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  1. I've been searching for an answer on this one for quite some time. I have fold my question in some other web bulletin boards, waiting for answers. I have also gotten some. However, it still feels somewhat like in a confused maze, so I shall try dropping the question here as well, sort of like a carpet of bombs (to get as many answers as possible).

    So, question is simple. I have 16:9 PAL DVD material. I wish to transcode it into square pixel MPEG-4. I am a purist, so I want to ensure that natural aspect ratio is preserved. How shall I proceed? When I DirectShowSource("random.VOB").AutoCrop() my cropped output result becomes 720x554. When I read IMDB, it says the intended AR of this particular movie is 1.85. So..

    720/554*64/45=1,8483754512635379061371841155235

    (versus)

    720/554*16/11=1,8903839842468001312766655726945


    The first calculation returned ~1.85, so it felt kind of satisfying. However, according to several people, ITU-standards make dvd-players begin with cropping 16 horizontal pixels, and then apply 16:11 on the remaining 704x576. Overscan-isch.

    In fact, 704/554*16/11 gives me the same nice numbers as my first calculation. Only thing is, I know for a fact that I have just cropped 16px worth of actual movie, so by all logic means, the final aspect ratio should no longer be 1.85, as stated on IMDB.

    Does any of you guys follow my thoughts on this?

    Someone suggested that in case of no padding to the left & right (i.e. all 720 pixel samples contain material), it's safe to presume that the DVD author has already slightly cropped the source from its top slash bottom, to sort of "make up for" the unavoidable loss of 16px during playback on any DVD player. Could anyone here verify this? Would be sort of a nice clarification.

    Bottom line question.
    Is it so that PAL DVD players does have one SAR flag and one SAR flag only, which is 16:11. Meaning I should totally forget about 64:45, since it will only generate slight AR errors..?

    If this is the case, I would (for example) disregard from the fact that IMDB says movie AR is 1.85, and since MPEG-4 playback doesn't have overscan issues, I would not even have to crop down to 704, but simply (from my example far above) run 720/554*16/11=1,8903839842468001312766655726945, and interpolate/resize this into some mod16-ok resolution.

    Does this look okay to you?

    Any points of view are very much appreciated.
    I'm very grateful for your help!
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  2. Member Alex_ander's Avatar
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    Originally Posted by Gew View Post
    So, question is simple. I have 16:9 PAL DVD material. I wish to transcode it into square pixel MPEG-4. I am a purist, so I want to ensure that natural aspect ratio is preserved. How shall I proceed?
    There is just one mandatory flag of AR (4:3 or 16:9) in addition to presence of known resolution numbers. MPEG2 standard (H.262) also mentions sequence_display_extension parameter, which may be present or not present. When it is not, proportions are defined by the mentioned numbers (720x576 must be stretched to full screen with specified DAR). When it is present (in form of sample numbers corresponding to 4:3 area on display, like ITU's 702x575 for PAL), proportions must be re-calculated by mpeg2 decoder.
    So if your DVD files don't show sequence_display_extension in GSpot (the display box for it is black), you can assume full 720x576 corresponding to 16:9. Simply crop to visible and stretch horizontally in 1024/720 proportion. In case the manufacturer did it correctly, you'll get proper result. In case you know the DVD is only shown correctly by ITU, you can adjust the horizontal size additionally by factor of 720/704.
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  3. Originally Posted by Alex_ander View Post
    Originally Posted by Gew View Post
    So, question is simple. I have 16:9 PAL DVD material. I wish to transcode it into square pixel MPEG-4. I am a purist, so I want to ensure that natural aspect ratio is preserved. How shall I proceed?
    There is just one mandatory flag of AR (4:3 or 16:9) in addition to presence of known resolution numbers. MPEG2 standard (H.262) also mentions sequence_display_extension parameter, which may be present or not present. When it is not, proportions are defined by the mentioned numbers (720x576 must be stretched to full screen with specified DAR). When it is present (in form of sample numbers corresponding to 4:3 area on display, like ITU's 702x575 for PAL), proportions must be re-calculated by mpeg2 decoder.
    So if your DVD files don't show sequence_display_extension in GSpot (the display box for it is black), you can assume full 720x576 corresponding to 16:9. Simply crop to visible and stretch horizontally in 1024/720 proportion. In case the manufacturer did it correctly, you'll get proper result. In case you know the DVD is only shown correctly by ITU, you can adjust the horizontal size additionally by factor of 720/704.
    Thanks for your quick response alex_ander. Much appreciated!
    So. I just loaded one of the main VOB's in GSpot.
    This is a snapshot of what I get.

