[MPlayer-users] Cropping in original-size vs reduced-size

[Jorge Fábregas]

Hello guys,

I would like to downscale some NTSC clip from a DVD to 640x480. I like this 
resolution since it's 4:3 and multiples of 16.  However, I need to crop 16 
pixels on each side.  I plan to:

1- downscale to 640x480.  At this point the video is 4:3 and already 
vertically compensated (ready for square pixels).

2- now I'll crop 32 pixels horizontally (16 in each side) and I end up with 
608x480. It's not really 4:3 but it doesn't matter as the video was already 
vertically compensated in previous step.

Now, my problem is... How do I go from step #1 to step #2 in just one 
encoding?  It's not the same thing to crop in full size than to crop on the 
downscaled version. How can I know beforehand that what I want actually is 
608x480?

I'm assuming that we should downscale to 4:3 considering the WHOLE frame (in 
this case 720x480) including the black bars. If I crop in original size, and 
I just leave THE ACTUAL CONTENT, the aspect ratio of that content may not be 
4:3 right?  The quesition is: Should you do the 4:3 conversion considering 
the whole frame or JUST the actual content (geting it by cropping)?

Please help me understand this. 

Thanks for your comments.

Best regards,
Jorge