Anamorphic NTSC displayed at PAL resolution?

[makkia]

It's not actually a big problem, rather a puzzling behaviour.

A 4:3 NTSC mpeg-2 stream plays at 720x540 resolution on VLC.
The archived resolution is 720x480 and AFAIK on U.S. TV sets it's supposed to be displayed anamorphically at 640x480.
I know that 720x540 is as perfect 4:3 as 640x480 is, and that LED displays can use only square pixels. What puzzles me is:
As I understand it, in 4:3 NTSC pixels are stored in such a ratio that you keep the vertical resolution and - huh - "compress" them horizontally to fit 640.
VLC instead apparently keeps the horizontal 720 and stretches vertically to match 4:3 at 540 (resulting in an "almost PAL" resolution).
Which confuses me: those 720 pixels have the "wrong" (non square, slanted) aspect. So in order to display correctly VLC must first compress horizontally (to 640x480 square pixels) and then upscale both axis to 720x540.

Why is this? And can this behaviour impact on image clarity?

I'm asking because MPEGstreamclip plays that same MPEG at 640x480 and comparing png captures from both players shows a slight difference: VLC's snaps exibit more grain and fuzzier details. Not a big difference, but detectable.

BTW This happens on VLC 2.1.3. I tried the same movie in VLC 0.9.10 and it plays @720x540 but snapshots are saved @640x480 (?!?)


I don't know if this behaviour is OS related, but just for the record:
MacOS 10.6, Language italian (PAL country)

[Rémi Denis-Courmont]

You can change the pixel aspect ratio in VLC preferences (monitor-par) if you don't want 1:1.

[makkia]

Thanks for the reply, but I didn't mean "I don't want anamorphic rendering".
I must have worded my question in a very confused way (apologies for my non native english BTW).

Trying again:

I would just expect the "normal size" item in "video" menu to display the intended resolution.
Which is VLC's behaviour with any other video content, including PAL anamorphic dvd's (that get correctly displayed @576p).
Only NTSC anamorphic material is arbitrarily upscaled from the intended 480p to 540p.
It's down to earth behavioural consistency I'm talking about.

Looking at a movie without resizing/upscale helps in evaluating quality and defects.

AFAIK if VLC "decides" that its default size may not be the real one, there's not a preference that delivers a complex instruction such as: "display NTSC StdDef mpeg-1/2 streams @480p, including the tag-induced deinterlacing and anamorphic compensation, but leave any other thing at their default size".


Hence the question: why is that?
A coders' decision?
A bug?
A needed workaround for some obscure defect?
A known problem?

[Rémi Denis-Courmont]

If the pixel aspect ratio of the video and that of the monitor do not match, you cannot get the correct size. Period.

It might be that the current VLC code ends up matching the number of column rather than the number of lines. Frankly, whichever is the result, I would not consider it a bug. If you want a specific height, set a specific height.

[makkia]

If the pixel aspect ratio of the video and that of the monitor do not match, you cannot get the correct size. Period.

But you still have the option of following the standards in how to compensate the differences. Or, you can pay no mind to standards.
And that's not a "Period", it's a meanigful choice. Impacting - one out of many things - custom subtitles appearance.

It might be that the current VLC code ends up matching the number of column rather than the number of lines.

Actually, it does neither:
NTSC "columns" are 854 (square) pixs [16:9] and 640 [4:3]. VLC does 960 [16:9] and 720 [4:3].
NTSC lines are 480. VLC does 540.

The only rationale I can fathom for pulling 540p out of the air is that it makes half FullHD (and yet, PAL's 576p is not... so it's still a tad fuzzy a logic).

Frankly, whichever is the result, I would not consider it a bug.

Got it. And this one looks more like a "Period".

If you want a specific height, set a specific height.

Or change player for serious work. More straightforward.


Thanks for your attention.