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1952x1080 not 1920x1080

Discussion in 'Desktop & Laptop Computers Forum' started by cybersoga, Mar 7, 2004.

  1. cybersoga

    cybersoga
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    Did you know that if you resize 720x576 anamorphic dvd image to 1080p on a square pixel display you should use 1952x1080? If you resize to 1920x1080 without first resizing to 1952x1080 and cropping 16 pixels off each side, the image is squashed horizontally (faces look thin) - try it with the 16:9 aspect ratio circle test of THX or Digital Video Essensials. Another way of looking at it is to crop 8 pixels from each side of the 720x576 image then stretch it out to 1024x576 so it's the right aspect ratio then resize to 1920x1080.

    To get from a "4:3" 720x576 screen shot to the correct aspect ratio, you have to crop 8 pixels from each side (704x576), then resize it to 768x576. Another way of doing it is to resize the image to 784x576 then crop it after.

    It means that power dvd doesn't display the image at quite the right aspect ratio on square pixel displays because it doesn't crop pixels off the sides or compensate for it by stretching it out a bit more. If you use an 720x576 anamorphic overscan test pattern on a 16:9 (1.78:1) screen, the display should crop more from the sides than from the top/bottom to maintain aspect ratio. I wonder how many displays are getting this wrong, especially with digital connections...

    http://www.uwasa.fi/~f76998/video/conversion/#faq

    apologies for the edits i'm just getting this right in my head :) comments?
     
  2. Mr.D

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    PAL D1 16x9 resolution 720x576 = 1024x576 square pixel 1.7777778 ratio
    1920x1080 square pixel= aspect ratio 1.7777778

    720x576 needs to be displayed at 768x576 to make it square pixel.
    This is inclusive of the blanking.
    croping it to 704x576 then resizing it to 768x576 will give you an image thats too wide in the x-axis. You would need to resize the image to 751x576 ( nearest pixel)

    You have to include the blanking if you don't you would have to crop into the image slightly top and bottom to get to the correct ratio on square pixel display.(13 approx)

    Personally I'd either leave the blanking alone and push in slightly on the display in single pixel increments or setup a custom aspect ration in TT. What makes the latter even more likely is that quite a few movies have additional blanking on them anyway beyond the 8 pixels either side. Dealing with similar issues on NTSC material also favours the latter method. The important thing is to get the material onto the display correctly as square pixel ( includng devices with non-square pixels , think about what I'm saying). Then its just a question of straight resize and/or crop if you want to hide any additional blanking/padding)

    Get it all square pixel blanking and all , then decide what you want to do to it.
     
  3. cybersoga

    cybersoga
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    All the tests I have done indicate that i'm right, i've measured the circles to make sure they are round with a pice of paper and pen, and they are not round if you go from 720x576 to 768x576 without cropping the sides first, here are some screen shots...

    Resized from 720x576 to 768x576, the circles are not round:-
    [​IMG]

    Resized from 720x576 to 784x576, circles are round:-
    [​IMG]

    It's the same story with going from anamorphic widescreen 720x576 to 1042/1024x576 or 1952/1920x1080 (the circles are only round with 1042x576 or 1952x1080)

    What I have done is adjusted my displays so I can see as much as I can of the picture but the circles are still round and the cross is in the center (I don't watch DVD's on the PC only analyise them!).
     
  4. MikeTV

    MikeTV
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    ... and just when you thought you knew everything there was to know about aspect ratios!

    Interesting information. Thanks.
     
  5. Mr.D

    Mr.D
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  6. cybersoga

    cybersoga
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    Certainly seems to confirm my findings...

    this is the bit I like from that site you just found talking about SMPTE RP 187

    "They result in a non-integral numbers of luma and chroma sampling instants per line, which makes hardware people jump out of high windows"

    There are probably some DVD's which follow SMPTE RP 187 (most?) and some which do not... nightmare!
     
  7. Mr.D

    Mr.D
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    Everyone I've spoken to on this is adamant that the 768 unsquish is correct: but that doesn't necessarilyprove anything.... still trying to find another opinion.
     
  8. Mr.D

    Mr.D
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    What is your display by the way?
     
  9. cybersoga

    cybersoga
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    Main one is a Sony KE-32TS2 plasma, it lets me adjust position and size of the picture from the user menu and saves settings per mode like a multiscan monitor which is quite handy. The active area of the screen measures exactly 720mm x 405mm, which works out at 1.7777777777777777777777777777778:1.
    I have it adjusted so I can see as much as I can of the picture while keeping the circles round with PAL DVE and NTSC DVE, that way I get a higher resolution image, i'm not worried about the odd dvd which doesn't go all the way to the side edges. On most DVD's, I can see small black borders top/bottom on 1.85:1 DVDs, wheras 1.78:1 dvd's fill the active area of the screen.

    Do you think that's right or should I have it adjusted so I can see an equal amount of the sides as the top/bottom on DVE test patterns? that results in non round circles like the first screen shot above. I'd be interested to hear your thoughts when you've reached a conclusion!
     
