cropping interlaced YV12 video caused chroma error

[dphirschler]

Today I solved a problem I see occasionally. I've seen this before and didn't understand it. But today I figured it out, so I thought I would post about it here.

I capture and record a lot of shows and I almost always remove 3:2 pulldown so I can have progressive 24 fps frames. Occasionally I see these weird chroma interlace lines that bleed from the previous or next frame. Here is an example of it:



I've solved it in the past by converting the colorspace from YV12 to YUY2. But now I understand it better. After investigating it, I determined the cause of this error is cropping too early in my process. Typically, my process goes something like this, but not always in this order (which is why the error was sporadic):

1) Crop two lines off the top to get CC info out of frame
2) Add two black lines to the bottom
3) Remove pulldown (inverse telecine)

Since I am removing pulldown, I am converting field-based video (interlaced) to frame-based video (progressive). But cropping before that conversion causes the chroma to get out of sync. Maybe it's an AviSynth bug, or maybe it's just typical of YV12 video. Changing the order like so solved it:

1) Remove pulldown (inverse telecine)
2) Crop two lines off the top to get CC info out of frame
3) Add two black lines to the bottom

Here is the same frame without the error:




Darryl

[jagabo]

Interlaced YV12 has interlaced chroma channels. Cropping two lines off the top of the frame results in only one line of the luma channels being cropped. That reverses the field order of the chroma channels (but not the luma channel). Hence the errors -- the chroma channels are being applied to the wrong fields of the luma channel. Always crop interlaced YV12 mod4. Or ConverToYUY2(interlaced=true) first.

[Gavino]

Maybe it's an AviSynth bug, or maybe it's just typical of YV12 video.

Interlaced YV12 should only be vertically cropped by multiples of 4 (because of field chroma placement, as you discovered). Avisynth doesn't know if a video is interlaced, so cannot prevent you doing it wrong.