[MEncoder-users] How to produce VCD\DVD content from a raw AVI video ?

[Ross Clement]

I see. That explains some general weirdness that I blamed on my DVD
player. I'll look further into dsize and scale filters. And revise the
mencoder documentation page given before.

On 6/30/08, Nicolas George <nicolas.george at normalesup.org> wrote:
> Hi.
>
> Le tridi 13 messidor, an CCXVI, Ross Clement a écrit :
>> One thing I found confusing about that page was that it only really
>> mentions
>> the expand filter in passing. The complete examples of creating DVDs down
>> the bottom only use scale. This means that if your original videos are not
>> in the correct (e.g. for PAL) ratio 720x576, then the picture will get
>> pulled out of shape. Nowdays when I'm using video that's not already in
>> the
>> 720x576 *ratio*, I use a combination of scale and expand. For example, if
>> I
>> want videos from my "still" camera (640x480) to become DVD sized, I first
>> scale to 720x480 and then expand to 720x576, effectively making the video
>> slightly widescreen-ish. Though, as my DVD player doesn't show all of the
>> 720x576 screen anyhow, I sometimes scale less (or not at all), and expand
>> more. This is only for "home videos", and hence "perfection" is not
>> required.
>
> On the general principle, you are right: people should be aware of the need
> for the expand filter to adjust the aspect ratio.
>
> But in the particular case you are taking as example, you are totally wrong.
>
> You work under the false assumption that de ratio between the width and the
> height of the video in pixel is the same as the ratio between the width and
> the height of the video in physical units when it is (correctly) played.
> That is not true.
>
> That assumption is mostly true for nowadays screens and resolutions: they
> have square pixels, although it was not always the case (some fifteen years
> ago, on 4/3 screens, 320×200, 640×400, 640×350 and 640×200 were quite
> common). The same is probably true also for your camera.
>
> But most video containers have a field to specify the desired display aspect
> ratio. If this aspect ratio does not match the actual pixel aspect ratio of
> the screen, players are supposed to prescale the video to adjust for it. For
> example, when you play a DVD with mplayer, you can read:
>
> Movie-Aspect is 1.33:1 - prescaling to correct movie aspect.
> [swscaler @ 0xba2230]720x576 -> 768x576
>
> A lot of players are bogus, and do not honor this field; thus, a lot of
> video randomly lying around are set up for square pixels, so that they will
> display correctly in most cases.
>
> In the particular case of a PAL DVD, the resolution must be 720×576
> (although a few other lower resolutions are possible, almost nobody uses
> them), but the display aspect ratio must be either 4/3 or 16/9.
>
> In your particular case, your camera uses the 4/3 aspect ratio, so there is
> no need for expand. In fact, if you expand as you said, the resulting image
> will be horizontally stretched and vertically squished in a 16/15 ratio.
> This is almost unnoticeable, but this is not correct.
>
> The arithmetic to do everything properly is quite confusing. I think it is
> possible to let mplayer do it by using the correct combination of dsize and
> scale filters, but I did not manage to do it yet.
>
> Regards,
>
> --
>   Nicolas George
>