The file I'm talking about was downscaled from 1080i to 720p causing a BFF-TFF pattern that repeats every 4 lines, but here's a better (double) example to illustrate:
Upper left: downscaled from 576i to 540p, causing blend-BFF-blend-TFF-blend pattern that repeats every 34 lines
Lower left: failed deinterlacing (wavy edges every 34 lines)
Upper right: upscaled from 576i to 720p, causing blend-BFF-blend-TFF-blend pattern that repeats every 10 lines
Lower right: failed deinterlacing (wavy edges every 10 lines)
Combining the two
should recover
most of the field information for 86% of the lines if my math is correct, the rest would havebe unrecoverable overlapping blends. I think.