  10. Mr.D

    Mr.D
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    Well on my pannay 37pw5 I feed it 856x480. The pany panel is only 852x480 so for correct 1:1 pixel match you'd expect to lose a couple of pixels either side which is exactly how I have it set up.

    Someone had just done a test for me pulling material off D1 at 720x576 ....looking at it resized to 768x576 it looks correct ....looling at it 785x576 its too wide. I'm going to try my own testing when I get the time.
     
  11. cybersoga

    cybersoga
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    On a side note, i've just taken a screen shot of NTSC DVE 1.78:1 SMPTE RP 187 geometry pattern with Power DVD (720x480), resized to 852x480 the circles arn't quite round (the larger circle is 312x308 pixels in photoshop). If I add 3 lines of black pixels to the top and bottom of the 720x480 image making 720x486, then resize to 852x480, the large circle is a perfect 308x308 pixels.

    720x480 resized to 856x480 the circle is 310x312 pixels. 720x486 (with 3 blank lines top/bottom) resized to 856x480 the circle is 310x308 pixels.

    720x480 resized to 863x480 the circle is 313x313 pixels.
     
  12. cybersoga

    cybersoga
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    PAL DVE Anamorphic pattern:

    720x576 resized to 872x480 the circle is 314x314 pixels.
    720x576 resized to 1044x576 the circle is 376x376 pixels.
    720x576 resized to 1963x1080 the circle is 708x708 pixels.

    THX Optimode 16:9 circle test on R2 Star Wars AOTC:

    720x576 resized to 868x480 the circle is 342x342 pixels
    720x576 resized to 1041x576 the circle is 412x412 pixels
    720x576 resized to 1952x1080 the circle is 772x772 pixels

    Which is more accurate, DVE or THX? I don't know, but they are quite similar. My head hurts!
     
  13. jon2099

    jon2099
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    According to this link, the true active area for a PAL image is 702x576. As 702 is not a multiple of 16 (the macroblock size in MPEG), the next acceptable figure is 704, hence 704x576. So in fact to get the correct aspect ratio, you need to crop 9 pixels on each size. As long as you're not planning to recompress it with another MPEG compressor, this is fine (as the macroblock boundaries will no longer line up, leading to more blocking artefacts).
     
  14. cybersoga

    cybersoga
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    here is a little bit of info from Joe Kane's website, have a look at point number 3.
     
  15. cybersoga

    cybersoga
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    What I don't want to do is encorage people making dvd's and dvd players to stop at 702, as that would mean black pillar borders on some displays, it would be better to use the whole 720 and keep in mind while making the dvd that more than 702 will probably be cropped *by the display* but still maintain the right aspect ratio. Some DVD's don't go anywhere near 720, Blade Runner is the worst (or best?) example, using 655 with wobbly soft edges.
     
  16. cybersoga

    cybersoga
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    SMPTE RP 187 says that PAL 4:3 aspect ratio is 1132/1035, which is 1.0937198067632850241545893719807 (roughly, the calculator can't display the whole number).

    702 x (1132/1035)
    =

    702 x 1.0937198067632850241545893719807
    = 767.79130434782608695652173912996


    720 x 1.0937198067632850241545893719807
    = 787.4782608695652173913043478256


    SMPTE RP 187 says that PAL 16:9 aspect ratio is 4528/3105 (derived from 4:3), which is 1.4582930756843800322061191626409.

    702 x 1.4582930756843800322061191626409
    = 1023.7217391304347826086956521733

    720 x 1.4582930756843800322061191626409
    = 1049.9710144927536231884057971008


    SMPTE RP 187 says that NTSC 4:3 aspect ratio is 160/177, which is 0.90395480225988700564971751412429.

    SMPTE RP 187 says that NTSC 16:9 aspect ratio is 640/531, which is 1.2052730696798493408662900188324.

    From this I conclude that I was pretty much spot on dont you think?
     
  17. Mr.D

    Mr.D
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    http://www.uwasa.fi/~f76998/video/conversion/

    there is a good chart in here that illustrates the pixel aspect ratios for a number of systems. It certainly states that the 704x576 area is the 4x3 area ( or 16x9) exclusive of blanking as well as giving the unsqeaze to be 787 for 720 ( I think from memory) I'm just trying to verify if his facts are correct , which is proving quite difficult as all the usual sources continually trot out the 768 aim.

    We really need a definitive answer to this ( well I do anyway) . Video standards can historically be quite sloppy but pixels are pixels after all.
     
  18. cybersoga

    cybersoga
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    704 x 1.0937198067632850241545893719807
    = 769.97874396135265700483091787392
     
  19. Mr.D

    Mr.D
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    > * EBU Technical Recommendation R92-1999: Active picture area and
    > picture centring in analogue and digital 625/50 television
    > systems

    So the width is 702 not 720 ... for the 4:3 area for 625 if you go by
    BT.470 and R92-1999 but 720 acording to BT.601, I guess the majority
    wins, depending on what other people do .

    from someone else who hitherto was a 768 believer
     
  20. jon2099

    jon2099
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    Actually both are correct. See section 4.1 of the link I posted. The active number of pixels is 702, but 720 pixels are sampled to allow for signal rise and fall times (apparently). Thus each line has 720 pixels, but only 702 of them actually contain useful information.

