Careful with that --aspect
I've talked to several people on IRC recently about
getting the proper aspect ratio when encoding theora video, and wanted
to summarize here.
The short answer is, when you're encoding from DV (or
most other formats) with ffmpeg2theora
don't use the --aspect switch. The program
calculate the correct aspect ratio based on the source and
whatever target resolution you give it, either through a
profile or explicit -x -y switches.
The --aspect switch is for overriding the default
calcuation, usually because the input source video is
incorrectly marked. But video aspect ratios are
confusing, and it's easy to mess up. For example, if you
take a DV source video and encode with -x 320 -y 240
--aspect 4:3, you will get a video that says its
pixels are square, but in fact they are not, so playback
will be distorted. Without the --aspect switch,
ffmpeg2theora will mark the file with the correct (non-4:3)
Huh? 320x240 is 4:3! Yes. DV, whose native resolution is
720x480 (NTSCish) or 720x576 (PALish) contains some overscan
area, and so the full frame is not a 4:3 image. It roughly
contains one, and the theory is the extra bits get masked
off by the edges of your CRT. Computers have nice, square
pixels, so everything is much easier, but digital video
imports a lot of the complexity of the analog technology it
developed from, and had to interoperate with.
Since the full DV frame isn't actually 4:3--or 16:9 if
you're shooting in wide screen--you have to crop if you want
to make an actual, standard ratio, square pixel file.
ffmpeg2theora -x 320 -y 240 --cropleft 8
--cropright 8 -o output.ogg input.dv
will give you square pixels with a 4:3 frame aspect ratio.
For 16:9, use something like:
ffmpeg2theora -x 640 -y 360 --cropleft 8
--cropright 8 -o widescreen.ogg widescreen.dv
The same thing goes for the --deinterlace flag. It forces
using the deinterlace filter regardless of whether the input
is marked as interlaced or not, and so can degrade quality
on progressive material. DV is a very sane format and all of
these things are reliably marked. In general, trust the