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Old 18th November 2023, 23:10   #1  |  Link
-Radar-
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Are interlaced fields encoded separately?

This is a question regarding interlaced video that I've never received a firm answer on.

Question is: are the fields in interlaced video actually encoded separately? This would mean that the fields exist separately in the video stream and are recombined by your video player. Or, are the fields encoded into the same frame as a progressive video, and the term "60i" is simply used to describe how the video is visually perceived?

If the former, how can I check that a video is truly interlaced, and not just a progressive video with interlaced frames?
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Old 18th November 2023, 23:27   #2  |  Link
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Question is: are the fields in interlaced video actually encoded separately?
That depends on how you encode them

You can always encode interlaced video "as if it was progressive", i.e. weaving each pair of consecutive fields and then encoding the "interlaced frames" just as if they were progressive video. The result will not be good, obviously

That's why video formats like H.264 have a dedicated "interlaced" mode, where the fields will indeed be encoded separately. But, usually, you need to explicitly enable this!

Note that H.264 actually has two different "interlaced" modes, called PAFF (Picture Adaptive Field Frame) and MBAFF (MacroBlock Adaptive Field Frame). PAFF allows to decide, on a frame basis, whether to encode each frame as a frame or as two fields. MBAFF allows to decide, on a macroblock basis, whether to encode as field or frame. Meanwhile, in H.265, encoding of "interlaced" material has been greatly simplified, pretty much treating each field as a separate picture.

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If the former, how can I check that a video is truly interlaced, and not just a progressive video with interlaced frames?
You can check how it was encoded. But, unfortunately, there is no trivial way to figure out what was the nature of the original material

It is possible that progressive material was encoded as interlaced, which is inefficient but doable. And, just as well, it is possible that interlaced material was encoded as progressive, which definitely should be a no-go but you never know!

(Note that if any tools like MediaInfo and friends show "interlaced" vs. "progressive", then this always indicates how the video was encoded, not what the nature of the material is)
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Last edited by LoRd_MuldeR; 18th November 2023 at 23:50.
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Old 19th November 2023, 20:03   #3  |  Link
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Originally Posted by -Radar- View Post
how can I check that a video is truly interlaced, and not just a progressive video with interlaced frames?
Select a scene with motion. Separate the fields and step through the fields. If there is motion with every field advancement it is true interlaced. If there is motion with every 2nd field only it is progressive video.
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Old 21st November 2023, 04:18   #4  |  Link
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Sorry for not responding earlier. I actually did respond within the first day but I got a message about the post requiring moderator approval, but that never happened so I'm just re-writing my post.

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Originally Posted by LoRd_MuldeR View Post
That's why video formats like H.264 have a dedicated "interlaced" mode, where the fields will indeed be encoded separately. But, usually, you need to explicitly enable this!

Note that H.264 actually has two different "interlaced" modes, called PAFF (Picture Adaptive Field Frame) and MBAFF (MacroBlock Adaptive Field Frame). PAFF allows to decide, on a frame basis, whether to encode each frame as a frame or as two fields. MBAFF allows to decide, on a macroblock basis, whether to encode as field or frame. Meanwhile, in H.265, encoding of "interlaced" material has been greatly simplified, pretty much treating each field as a separate picture.
I'm mostly interested in investigating DVD ISOs. Is it true that DVDs with hard telecine are encoded as interlaced? And soft telecine DVDs are progressive?

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(Note that if any tools like MediaInfo and friends show "interlaced" vs. "progressive", then this always indicates how the video was encoded, not what the nature of the material is)
What if MediaInfo doesn't show anything? There is a DVD remux I'm watching with one episode that has interlaced lines, but the MediaInfo doesn't specify interlaced or progressive.

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Originally Posted by Sharc View Post
Select a scene with motion. Separate the fields and step through the fields. If there is motion with every field advancement it is true interlaced. If there is motion with every 2nd field only it is progressive video.
What are you using to separate the fields? I can only advance frame by frame using vlc or mpv.
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Old 21st November 2023, 22:40   #5  |  Link
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Select a scene with motion. Separate the fields and step through the fields. If there is motion with every field advancement it is true interlaced. If there is motion with every 2nd field only it is progressive video.
Specifically, telecine shows a repeating sequence of three progressive frames and two interlaced frames.
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Old 21st November 2023, 22:54   #6  |  Link
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Originally Posted by -Radar- View Post
I'm mostly interested in investigating DVD ISOs. Is it true that DVDs with hard telecine are encoded as interlaced? And soft telecine DVDs are progressive?
Most "24p" DVDs are nominally interlaced, but the encoder sets the field/frame tags so the encode is efficient as a native 24p encode (which DVD doesn't actually support).

Some very old and very low budget TVs take 30i source with 3:2 pulldown and just encode it as interlaced, which delivers worse quality. But inverse telecine is still often possible.

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What if MediaInfo doesn't show anything? There is a DVD remux I'm watching with one episode that has interlaced lines, but the MediaInfo doesn't specify interlaced or progressive.
Weird. What does the fps say? If it is 29.97 is is normally interlaced, 23.976 proper telecine.

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What are you using to separate the fields? I can only advance frame by frame using vlc or mpv.
VLC deinterlaces by default. You can turn it off in settings to see the actual frames.

Note that lots of tools treat a properly encoded 3:2 pulldown streams as straight-up 24p, and report it as such, despite DVD not having a true 24p mode.
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