[MPlayer-users] MPlayer-1.0pre8 detecting wrong aspect ratio

[Nico Sabbi]

Giacomo Comes wrote:

>On Mon, Jul 31, 2006 at 10:31:18PM +0200, Nico Sabbi wrote:
>  
>
>>>      
>>>
>>try to use -vf scale=100:200 -lavcopts vcodec=mpeg1video:aspect=4/3 and 
>>see what happens :)
>>    
>>
>
>Let's try to clarify the situation.
>I understand that you are now using in order to determine
>the aspect ratio the pel aspect ratio of the mpeg1 standard:
>3   0.7031   16/9 PAL
>6   0.8437   16/9 NTSC
>8   0.9375   4/3  PAL
>12  1.1250   4/3  NTSC
>
>and you determine the aspect ratio using the exact coefficent
>present in the second column.
>But it is the proper way to do it?
>  
>

lavc is using the right method. Unfortunately mpeg1 (that uses PAR 
instead of DAR)
can't specify a DAR corresponding to a PAR > 1.2+ something, so 100x200 
@ 4/3
can't be expressed. My fault, I should have remembered

>Let's consider an example.
>
>PAL VCD is encoded at 352x288.
>When you watch the VCD on a (4/3) TV, the video is expanded to fit
>the displayed aspect ratio: 384x288.
>This is how mpeg2 is handled.
>For mpeg1 now mplayer expand the video to (352/0.9375=376) 376x288.
>The aspect ratio is not anymore 4/3 but 13/10.
>It means that if I make an mpeg stream and I connect my computer to a
>TV and I use mplayer to watch the movie, the result will be a little
>bit different if I encode to 352x288 mpeg1 or 352x288 mpeg2
>because mplayer will play the mpeg1 movie with 1.30 A/R and the mpeg2 movie
>with 1.33 A/R.
>In my opinion this should be considered wrong.
>
>PAL  352x288 mpeg1 (aspect 8)  should be played as 384x288 (Movie-Aspect 1.33:1)
>                                            not as 376x288 (Movie-Aspect 1.30:1)
>NTSC 352x240 mpeg1 (aspect 12) should be played as 352x264 (Movie-Aspect 1.33:1)
>                                            not as 352x270 (Movie-Aspect 1.30:1)
>PAL  352x288 mpeg1 (aspect 3)  should be played as 512x288 (Movie-Aspect 1.78:1)
>                                            not as 500x288 (Movie-Aspect 1.74:1)
>NTSC 352x240 mpeg1 (aspect 6)  should be played as 426x240 (Movie-Aspect 1.78:1)
>                                            not as 418x240 (Movie-Aspect 1.74:1)
>
>I know it's a bit confusing but I hope I was able to explain what I meant.
>
>Giacomo
>  
>

while mpeg2 specifies a plain DAR (that is, display width / display 
height, irrespective
of the picture resolution)  mpeg1 specifies the aspect code as  1/PAR 
(decode h/ decode w),
so  



+    float par[] = {
+                   0, 1, 0.6735, 0.7031, 0.7615, 0.8055, 0.8437, 0.8935,
+                  0.9375, 0.9815, 1.0255, 1.0695, 1.1250, 1.1575, 1.2015
+    };
+
+    if(picture.mpeg1) {
+        if(picture.aspect_ratio_information > 0 && 
picture.aspect_ratio_information < 15)
+           *aspect = picture.display_picture_width / 
(par[picture.aspect_ratio_information] * picture.display_picture_height);
+       else
+           *aspect = 0.0;
+    } else // MPEG2



thus if aspect_info = 8,  *asepct =  352 / (0.9375*288) = 1.3037...

I understand that having fixed the code spread out a can full of 
previously hidden worms,
but since bugs had better be fixed than hidden I still maintain that I 
did the right thing.

Of course, as I wrote yesterday, since correct playback is more 
important than the formal
correctness of the code, I'm favorable to the introduction of an option 
to restore
the old behaviour.
   
       Nico