Video
Field & Frames - 3
Capturing stills
If you want to capture a still image of a single frame from an AVI file, then the multiple fields can be a serious problem. You can use the MSP Video Capture program to do this - open the AVI file, then play and/or step to the required frame and use the Capture Frame menu or button. But Video Capture is using whole frames, and if there is any significant motion present the difference between the fields shows up as a blurring. You might want this of course, but let's assume you don't!
One solution is to re-capture the sequence using single-field mode in the capture driver. However, this may lower the resolution too far, or you just may not be able to re-run the original. Much simpler is to use the Video Editor to create the image, and it will look after the field problem at the same time.
One of the editor's output options in File-Create is an Image Sequence. This creates a separate image file for every output frame of the video. If you use the preview marker to select just one frame, and create an image sequence of the preview area you will get just one image file of the selected frame. If the clip is set to field mode, MSP uses only the first field to create the image, thus avoiding the field-blur problem.
It's unlikely that you'll be concerned about only having the first field from each original frame, but if for some odd reason you really need the second field in a frame you can get at it by doing the same as above, but also slowing the clip down to 50% speed. As we saw before this has the effect of turning each input field into one output frame. You can now grab exactly the image you want.
Freeze frames
MSP has a freeze-frame function that allows you to freeze the start and/or end of a clip, but it runs into problems with dual-field clips. Freeze-frame works by repeating a single frame of source for the required number of frames of result. But this means repeating the two fields, A-B-A-B-A-B-... etc, so again it looks jittery. An obvious solution is to check Deinterlace in the Field Options, but this means you are de-interlacing the entire clip, which is probably not what you want.
You can improve on this by pre-creating the frozen section as a separate AVI file, with de-interlace set. You then load this AVI back into the timeline to replace the section where you've used the freeze effect. (This technique is covered in more detail in the Tutorial).
Other editors
The principle of fields is not of course specific to MediaStudio, I've just used that to illustrate since it's the editor I use and know. All good editors, such a Premiere, offer ways to handle fields, although the details differ between products. If you understand how fields work then it should not be difficult to decide how to use the features of any particular editor.
Summary
If you capture clips using both fields, then set the default import mode in MSP to the correct field-order for your capture card. Also set the same field-order in Create Video File Options, and in Preview Options. You shouldn't then get any peculiar effects caused by the existence of two fields per frame.
If you're doing anything more subtle, especially working with still images, then understanding what fields are all about will help you get the best results
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