Apparently some image processing programs, like Photoshop, expect a DPI setting,
and when one is not supplied they assume 72 DPI.
Is there a way to have POV-Ray write a specified DPI value to the file, or is
that a big NOPE ?

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That's an interesting question. Which I don't know the answer to :-P

But I work around it in Photoshop: If I need to change an image's DPI, for
printing or whatever, I just go into IMAGE/IMAGE SIZE and fiddle with the
resolution there, then re-save it. (If the image is PNG, that's a lossless
re-save, quality-wise; but if JPEG, then a 2nd compression quality-loss is
incurred... which I usually live with.)

That said, it would be nice to be able to specify a DPI setting in POV-Ray.

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"Kenneth" <kdw### [at] gmailcom> wrote:
> That's an interesting question. Which I don't know the answer to :-P
>
> But I work around it in Photoshop: If I need to change an image's DPI, for
> printing or whatever, I just go into IMAGE/IMAGE SIZE and fiddle with the
> resolution there, then re-save it. (If the image is PNG, that's a lossless
> re-save, quality-wise; but if JPEG, then a 2nd compression quality-loss is
> incurred... which I usually live with.)
>
> That said, it would be nice to be able to specify a DPI setting in POV-Ray.

You do realise that it is pointless to add some arbitrary number to an image
file? Apart from the fact that misc image formats, JPEG included, don't have a
standard DPI flag? - It isn't surprising either, as there is absolutely no use
for it ... I might also add that this discussion comes up every few years. So
maybe search the newsgroups for the many answers in that regard.

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Am 02.04.2017 um 06:08 schrieb Thorsten Froehlich:
> "Kenneth" <kdw### [at] gmailcom> wrote:
>> That's an interesting question. Which I don't know the answer to :-P
>>
>> But I work around it in Photoshop: If I need to change an image's DPI, for
>> printing or whatever, I just go into IMAGE/IMAGE SIZE and fiddle with the
>> resolution there, then re-save it. (If the image is PNG, that's a lossless
>> re-save, quality-wise; but if JPEG, then a 2nd compression quality-loss is
>> incurred... which I usually live with.)
>>
>> That said, it would be nice to be able to specify a DPI setting in POV-Ray.
> 
> You do realise that it is pointless to add some arbitrary number to an image
> file? Apart from the fact that misc image formats, JPEG included, don't have a
> standard DPI flag? - It isn't surprising either, as there is absolutely no use
> for it ... I might also add that this discussion comes up every few years. So
> maybe search the newsgroups for the many answers in that regard.

If the image is intended to be printed, there /is/ use for it. You might
argue that it's not a strong enough case, but just because you don't see
a use case doesn't mean there is none at all.

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Am 01.04.2017 um 22:03 schrieb Bald Eagle:
> Apparently some image processing programs, like Photoshop, expect a DPI setting,
> and when one is not supplied they assume 72 DPI.
> Is there a way to have POV-Ray write a specified DPI value to the file, or is
> that a big NOPE ?

It's a small nope: POV-Ray currently does not provide such a feature.
And it is questionable whether it is worth the effort, given that
virtually every image processing program that gives a dingo's kidney
about DPI provides a means to adjust that setting.

But theoretically it could be done -- at least for image file formats
that actually support such a setting.

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clipka <ano### [at] anonymousorg> wrote:
> Am 02.04.2017 um 06:08 schrieb Thorsten Froehlich:
> > "Kenneth" <kdw### [at] gmailcom> wrote:
> >> That's an interesting question. Which I don't know the answer to :-P
> >>
> >> But I work around it in Photoshop: If I need to change an image's DPI, for
> >> printing or whatever, I just go into IMAGE/IMAGE SIZE and fiddle with the
> >> resolution there, then re-save it. (If the image is PNG, that's a lossless
> >> re-save, quality-wise; but if JPEG, then a 2nd compression quality-loss is
> >> incurred... which I usually live with.)
> >>
> >> That said, it would be nice to be able to specify a DPI setting in POV-Ray.
> >
> > You do realise that it is pointless to add some arbitrary number to an image
> > file? Apart from the fact that misc image formats, JPEG included, don't have a
> > standard DPI flag? - It isn't surprising either, as there is absolutely no use
> > for it ... I might also add that this discussion comes up every few years. So
> > maybe search the newsgroups for the many answers in that regard.
>
> If the image is intended to be printed, there /is/ use for it. You might
> argue that it's not a strong enough case, but just because you don't see
> a use case doesn't mean there is none at all.

At least I am in good company:
http://news.povray.org/povray.general/thread/%3CpdbNOSPAM-66C5AC.10595417092005%40news.povray.org%3E/

