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>so long story short: what are the settings to make animations frame based but to maintain the sources fields?
A comp will mix interlaced and progressive footage without problems. You'll get better results if you separate fields on all interlaced footage especially if you move or scale the source footage in the composition. ALL NTSC and PAL output is interlaced when it is broadcast. If you've chosen to render without fields and you send your video to a TV set via any means the video on the screen will be interlaced but each pair of fields will be exactly the same moment in time.
If you render with fields and your source files are interlaced (fields) and progressive (frame based) and fields are properly interpolated with fields separated, then the portions of your rendered video where your source footage is progressive will have pairs of identical fields and the portions of your video that has interlaced footage will have field pairs that are not the same moment in time.
The most important rule when working with interlaced footage is that you MUST separate fields if you are going to move or scale the footage in the composition. If you do not separate fields and you move the footage up or down an odd number of pixels you will reverse the field order in an interlaced render and significantly soften the image in a progressive render. After Effects built in de-interlacing tools do a fine job but 3rd party tools like Magic Bullet and ReVision Effects fields kit do a much better job. If you're doing a lot of work with interlaced footage I'd suggest you look into one of these products.
You can't set up a composition to render part of the footage as field based and part as progressive. Fields are introduced in the render cue and not in the composition. If you have interlaced original material and you want to maintain the temporal quality (smooth motion or video look) then you set your render settings to introduce fields in the proper order. If you like the look of progressive footage on TV then don't add fields to your render.
You also must consider how the frame rate and interlacing effects the timing of the motion of your elements. If you're outputting for anything other than NTSC or PAL displays or one of the HD i formats, then the renders must all be progressive and square pixels. Temporal artifacts (judders) will be different for each frame rate and frame size. To be the best that your video can be you have to adjust the timing of your animations for each of the different frame rates and sizes you choose.
Most of my projects are HD. About half the clients want progressive (P) and the other half want interlaced (i). If I get a request from a client to deliver in both formats I usually have to tweak the speed of moving elements and adjust the amount of motion blur to achieve optimal results. This means more work and should mean a higher charge to the client. It's the only way to avoid the judders in a 24P render that present no problems in a 30i project. It's also the only way to get a filmic look in a 30i project.
I hope this helps. It's too bad that production isn't getting simpler and that there are not one click solutions for all problems.
Here's a link to a page in After Effects Help that explains fields and links to other resources that go into even more depth: "Interlaced video, noninterlaced video, and progressive scanning"
Here's a link to a page in After Effects Help that explains how and why to separate fields when you import footage: "Separate video fields"
thanks guys. that's really appreciated.
I haven't had the chance to digest what you've written yet, but i just wanted to say thanks.
it very much looks like that should clear it up for me.