Resolution of JPG's to make a DVD slideshow for TV
Forum Graphic & Displays : TV/Video Cards - Resolution of JPG's to make a DVD slideshow for TV
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csaagpDIESPAMDIE@yahoo.com wrote:
> So, if I get this straight -
>
> if there's no conversion from square to non-square by the slideshow
> app, then I could , on my own, 1) crop to a 720x527 ratio (the
> original pics are something like 2000x1500 pixels so I can't just
> crop to 720x527), then interpolate to 720x480
Yes. That should get you a full 720×480 frame, nicely filled up with
correctly-shaped DV (ITU-R BT.601) pixels.
One thing to note, however, is that tv sets - unlike computer
monitors -are generally set to overscan the picture: that is, the image
fills the whole screen but the border areas go over the edges of the
tube.
If you want to fill up the whole tv screen from top to bottom, edge to
edge, you need to make sure that nothing important in your original
images is lost at the very edges due to overscan. If you can't solve
this problem by creative framing (i.e. if there is something very
important at the very edges of the original image that absolutely
_needs_ to be visible on the tv, too), you could always add borders
around the image.
Be warned, though, that adding borders is not necessarily an ideal
solution: different tv sets from different manufacturers may have been
set for a different amount of overscan. Even if you adjust the border
width to be minimal on your set, they might show up larger (or
non-existent) on some other sets. Sometimes the picture was not even
centered correctly when the set left the factory, which may manifest
itself as those added borders not being visible on all sides of the
image (or in some cases, at least not being of equal width), which is
usually somewhat distracting.
This site wraps up all things "overscan" quite well (see especially the
last two images on the page):
<http://scanline.ca/overscan/>
One more thing: interlacing may make certain kinds of picture content
flicker quite badly on a domestic tv set. This usually has to do with
thin horizontal details, and horizontal details in general. For example,
an image of venetian blinds, such as this one
<http://www.cintacor.es/CATA/VENE/BVENE1.JPG>
may flicker quite much. Reasons for this can be found from this article:
If needed, this can be fixed by mildly low-pass filtering (or blurring)
the image in the vertical direction, making for a smoother transition
between the odd and even fields. For example, you could use the Motion
Blur filter in Photoshop, with direction set to vertical and the amount
to 2 pixels.
> (btw I found a program called Irfanview that has a batch
> resample function which is very quick and seems to give good
> results).
Irfanview is a great tool, but you can also do these type of batch jobs
with the ImageMagick command-line tools, which I suggested in the other
message.
> 2)I'm trying to figure out how to generate (30 frames * desired
> picture length) per each picture with AviSynth so I don't have
> to worry about whether or not a slidehow app does an internal
> conversion.
Are you sure you want to make the slideshow a video at all? DVDs can
also contain MPEG still images [1], and slideshows can be constructed by
timing the display of these pictures (by means of DVD scripting, instead
of making an actual movie out of it.) There can also be an audio track
on the background.
Then again, if you want transitions and other visual effects in your
slideshow, you _must_ make it into a movie of sorts.
> Or does the square/nonsquare logic come in the authoring
> process and not in the conversion to mpeg2 process?
Once you have encoded it into a DVD-compliant MPEG-2 file (with the
aspect ratio you want), it will stay that way through the authoring
process.
(Exception: some authoring programs - especially the lower-end
all-in-one style offerings - may try to second-guess you, and may want
to re-encode the MPEG-2 files with their built-in MPEG-2 encoder if they
detect there is something wrong with the file you originally offered
[i.e. if the authoring program thinks the file you offered is _not_ DVD
compliant, for some reason].)
> Guess it would be easier if the typical $50 slideshow apps would
> given an option about square/non-square. But I'm not sure if they
> do, which is why I'm going thru this exercise. At the least I'm
> learning something (I think)
I think practically _all_ of the DVD slideshow apps - at least those
aimed for normal home users and hobbyists - will automatically do the
square-to-non-square conversion for you.
(Hideously expensive and complicated professional DVD authoring
applications may be another story.)
> Windows Movie Maker has profiles you use when you save in their
> .wmv format; those formats have a 'use non-square pixels' option.
> I'm guessing that if I give it a 720x480 made as desribed above
> & use a profile with that option checked, then the app shouldn't
> be doing any internal conversions.
Sounds plausible, but I have not really used Windows Movie Maker, so I
can't confirm that. (You probably wouldn't want to save intermediate
files to .wmv format, though.)
> If I use an app that does do an internal square to non-square
> conversion then I would just crop to a 4:3 ratio (and possibly
> resize myself to 640x480) and feed that to the slideshow
> application.
>
> Is all that correct?
Yup, though I don't think you would even need to crop to any specific
ratio, much less resize to 640×480. Most of these simple slideshow apps
are designed so that they treat everything thrown at them "square
pixels" and resize and add borders accordingly. If you have an
odd-shaped image, they will probably let you decide whether you want it
cropped or if you would like to have borders added around it instead.
> That paper of yours is incredibly informative, although it took 2
> tylenols after trying to digest it the first time <g>
Ha! 
I've been planning to update and rewrite that paper for some time
now. (For example, it doesn't have anything about the 16:9 aspect ratio,
or HDTV resolutions, and I think it could also benefit from a couple of
explanatory illustrations.) Haven't really had the time for that yet.
> Also, what are some of those professional apps you mentioned that
> allow you to work in native format?
Well, at least <http://www.sonic.com/products/scenarist/default.asp>. As
far as DVD authoring goes, you can't get much more professional (and
expensive.)
_____
[1] Yes, there are still-image MPEGs, too, although the photo DVD and
slideshow authoring apps usually handle the conversions from JPEG etc.
transparently for you (so you do not necessarily need to know that MPEG
stills exist, or handle the stills directly in MPEG format at any point,
because the authoring app will take care of all that for you behind the
scenes.)
This is what the DVD FAQ (http://www.dvddemystified.com/dvdfaq.html) has
to say on the subject: "Still frames (encoded as MPEG I-frames) are
supported and can be displayed for a specific amount of time or
indefinitely. These are used for menus or slideshows. Still frames can
be accompanied by audio."
--
znark
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