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Thread: horizontal lines

  1. #1
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    Default horizontal lines

    After rendering my to WMA, the picture has horizonal lines that appear on anything that is moving. Stationery objects are perfect.
    Any ideas please.

  2. #2
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    Sounds like interlace artifacts. Try de-interlacing and see if that sorts it out.

  3. #3
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    That sounds great. Sorry don't know what you mean LOL

  4. #4
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    It's interlace combing, one of the side effects of using interlaced video.

  5. #5
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    Still sounds a bit chinese to me Nik, However I've managed to cure a great deal of it by matching the resolutions.

    I really am a babe at this game but am learning something everyday.

    Thank goodness for you guys

  6. #6
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    There's a mountain of forums and information just about that one subject, if you have a spare 5 minutes some day.

  7. #7
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    Default

    Thanks for clearing that up...

  8. #8
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    As usual Smifis has got it wrong again. De-interlacing is NOT the same as progressive. Similar effect in the end but not the same as recording in progressive mode.

    For your info (and Smifis) a simplified explanation.

    Think of showing video like writing a page. You start at the top left and work your way down, letter by letter, finishing at the bottom right. Instead of letters video has different coloured dots and you end up with a picture which you see in one glance. With a television (the older "tube" screens) the dots were "written" by shining an electron beam onto the screen which was coated with a material which glowed when it was hit with these electrons. It stayed glowing for a short while but... By the time the beam had got to the bottom of the screen, either the glow was still there at the top of the screen, just when the next "page" needed to be displayed or it had disappeared completely, leaving a black hole and this added to the impression of flicker which resulted by showing pictures at 25 or 30 frames per second.

    So the idea of interlacing came to the rescue.

    In essence the video records (and transmits) the image in two parts. The first half displays lines 1,3,5,7 etc and then it displays lines 2,4,6,8, etc. because the lines are so close together, you don't notice the alternating fields (as they're called to differentiate them from frames) This overlap avoids the problem of the "glow" which fades too slowly or too quickly and the apparent doubling of the frame rate helps to reduce flicker.

    Computer screens work differently and show the whole picture in one go (not quite but it makes it easier to understand if you think of it like that) and so, you can see that showing an interlaced picture means showing two "fields" at the same time. If the subject has moved between the recording of the odd field and the even field it gives an effect described as "combing" because edges look like the teeth of a comb.

    So, if your picture is being viewed on a computer screen you may see these artifacts. By de-interlacing you delete one of the fields (and half of the information) and lose this "combing". As you can see, recording in interlace mode effectively gives you 50 fps although each "field" only has half the information needed to make a complete picture. When you de-interlace, one of these fields is deleted and picture is made up with information from the remaining field.

    Progessive, on the other hand is where you record the whole picture as you would write it, in one go. No half-frames or fields. It is still "transmitted" as two fields so that older televisions can deal with it. Since it displays 25 different frames per second, as opposed to 50 different fields, it tends to have a slight "film look" when it comes to motion.

    With progressive recording you have all the information recorded on one frame originally and don't have the "two into one" scenario.

    If your video/dvd is going to be viewed mainly (or completely) on televisions, then the artifacts you see when editing on the computer will not be visible when you view it on the telly and there's no need to de-interlace. If however, the footage will be seen on computers, then de-interlacing will remove the combing. In future, if you know that the footage is intended only for computer viewing when you shoot it, it's often better to use "progressive" mode.

    Be aware though that some camcorders don't really have a true progressive mode and do a form of in-camera de-interlacing which results in a loss of quality.

    So, as you can see, de-interlacing isn't the same as progressive.

  9. #9
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    Phew! Thanks

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