeren banden omspannen, met de
stoomvaartlijnen die als draden over de zee�n zijn getrokken,
Dezen wereldkloot in zijn tuimeling door het heelal breng ik U.
  "Grashalmen", Walt Whitman, http://www.gutenberg.org/etext/14281

Hello Branko,

> You cannot do that without a solid restructuring of the laws of
> physics.

;-) and the i get the nobel price!

> However, you can change the number of pixels in the image, using
> regular scaling.

Ok, that sounds comprehensible.

> Currently I am entertaining one of two possible scenarios:

> 1) Somebody told you the image needs to be so and so large, using a
> resolution of X DPI.

I have an image "x by y" pixel, and it should be printed "a by c"
millimeter. The reproduction unit prints at 300 dpi. Rescaling the print
size gets ~170 dpi. But you're right, it doesn't make the picture better
to imterpolate pixels.

Best wishes,
  Christoph
-- 
True misery for a man is when there are no more problems to be solved.

Christoph Conrad <nospam@spamgourmet.com>, you wrote on Wed, 06 Apr
2005 20:53:04 +0200:

>> Currently I am entertaining one of two possible scenarios:
>
>> 1) Somebody told you the image needs to be so and so large, using a
>> resolution of X DPI.
>
>I have an image "x by y" pixel, and it should be printed "a by c"
>millimeter. The reproduction unit prints at 300 dpi. Rescaling the print
>size gets ~170 dpi. But you're right, it doesn't make the picture better
>to imterpolate pixels.

Well, there's not much you can do then. Try and get higher quality
originals (read: more pixels) if you can.

-- 
Deze geheele aarde door ijzeren banden omspannen, met de
stoomvaartlijnen die als draden over de zee�n zijn getrokken,
Dezen wereldkloot in zijn tuimeling door het heelal breng ik U.
  "Grashalmen", Walt Whitman, http://www.gutenberg.org/etext/14281

Branko Collin wrote:
> Christoph Conrad <nospam@spamgourmet.com>, you wrote on Wed, 06 Apr
> 2005 11:14:01 +0200:
>
> >i want to change the size of a picture, both print size in mm AND
the
> >resolution. How to do that? They seem to be coupled together.
>
> You cannot do that without a solid restructuring of the laws of
> physics.

You can do this without upsetting generations of physicists...

> However, you can change the number of pixels in the image, using
> regular scaling.

.... by combining scaling the image and changing the ppi into one
operation. Just wrtie a script that allows the user to input the target
size and the target ppi and do some math. Nice excercise for someone
who want to learn about unit conversion, btw :)


HTH,
Michael