Advanced AVISynth Commands and IVTC


Advanced AVISynth Commands:
Now that we dealt with the simpler aspects of just loading our video, we will now see how to IVTC and further process our video.

MJPEG Luma Range:
Luma (brightness) in MJPEG decoders is often decoded incorrectly to the YUV luma-range, a special plugin was written specifically to fix this issue with MJPEG videos. I have not verified if this problem exists with all MJPEG Video Codecs, but the two most popular codecs (PicVideo & Morgan) both suffer from this issue (only when decoding to YUV, decoding to RGB in programs such as VirtualDub doesn't require this fix).

Here's a simple script that fixes this issue:
LoadPlugin("MJPEGcorrect.dll")
AVISource("my_mjpeg_capture.avi")
MJPEGcorrect()

You can obtain the MJPEGCorrect plugin either here or here.

IVTC (Inverse Telecine) and Field Matching:
As many of you that have experimented in IVTC in the past know, IVTCing a video is often a slow and problematic issue. Especially when your source is erratic and possibly even noisy (such as Anime or TV Capture).

Up until late 2001, the only way to properly IVTC video was using TMPEG Encoder or AVIFile. Both these systems were not perfect. They had issues with certain hardware and often broke down, not to mention the IVTC processing speed left much to be desired.

Then Tom Barry, one of the authors involved in the dScaler project took it upon himself to convert his dScaler field matching plugin to the AVISynth platform. The filter is GreedyHMA. The benefits of this filter is that it was CPU Optimized and was quite adaptable.

A few months later, Donald A. Graft, the Author of Telecide (A field matching filter for VirtualDub) converted his filter to the AVISynth platform and modified the code to allow for dropping of duplicated frames (part of the IVTC process) and further optimized it so that it too was quite a bit faster than the older VirtualDub version. His filter is called "Decomb".

Both these filters work well, but Decomb is currently more evolved and as such will be the plugin I will discuss in this article. At this time, the latest version of Decomb is v3.6 and as such some of the parameters may change in a future version. The latest version of Decomb can be found here and possibly on doom9.org as well.

The Decomb plugin is built from two sections:

1. Telecide, which matches fields in order to reconstruct the original progressive image and filters out fields that can't be matched).
   
2. Decimate, which matches duplicate frames after the Telecide process and removes one of these frames to restore the original 23.976 frame rate.

• Reverse: Reverse Field Matching (true/false, Default "False").
  Some badly authored DVD content or sources captured using a Capture card may have the prominent field reversed which may cause interlaced output even though the video was originally progressive. This setting reverses the matching order and may resolve these issues.
• Swap: Swap Fields (true/false, Default "False").
  Swapping Fields is the process where every second line is swapped with the line above it. Some capture cards capture the fields in the wrong order and using this setting you can fix the issue.
• FirstLast: Blend First/Last Frames (true/false, Default "False").
  In a lot of cases, the first and last frames of a video will have a missing field, which would prevent field matching. If you want to ensure that these frames are blended, enable this parameter. However, in most cases due to editing the first/last frames of a video is usually black and won't require this setting.
• Post: DeInterlace Non-Matching Frames (true/false, Default "True").
  In certain conditions it may be impossible to reconstruct a progressive frame (scene changes, bad editing on the original source material). When this setting is enabled, it will compensate for this by either Blending the fields to smooth the artifacts or by Interpolating the fields to estimate motion (according to the "blend instead of interpolate" setting). Before enabling this setting, it's usually wise to load the clip with it disabled so you can see if your field order is correct. Otherwise, you would get a blurry video. Once you inspected that most of the frames match properly, re-enable this setting to make sure no interlaced frames sneak in.
• Threshold: Field Detection Threshold (0-255, default "15").
  This is the value that determines which area is considered interlaced and which is considered progressive. The lower the value, the more the image is considered interlaced.
• dThreshold: Field DeInterlace Threshold (0-255, default "9").
  When combing is detected after the field matching process, this setting determines the detection sensitivity of the remaining interlacing that will later be blended or interpolated.
• Blend: Blend instead of Interpolate (true/false, default "True").
  Blending gives a smoother image with better motion but with slight ghosting, an interpolated image may have a sharper image but motion may be jumpy and the image may have additional noise. On a whole, Blending provides a better image quality but is slightly slower (only when non-progressive frames are found).
• Chroma: Use Chroma Search (true/false, Default "false").
  By default only the Luma (brightness) of an image is used to check for interlaced data. However, in certain types of content (mostly in animation) it may also be wise to look at the Chroma (Color). Unless you're using Animated content, I recommend leaving this disabled.
• Y0: Exclude Line Start (0-4095, Default "0").
  In some cases where you may have an on-screen logo, or perhaps subtitles, this overlayed data (that isn't part of the original show) may confuse the field matching code. In order to exclude this area of the screen from being checked for matching fields, you use the Y0 and Y1 parameters. The Y0 parameter defines which starting line should be excluded.
• Y1: Exclude Line End (0-4095, Default "0").
  The Y1 Parameter defines which ending line should be excluded. Any lines in between Y0 and Y1 are ignored. If Y0 and Y1 have the same value, the excluding setting is disabled. Since by default their values are both "0", the entire image is checked for field matches.

• Cycle: Cycle 1 in [n] Frames (2-25).
  Basically you tell Decimate to remove one frame every [n] number of frames. For use with IVTC you would like to remove 1 duplicate frame from every 5 frame sequence, so the value would be "5".
• Mode: Decimate Mode (0-1).
  In mode "0", 1 every [n] frames will be removed from the video. In mode "1", instead of removing the frame, the duplicate frame is blended with the previous frame and thus motion becomes smoother while remaining at the same frame rate. This is useful if your source video is a hybrid of 24fps and 30fps content and you don't want to make the 30fps content jerky by discarding one of it's frames. This is the case in quite a few series that incorporate live-action with computer generated images (such as Andromeda and Babylon 5).
• Threshold: Hybrid Threshold (0-99).
  When using mode "1" to blend duplicate frames on hybrid 24fps/30fps content, you don't want blended frames on the 30fps content. To prevent that from happening, you define a threshold which determines the difference between frames for decomb to deduct which frames should be blended. A value of 25-50 should be ok for DVD content. You may want to experiment to see what works best for you.