Clarification of FAQ Q64 (overscan)?

[Matthew]

"1) For anamorphic (16:9) PAL DVD source change your AviSynth script to look like this:
...
Crop(24,0,672,576) <= insert this line
SimpleResize(448,432) <= this line depends on the resize method used
AddBorders(16,72,16,72) <= add this line if not present"

I'm confused, because the overscan area is assumed to be 16 pixels each, correct? But unless I'm mistaken this avisynth script is cropping 24 from the left and 24 from the right.

Now I know I'm mistaken, I would just like to know why ;)

I did use the search feature, etc and read a bunch of posts, so call me dense but I just couldn't figure it out.

And while I'm here I'll plug my earlier 0 reply thread
http://forum.doom9.org/showthread.php?s=&threadid=29612
:D

[Wilbert]

I'm confused, because the overscan area is assumed to be 16 pixels each, correct?

Yes. The overscan area is different for each TV, usually a little bit more than 16.

But unless I'm mistaken this avisynth script is cropping 24 from the left and 24 from the right.

Yes. You can figure that out with FitCD. But this crop has nothing to do with the overscan area (just for maintaining the correct aspect ratio). In the last line (addborders ...) black borders are added to end up with 480x576. So on the left and right there are two borders of 16 which take the overscan area into account.

As for your other question. Also here you must crop to maintain the correct aspect ratio. If you don't want to crop you have to resize to 448x400.

[gerti67]

Hi Matthew,

as Wilbert already explained, the cropping of 24 "pixels" from the left and the right of the source has nothing to do with the TV overscan area. Adding a 16 "pixel" border around the rezised material is the step that saves you some bitrate for important parts.

The source width is 720 "pixels" and has to be resized to 480 for SVCD. So the conversion factor is 720 / 480 = 1.5. With this calcuation in mind we know that a border of 16 "pixels" after the resize must be equal to a border of 24 (16 * 1.5 = 24) "pixels" with the source material. That's the whole secret. ;)

And yes, a border of only 7 "pixels" will need some bitrate, as the macroblocks are not totally black and even totally black macroblocks need a little "bandwith" but not as much as if there's some picture information in it.

Hope this helps,
Gerti

[Matthew]

Thanks for the replies. After reading Wilbert's reply last night I was still confused and then I understood in bed at 5:51 this morning ;) i.e. 16 pixels at SVCD resolution = 24 pixels at DVD resolution. And as the cropping occurs before resize...

In regards to the 7 pixels, I knew it needed some bitrate, it's just that apparently with divx, a black border of X pixels which is not divisible by 16 requires more bitrate than a black border of Y pixels that is divisible by 16, where Y>X.

Another question, if I may. When (if) I get a widescreen television will overscan still be an issue? I'll be watching these SVCDs in 4:3 mode, but will the image protrude into the "side bar" area or will this overscan area (16 pixels) simply not be displayed at all. If it's the latter then I might as well do this for all films.

Thanks again ;)