Cropping DV

[bsuska]

I can't seem to find what would seem to be a pretty common issue. I've got DV video (720 x 480 - .9 PAR) that needs to be resized to 320 x 240. I'd also like to crop a few of the edges. Since my encoder will be translating the .9 pixels into square pixels what is the proper way to enter cropping dimensions that will not distort the image? From what I understand a 640 x 480 square pixel movie is the only true representation of the 720 x 480 signal. No resizing here of the cropped video of course. I've heard that one should use the same aspect ratio as the source footage. 720/480 = 1.5 Does that mean I would add the pixels I'm subracting from the left and right and divide them into the dimension from doing the same for the top and bottom? Trying to arrive at the 1.5 aspect ratio?

Thanks in advance...

-Brock

[edDV]

This gets complicated if you want precision aspect ratio.

First, it depends on your source. Back to basics...

CCIR-601 (now ITU Rec-601) defined 4:3 as 704x480 and this is still the basis for all digital broadcasting. 8 pixels were added to each side to protect the image from horizontal shifts through the transmission process.

704 divided by 2 gets you 352x480 or half horizontal sampling. This gets padded out for display so it is still 4:3 aspect. Rec-601 provides for alternate 16:9 aspect at 704x480 but 352x480 (and lower) is only defined as 4:3 aspect.

Quarter resolution (still 4:3 aspect) is 352x240. 320x240 is quarter resolution 640x480 (square pixel) they are in sort of parallel universes.

I could go on about how the DVD standard and DV camcorders spoiled this perfect world by moving to full width 720x480 with slightly different PAR ...


PS: Equivalent PAL resolutions are 704x576, 352x576 and 352x288 (aka: CIF)