    Click image for larger version

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    The "seq ext" field is blank, as you see. Also, the movie stretches over all 720 horizontal pixel samples, i.e. no slight padding far left/right. Does this mean that I should apply 64:45 to get natural AR? (i.e. 720*64/45=1024).
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  4. Member Alex_ander's Avatar
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    Yes, and you can get the same result without 64/45: 576x16/9=1024. That's where all those numbers in tables come from .
    Or if you want to keep width 720, you can you can adjust the height instead (full or cropped to visible), in proportion x720/1024.
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  5. Ok, cool!

    When speaking of, I just ran across a strange piece of DVD. It's Platoon, the 1986 war flick. When I launch the VOB file in Media Player Classic, it is shown with DAR 16:9, some top/bottom padding present. According to IMDB, Platoon is in AR 1.85. When I DirectShowSource() the .VOB, and furthermore AutoCrop() it's content, I get an output of 716x556. Furthermore, 1,85*556=1028,6; i.e close to a perfect ~1.85 (1024) match.

    However, when I load the VOB in GSpot, I see a value of "540x576" in that "seq ext" field you mentioned earlier. What's up with that? Is this ignored somehow? How would it look? It means the DAR of the whole thing should be squeezed down to 540x576??

    Also, even weirder. I run MediaInfo on the very same VOB file. I notice "Display aspect ratio: 2.35:1". What's up with that?? The movie is only supposed to be 1.85:1. Really weird. Do you have some kind of logical explanation? Would come in handy, really.

    --EDIT--
    I think I ran across some article that taught me about what that "seq ext" field is really about. Stated as follows:

    You can add Pan and Scan information to MPEG streams during encoding or even to an existing stream. This information instructs a decoder what subsection or supersection of each MPEG decoded frame to display. Pan and Scan information is added to the stream as two parts. The MPEG Sequence Extension should contain the horizontal and vertical dimensions of Pan and Scan decoder for this sequence. These dimensions can be smaller or larger than the source frame size. In addition, each decoded frame can include a pan and scan offset for that frame. The offset is presented in terms of horizontal and vertical pixel displacement from the center of the frame. A 0 horizontal and vertical offset for example would instruct the decoder to use the center of the decoded frame as the center of the Pan and Scan image. The offsets can be updated each frame and are stored in the MPEG Picture Coding Extensions.
    It was actually kind of logical. I fiddled with my dear calc.exe, and found that 540/576*64/45=1,333... (4:3), which is supposedly the desired Pan&Scan resolution. So, I guess we've got that one clear. However, the messy AR value of 2.35 reported by MediaInfo is still a mystery.
    Last edited by Gew; 28th Jun 2010 at 13:48. Reason: got some facts straight
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  6. Member Alex_ander's Avatar
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    Yes, 540x576 are pan&scan numbers, there was a thread last week with exactly the same question. Again, you don't need 64/45 numbers. The 540 comes directly from relationship between 4:3 and 16:9 images. Independently of pixel type, 4:3 is obtained from 16:9 by cropping exactly 1/4 of total width, so with 720 the remaining width is 720x3/4=540.
    "Display aspect ratio: 2.35:1" is really an unexpected thing to see (some VOB metadata?) since mpeg2 has just one more additional AR value 2.21:1 (not used for DVDs). Although there's enough space (4 bits for mpe2 AR setting) to specify one more value, H.262 says other bits must not be interpreted.
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