[MEncoder-users] frame size confusion

[Mark Himsley]

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 
video files.

Your 'widescreen' will be 16:9 with anamorphic pixels, i.e. the pixels are 
not square.

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 [1].

> 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 [2]. 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 [3].

Here's where I insert a rant about loosing data because the number of lines 
has been reduces instead of the width being increased.

HTH


[1] it's slightly more complex but that's close enough
[2] I get really annoyed at Quicktime that completely lies about movie 
sizes!
[3] 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.


-- 
Mark Himsley