gregjohn wrote:
> Warp <war### [at] tagpovrayorg> wrote:
>>   So my question stands: Why can't you do the exact same thing with a
>> povray-generated image as you would do with a photo?
>>
> 
> 
> I have published products at zazzle.com where I have povray generate the white
> border around the image. Some of these are 1:1, some are 2:1.  It soon becomes
> a labor-saving device when you generate 100 of these products. Before you call
> me petty, I've sold about a dozen of them.
> 
> If the docs were to empower someone to understand the camera, none of this
> discussion would be necessary-- it wouldn't have been started by me! :)  The
> docs fail because they don't teach someone how to create an object that
> precisely covers the screen, regardless of pixel aspect ratio or camera angle.
> Once I figured out how to do it, I wondered if it were a cleanly written
> algorithm. 4:3 is the tripping point.
> 
> One thing that troubles me in this discussion is the concept
> that there are petty uses of povray, that povray is only for certain noble
> tasks.

That has nothing to do with it.  It's not about the ultimate intended 
purpose of the image; its about making the image display correctly.

Changing the aspect ratio of an image can cause serious problems because 
of either cutting out portions of the image, or displaying objects / 
textures that weren't meant to be "on camera."

> Don't worry about anything that could also be done by mouse-dragging in
> photoshop (like cropping to get certain aspect ratios-- imagine doing THAT for
> a 2000-frame animation!!!)

It's not about what's easy to do in Photoshop; it's about what's easy to 
do, period.  In this case, adjusting an image for different aspect 
ratios has two possible solutions: Letterbox it (ie just leave it 
alone), or crop it (which is a solved problem when you use other tools 
like Photoshop or the GIMP).

-- 
Chambers

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> Alright then, let's ask this:
>
> What should the behavior be, when a scene is composed "without a second 
> thought for anyone else," and originally rendered in the default 4:3 
> aspect ratio... and you want to render it in a 3:4 aspect ratio for a 
> vertical poster?
>
> How should POV handle this?

IMO the default behaviour should be to preserve the horizontal FOV, as this 
is usually where the detail is modelled - more or less vertical FOV tends 
just to be more or less sky or emtpy ground.  So in your example the 3:4 
rendering would show more sky and ground (or whatever is in the image) than 
the original rendered at 4:3.

> Perhaps a sensible compromise would be that if only the width or height of 
> the file is set, and not the other, POV will assume square pixels and 
> adjust the other according to the camera aspect ratio.  That is, if you 
> set +w1600, and don't set height, POV will check the camera aspect ratio 
> (up and right vectors) and, in the case of the default 4:3, set +h1200 on 
> its own (and other values according to other ratios).  This would 
> certainly make scaling images easier.

So explain how you render a 3:4 version for a poster with your compromise?

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>> within POV or with some external tool).  If the default POV camera 
>> behaviour matched what most people want to do when showing a 4:3 image on 
>> a 16:9 device
>
> What do most people want to do in this situation?
>
> Some (myself) approve of letterboxing.  Others prefer pan&scan.  What 
> should POV do, that appeases both groups?

Assuming the scene file author has specifically written in the camera block 
that the aspect ratio should not be changed, then POV should give the person 
starting the render the choice of what to do from your choice - and an 
additional choice of changing the actual image resolution to match the 
aspect ratio fixed in the scene file.

Post a reply to this message

>> Of course you can, but it's usually by some contrived method
>
>  I wouldn't consider cropping an image "contrived", but whatever.

How do you work out what to type in for +w and +h to start with?  YOu need 
to go and find the correct camera block for the render, which is often not 
trivial for complex scenes due to multiple cameras being defined in 
different files.  Then you need to wait for the render, and start up a 
separate image editing program.  Is all that really necessary when you just 
want to put the image on your desktop?  It certainly seems unnecessary to 
me.

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Warp <war### [at] tagpovrayorg> wrote:
>   I'm sorry, but I just disagree with you. The original image composition
> should be preferred over anything else, unless the author *specifically*
> allows other aspect ratios to be used (which he can in the current POV-Ray).

To the contrary, I think that the default behavior should fit an "out of the
box" approach.

