QTGMC Deinterlacing Presets

[Jay123210599]

For 1080i videos, will they be at lower quality if I choose from "Fast" to "Ultra Fast" than they would be in the "Medium" as well as the "Slow" to "Very Slow" presets (or possibly the original interlaced video itself)? What does the "Draft" and "Placebo" presets mean and what do they do? What is the best preset to use for QTGMC?

Also, would they be at a better and higher quality if I keep the original frame rate or double it?

[Sharc]

For 1080i videos, will they be at lower quality if I choose from "Fast" to "Ultra Fast" than they would be in the "Medium" as well as the "Slow" to "Very Slow" presets

Basically yes

What does the "Draft" and "Placebo" presets mean and what do they do?

These are the extreme settings. They have little practical use and you can easily forget these.

What is the best preset to use for QTGMC?

The one which gives satisfactory results for your eyes at an affordable processing time.

Also, would they be at a better and higher quality if I keep the original frame rate or double it?

Use double rate if your player supports it. With original rate you would loose 50% of the temopral resolution (motion smoothness).

[Alwyn]

Also, would they be at a better and higher quality if I keep the original frame rate or double it?

Use double rate if your player supports it. With original rate you would loose 50% of the temopral resolution (motion smoothness).

Agree with this. On Video 8 and VHS captures the improvement in smoothness is quite noticeable at double framerate ie 50fps for PAL.

[jagabo]

Here's what you need to understand:

Truly interlaced video contains two half-picture in each frame, called fields*. For example, a 1920x1080 interlaced frame contains two 1920x540 images. Each of those images is intended to be displayed separately and sequentially. So a second of 30 fps interlaced video (30i) contains 60 different pictures that are to be displayed at 60 fields per second. Don't be fooled by the label 60i -- it's exactly the same thing as 30i, just with number inflation for marketing purposes.

Motion at 60 frames (or fields) per second is very smooth. If you deinterlace true 30i video to 30p you will lose that smoothness. Panning shots and shaky handheld camcorder video will get jerky and flickery. You'll want to deinterlaced to 60p -- unless you have some kind playback restriction (for example, a player that can't play 1080p60 but can play 1080p30).

Modern progressive displays need to convert interlaced frames to progressive frames for display. There are many different algorithms for doing so. None of them is perfect. For viewing purposes QTGMC usually gives the best image quality with smooth edges and little flicker. Lesser algorithms tend to deliver aliased edges that flicker or "buzz".

Films are shot at 24 frames per second. When converted to 30i video they go through a process call "pulldown". Film frames are duplicated to turn 24 pictures into 60 pictures. First a film frame is duplicated for two fields, then the next frame for three fields. And the process is repeated. So 24 film frames becomes 60 fields. The best way to handle this is to remove the duplicates to restore 24 progressive frames per second. 24 progressive frames per second compress better than 30i frames per second. Deinterlacing is sub-optimal for this.

Just because a video is encoded interlaced (a matter of how the video is handled internally by the codec) doesn't mean it contains interlaced frames. Progressive frames encoded interalced is common in PAL videos. Films are often sped up to 25 fps and then encoded interlaced for broadcast, even though the content of each frame is progressive. Then there is the variation where field are paired incorrectly -- resulting in every frame looking interlaced when there is motion. But the original progressive film frames can be restored simply by re-combining the correct fields. Deinterlacing is also sub-optimal for this type of video.

Then there are the myriad methods of converting PAL to NTSC and NTSC to PAL. Too many to go into here.

You'll want to learn how to deal with all of this if you want the best results.


* fields are spacially interleaved, not just temporally. One field is all the even lines of the original progressive frame, the other is all the odd lines of the original progressive frame. When played back they need to be displayed at the correct location as well as the correct time.