Aliasing when capturing VHS

[tugatomsk]

In a PAL ITU cap only a 702x576 portion of the frame corresponds to the 4:3 image. A little extra is captured (a total of 720 horizontally) in case the cap or source is off center. So you need to crop to 702 (704 is often considered close enough) before resizing to 768x576. Or you can resize to 788x576, then crop away the black borders. For an NTSC cap the active 4:3 picture is in a ~704x480 portion of the 720x480 frame.

Thank you for that remarkable insight! I'll try it tomorrow, as now it almost 3 am where I live.

[Cornucopia]

Also, a crop nearly always changes the Aspect Ratio*. If before a subsequent resize, it's changing the PAR*. If after a resize, it's changing the DAR.

Follow the examples as suggested (to 768x576), and then if you feel you need to crop further - DON'T!
Instead, mask sections that are bad.

If you truly cannot avoid additional cropping, it is best to do only 1 pass of cropping & 1 pass of resizing, and you should calculate your intended result ahead of time so the AR is automatically correct at the end.

The formula:

 H rez / V rez * PAR = DAR

Is your friend.
And when doing resizes, just remember you can do 2 instances of this formula (for source & result), match the parameters that match, combine them by equating, and plug things in to solve for the missing element.


Scott

*exception if the subsequent resize is doing equal ratios of both H & V.

[tugatomsk]

Also, a crop nearly always changes the Aspect Ratio*. If before a subsequent resize, it's changing the PAR*. If after a resize, it's changing the DAR.

Follow the examples as suggested (to 768x576), and then if you feel you need to crop further - DON'T!
Instead, mask sections that are bad.

If you truly cannot avoid additional cropping, it is best to do only 1 pass of cropping & 1 pass of resizing, and you should calculate your intended result ahead of time so the AR is automatically correct at the end.

The formula:

 H rez / V rez * PAR = DAR

Is your friend.
And when doing resizes, just remember you can do 2 instances of this formula (for source & result), match the parameters that match, combine them by equating, and plug things in to solve for the missing element.


Scott

*exception if the subsequent resize is doing equal ratios of both H & V.

Doesn't that confict with jagabo's suggestion of making a larger resize (788x576) due to the nature of the original video? Which one is the most correct?


What if I used AddBorders to the final cropped resolution to create a full-proof 1x1 PAR 4:3 PAL video?

original 720x576 resize to 788x576 -> crop to 756x564 -> AddBorders for 768x576

Although this would only be a source of concern when playing videos on older LCD. Using VLC player I now understand that I can playback a video with its original resolution with PAR 1x1, no matter how strange it is.

[jagabo]

There's no need to worry about the final dimensions unless you are creating video for a system that requires a specific frame size. You might as well leave your example video at 756x564.