[MEncoder-users] frame size confusion
mark at mdsh.com
Sat Dec 1 14:01:59 CET 2007
On 1 December 2007 12:18:26 +0900 Dave M G wrote:
> Here's where the confusion is. I thought "wide screen" was 720x480, and
> "regular" was "640x480". When I transfer files over to my computer,
> right click on them (I'm using Ubuntu 7.10 with Gnome Desktop) and look
> at their properties, that's exactly what they say.
It's probably to do with pixel aspect ratios.
You are in the USA, right? I guess so because of the 480 lines in the video
sizes you're quoting.
So 'regular' means 4:3. 480 lines * 4 / 3 = 640. So this file is square
pixels. Square pixels in not standard in TV but very standard in computer
Your 'widescreen' will be 16:9 with anamorphic pixels, i.e. the pixels are
I live in the UK so I'm know the figures for PAL TV, but in NTSC land
things are similar.
720x480 is the broadcast standard for widescreen NTSC TV. But you can see
that 480 * 16 / 9 does not equal 720 (1013 pixels is close) so the picture
is squeezed horizontally until it fits into 720 pixels .
> So I took a "wide screen" video, rendered a screen shot, then opened it
> in GIMP and the frame size was 720x405 pixels. Weird.
The software you're using to tell you the size of the image will know the
pixels are squashed and is compensating for this . It is keeping the
width and squeezing the height down by the same amount such that the image
will look correct (the picturs will not look tall and thin).
The maths for this is 720 / 16 * 9 = 405 .
Here's where I insert a rant about loosing data because the number of lines
has been reduces instead of the width being increased.
 it's slightly more complex but that's close enough
 I get really annoyed at Quicktime that completely lies about movie
 this calculation is wrong in PAL because the picture does not fill the
720 pixels but many people do not understand this!!! I cannot remember if
this is also an issue in NTSC.
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