To this end, the default would have to fit scenes where *no* thought whatsoever
has been put into aspect ratio - neither from the author, nor from the person
rendering the scene. (Unfortunately, this is not the case.)

If an author has put enough thought into the aspect ratio issue to favor a
particular one, I think it is reasonable to expect from him to *explicitly*
specify this in the scene file in some way or the other.

Therefore, I advocate the original composition should be preferred *only* if the
author specifically *disallows* (or shall we rather say, disocurages) other
aspect ratios. Which, in current POV-Ray, he can do - though I'd agree that a
simpler mechanism to do so would be helpful.

The biggest issue in this area, however, is IMHO the fact that current POV-Ray
has no mechanism whatsoever to detect the difference between a mismatching
screen aspect ratio and a non-square pixel aspect ratio.

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Warp <war### [at] tagpovrayorg> wrote:
> > No wonder POV is so slow to develop if you have to debate whether
> > using more meaningful keywords is a good thing or not :-(  Oh well.
>
>   Have you ever thought that adding new keywords can potentially break
> existing scenes? The SDL namespace is already heavily cluttered as it is.

Have you checked lately where we're discussing?

I guess POV 4 will break quite a lot of existing scenes anyway. Well, at least I
hope so, because I hope for a better SDL.

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>   If you have a 4:3 photograph and you want to fill your 16:9 screen with it,
> what do you do?
>
>   Why can't the same answer be applied to a 4:3 povray-generated image?

Yes, that's probably a sensible answer to the question for a default behavior:

- assume 1:1 pixel aspect ratio
- use a "virtual" field of view as specified in the scene file
- render only a subset of the image, so that it fits the specified output
resolution

Or, alternatively:

- assume 1:1 pixel aspect ratio
- reduce the field of view as needed to fit the specified output resolution


Note that the second approach is probably a bit easier to implement, but the
first one would be superior when it comes to preserving exact scene content, in
case the field of view is used as parameter to scene content creation.

Post a reply to this message

Chambers <Ben### [at] gmailcom_no_underscores> wrote:
> Perhaps a sensible compromise would be that if only the width or height
> of the file is set, and not the other, POV will assume square pixels and
> adjust the other according to the camera aspect ratio.  That is, if you
> set +w1600, and don't set height, POV will check the camera aspect ratio
> (up and right vectors) and, in the case of the default 4:3, set +h1200
> on its own (and other values according to other ratios).  This would
> certainly make scaling images easier.

I'm repeating myself here, but I think what POV-Ray *really* lacks is a separate
option to explicitly specify the output image pixel aspect ratio.

From there on, the door would be wide open to all sorts of "automatic fixing" of
the residual aspect ratio issues.


Even your suggestion would not catch cases where a user may want to preserve the
original image's aspect ratio, but render for a different pixel aspect ratio.

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"scott" <sco### [at] scottcom> wrote:
> IMO the default behaviour should be to preserve the horizontal FOV, as this
> is usually where the detail is modelled - more or less vertical FOV tends
> just to be more or less sky or emtpy ground.  So in your example the 3:4
> rendering would show more sky and ground (or whatever is in the image) than
> the original rendered at 4:3.

As a matter of fact, I recently did a submission to the TC-RTC where this
approach would fail if the aspect ratio was significantly narrower than my
design: It includes grass "planted" all over the visible section of the ground,
with the algorithm requiring that the top edge of the shot is still below the
horizon.

Post a reply to this message

scott wrote:
>>> Of course you can, but it's usually by some contrived method
>>
>>  I wouldn't consider cropping an image "contrived", but whatever.
> 
> How do you work out what to type in for +w and +h to start with?  YOu 
> need to go and find the correct camera block for the render, which is 
> often not trivial for complex scenes due to multiple cameras being 
> defined in different files.  Then you need to wait for the render, and 
> start up a separate image editing program.  Is all that really necessary 
> when you just want to put the image on your desktop?  It certainly seems 
> unnecessary to me.

It is unnecessary.  If you just want to put the image on your desktop, 
all you have to do is right-click, and choose "Use as desktop."  Problem 
solved, POV isn't even necessary.

-- 
Chambers

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