[FFmpeg-user] ffmpeg and use of the ildct flag in progressive encoding

[Nik Armstrong]

I've encountered a scenario where someone else has programmed an ffmpeg 
encoding command with flags that seem puzzling, and I'm having 
difficulty understanding their purpose.

The command in question is:

ffmpeg -loglevel info -y -r 29.97 -start_number 0 -i 
image_sequence_%4d.png -vcodec dnxhd -vf 
format=yuv422p10,colorspace=bt709:iall=bt601-6-625:fast=1 -b:v 220M 
-flags +ildct output.mov

I've been struggling to find information on what the addition of ildct 
does to the output. It appears to be influencing the media to perceive 
it as interlaced, even though it is, in fact, progressive. Using the 
flag ildct seems counterintuitive and unproductive.