I finding the way to get interlaced video from Houdini and I found description of the Field Merge compositing node in Houdini 10 manual. Can this node merge two consecutive frames rendered by Mantra into one interlaced frame? What is the correct way (pipeline) to use it? I have to use twice as much frames than usual in this case, and what to do with the time scale? Or is there another, the right way to get interlaced video from Houdini?
Thank You!

Doing fielded renders, in Houdini, has been a problem ever since the Fielded Render option was removed from the mantra ROP.

You can check out the H9 Mantra Field file on the exchange
http//www.sidefx.com/index.php?option=com_wrapper&Itemid=157

I will also attach an old file from H8 that had 2 work arounds for this problem.

To answer your specific question about the Field Merge COP - yes, you can use it to merge 2 rendered frames to create fielded frames. 2 frames become 1 frame, so your animated sequence is half as long.

you have the ability to add vm_field rendering property to your mantra ROP
that way can render just the fields you are after

so it seems to me that rendering sequence 1-100.5 with the step of 0.5
and vm_field set as 1(for integer frames) and 2(for float frames like 1.5,2.5…) with an expression, will give you 200 field renders which you can merge afterwards. That way you can avoid rendering 200 full frames for 100 interlaced frames

it's just how i see it may work, i have not tested it. I don't use fields and i do not know if there is more straightforward way. Just an idea

Thank You! I did it!
I added vm_field parameter into Mantra node, used this expression for it:
$N%2+1
and rendered with step 0.5. As result I received alternating odd/even fields and merged it with Field Merge compositing node. I was forced to use $N instead of $F for rendered frames name. With $F Mantra rewrites field with new one and I receive frames with even (top) fields only.