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Am 02.04.2017 um 10:27 schrieb Thorsten Froehlich:
> clipka <ano### [at] anonymousorg> wrote:
>> Am 02.04.2017 um 06:08 schrieb Thorsten Froehlich:
>>> "Kenneth" <kdw### [at] gmailcom> wrote:
>>>> That's an interesting question. Which I don't know the answer to :-P
>>>>
>>>> But I work around it in Photoshop: If I need to change an image's DPI, for
>>>> printing or whatever, I just go into IMAGE/IMAGE SIZE and fiddle with the
>>>> resolution there, then re-save it. (If the image is PNG, that's a lossless
>>>> re-save, quality-wise; but if JPEG, then a 2nd compression quality-loss is
>>>> incurred... which I usually live with.)
>>>>
>>>> That said, it would be nice to be able to specify a DPI setting in POV-Ray.
>>>
>>> You do realise that it is pointless to add some arbitrary number to an image
>>> file? Apart from the fact that misc image formats, JPEG included, don't have a
>>> standard DPI flag? - It isn't surprising either, as there is absolutely no use
>>> for it ... I might also add that this discussion comes up every few years. So
>>> maybe search the newsgroups for the many answers in that regard.
>>
>> If the image is intended to be printed, there /is/ use for it. You might
>> argue that it's not a strong enough case, but just because you don't see
>> a use case doesn't mean there is none at all.
> 
> At least I am in good company:
>
http://news.povray.org/povray.general/thread/%3CpdbNOSPAM-66C5AC.10595417092005%40news.povray.org%3E/

Quiz:

Q1: How many people involved in that thread had any actual experience in
the printing business?
A1: Only one, as it seems.

Q2: What was the opinion of that person?
A2: Not yours, as it seems.


In a nutshell, what that person wrote is:

If a person from the printing business asks you for an image with a
physical size of X mm by Y mm and a pixel pitch of P pixels per inch,
they _not only_ want the image to be (X,Y)*P/25.4 pixels in size, but
_also_ would like the image header to explicitly say "intended to be
printed at a pitch of P pixels per inch".

Whether this semi-requirement by the printing industry is deemed
unnecessary and confusing by most artists is irrelevant: Apparently it's
a _fact_ of the printing industry (or at leat it was, a dozen years
ago), and therefore /some/ people creating images may want to cater to that.

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"Thorsten Froehlich" <nomail@nomail> wrote:

>
> You do realise that it is pointless to add some arbitrary number to an image
> file? Apart from the fact that misc image formats, JPEG included, don't have a
> standard DPI flag?

I do see your point. I think the OP was curious if (to paraphrase him) POV-Ray
could output its image renders at, say, 300 dpi, for printing purposes. (I guess
that would mean an 'embedded' 300 dpi setting... which is necessarily different
from the on-screen resolution of the render, which itself depends on the screen
resolution of the monitor its viewed on.)

When I take a JPEG or PNG image to a printing house nearby, they always specify
that it be at 300 dpi resolution-- regardless of the actual pixel size of the
image-- simply because the printing equipment is made to work that way, I
suppose. Currently, I have to do that in Photoshop, as a post-processing step
(with *any* image, not just POV-Ray's, as most images seem to have been created
with a default rez of 72dpi-- if 'default' is the right word.) Having a choice
of an actual *embedded* resolution in a POV image would be useful for that, if
only just to save a step.

But since Photoshop *can* convert a JPEG image from 72dpi to 300dpi (and the
printing shop's machines can 'see' it that way), doesn't this imply that JPEG
images can have an embedded resolution?

In any case, your comments have given me food for thought, about something that
seems like a curious paradox: A typical POV-Ray rendered image has NO stated
resolution info, just the width and height in pixels. However, when I take a
digital JPEG photo with my Canon Powershot camera, and take a look at the
image's 'properties' before any image-editing has been done to it at all, it
actually does state that the image already has a resolution of '180 dpi.' I had
always assumed this to be an actual 'embedded' resolution. Or is it simply
*metadata* that my particular camera has written to the file-- without any real
practical meaning? (With the exception being, how LARGE the image initially
appears in a particular image viewing/editing app like Photoshop, which has a
default resolution of 72 dpi... the size depending on the MONITOR'S own
resolution, of course.)

Although 'resolution' of an image may not be of practical concern most of the
time, it does have some importance.

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"Bald Eagle" <cre### [at] netscapenet> wrote:
> Apparently some image processing programs, like Photoshop, expect a DPI setting,
> and when one is not supplied they assume 72 DPI.
> Is there a way to have POV-Ray write a specified DPI value to the file, or is
> that a big NOPE ?

Hmmm.   It's always the little things, isn't it.

Although POV-Ray was and of course still is primarily a raytracing program for
creating photorealistic images, I think that over the last 25 years, its users
have demonstrated that there are other purposes - many directly related and
incidental to creating such scenes - and other less so.

I think that with some of the recent discussions about why people use POV-Ray,
and trying to increase interest in using POV-Ray, it would be of interest to the
community and the developers to make POV-Ray as user-friendly and appealing to
use as possible.

I think people would much rather tell others "I (choose to) use POV-Ray because
....  XYZ", rather than "Well, POV-Ray can't do that, but...."

I'm just saying that POV-Ray has a market to compete in.
How it will do that is something that we can have friendly discussions about.

:)

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"Kenneth" <kdw### [at] gmailcom> wrote:
> it
> actually does state that the image already has a resolution of '180 dpi.' I had
> always assumed this to be an actual 'embedded' resolution. Or is it simply
> *metadata* that my particular camera has written to the file-- without any real
> practical meaning? (With the exception being, how LARGE the image initially
> appears in a particular image viewing/editing app like Photoshop, which has a
> default resolution of 72 dpi... the size depending on the MONITOR'S own
> resolution, of course.)
>
> Although 'resolution' of an image may not be of practical concern most of the
> time, it does have some importance.

DPI is always just metadata in a bitmapped pixel image. It has no meaning for a
pixel. I suppose someone at Canon added it to some EXIF metadata block in order
to just get customers off their back :-) The 180 (or any other value for that
matter) certainly has no meaning for a photo camera...

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