    I suspect DVD players may crop 8 pixels on either side to make it 704 pixels and display that. After all, many of them also play VCDs, and that is only 352x288 (PAL) and doubling on both axes gives the full 704x576.

    The 768 pixels across is also correct- you just need to expand the active 702 pixels to 768. The formula cybersoga posted proves that.
     
  21. Mr.D

    Mr.D
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    I think that most dvds have 704 pixels only and they are padded to 720 by the player on output although you can encode a dvd with 720 if you so desire from stuff I've gleaned on this thread.

    I'm with cybersoga ( some people sitting next to me are still insisting its 720-768 ...bizarre).
     
  22. Mr.D

    Mr.D
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    So the big tamale is how does this effect us HTPC owners.....

    I don't think it does in all honesty.
    Assuming you are using a b it of software that either does the correct remapping or doesn't ... and there isn't any way you the punter can make it do the correct remap what are your options?

    Stick with those tried and tested resolutions rather than use new ones ( easy one this if you have a digital display 1:1 is 1:1 regardless of what your software is doing to a video image.So assuming you are nopt going to touch the signal coming out of your HTPC you will probably adjust the image size parameters for the overlay in software whislt referencing your hopefully 1:1 mapped display.

    I certainly do this in Theatertek ( quite often its a necessity as dvds seem to be quite inconsistent with regard to image size).

    So net result is you've probably tweaked it a bit anyway.

    However I will be rechecking my baseline settings with a suitable test pattern first chance I get.

    http://www.execulink.com/~impact/utes_imp.htm

    go here for a PAL D1 resolution pattern with red blanking areas : this is the one I used for tests. It seems to be correct given what we've discussed on here.
     
  23. cybersoga

    cybersoga
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    That PAL D1 test pattern looks consistant with my findings.

    here is a digital capture of BBC Test Card W, which also tallies up (although it isn't centered properly vertically probably due to the way it was captured)...
     
  24. Mr.D

    Mr.D
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    Its amazing how people just accept things when its stuck in some technical paper. Well done cybersoga for not being a zombie like the majority of us.
     
  25. cybersoga

    cybersoga
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    No prob, I was quite surprised myself until I realised that circles weren't looking quite round. JK said in his FAQ that a lot of dvd players don't display more than 704. My DVD player does display almost all 720 (and almost all 576) if you bring the picture right in on the display. A lot of DVD's do have picture data all the way or almost all the way to 720 (mostly mastered from HD?)... you can easily see if they have if you play them on a PC, as long as they get the aspect ratio right that's what really matters...

    I wonder what the situation with HD video is...
     
  26. Mr.D

    Mr.D
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    From my undersatnding the BT601 spec makes provision for having image all the way out to 720 however it does state that the extra 8 pixels either side do not make up the 4x3 aspect ratio only the 702 ( 704) do. What this basically says to me is that the pixel aspect ratio is only ever 702/768 ( or thereabouts as diiscussed... lets not rehash it my brain hurts).

    So you get dvds that are mastered as BT601 ( or rather from BT601 D1 format)
    and another bunch that effectively come from the other spec : the EBU ones mentioned previously that have blanking instead of image.

    I assume sometimes on dvd you could get 720 with image other times you get 702(704) with blanking but if the dvd player/software is only ever utilising the inset 704 pixels and unsquishing that to 768 then in theory the two different formats will display the same. I strongly suspect that either spec is not rigourously adhered to at the mastering stage and its highly likely that you can never refer to a given dvd ( unless its a test disc thats had its accuracy confirmed) as being "pixel perfect"

    I wondfer if its worth asking someone at theatertek how their software handles pixel unsquishing.
     
  27. cybersoga

    cybersoga
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    I agree there, analogue video (from which these standards were set) is rarely pixel perfect, one can expect a slight deviation either way from the standards due to the fact that it's anologue, it's a bit like the below black and above white argument, digital has pixel perfection and brick walls whereas anologue you can go slightly above or slightly below.

    It's only when anologue video is sampled and digitised and displayed on digital screens with perfect square pixels can we then get really quite anal about it!
     
  28. JohnAd

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    The cropping to 702 normally takes place as the images are encoded to analog, it's only us weirdos who use SDI or a PC that can ever see more than 702 pixels wide on a PAL DVD.

    The best way of working with CCIR video is probably to talk in terms of pixel aspect ratios and forget about the whole picture.

    This page has been referenced by a lot of the linux guys when discussing this recently and appears to be accurate

    http://www.mir.com/DMG/aspect.html

    Anyway PAL DVD pixels have an aspect ratio of 59:54 in 4:3 and 118:81 in anamorphic.

    John
     
  29. JohnAd

    JohnAd
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    Fast and loose. In the windows DirectShow world you normally assume that 720 and 704 widths are interchangeable and both either 16:9 or 4:3.

    john
     
  30. cybersoga

    cybersoga
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    That's *almost* the same as SMPTE RP 187 standards that i've already mentioned...
     

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