Hi there. I'm doing some experiments related to changing subtitle 
contents with variable-size boxes.

Images related: http://imgur.com/a/IGKnm

I've made a subtitle file in .ass format with the box outline mode and 
put it into a video with a green background with the following line:

ffmpeg -loop 1 -framerate 29.970 -i "Background.png" -y -c:v rawvideo -t 
00:00:40 -vf subtitles=SUBS.BOXBORDER.ASS Export\SAMPLE.CROMA1.AVI

The result is a clean and crispy bordered image, as the latter one in 
the link. Here the tricky part: Now I'm trying to keep a small part of a 
video enclosed in the white part, so after changing the original video 
from yuv420i to bgr24 (Just to be certain) I'm using filter_complex:

ffmpeg -i "ORIGINAL.M1V" -i "Export\SAMPLE.CROMA1.AVI" -y 
"[1:v]chromakey=0xFFFFFF:0.25:0.00[keyed];[0:v][keyed]overlay[out]" -map 
"[out]" -c:v rawvideo -pix_fmt bgr24 Export\SAMPLE.FINALCROMA.AVI

FFmpeg does enter the original video under the croma one, but the 
borders are not crispy and perfect, as the first image in the link 
shows. Now, I've read a bit about yuv420 and yuv444, and that yuv420 has 
the tendency of blurring the edges.

I need clean edges for the chroma vid as I'll latter use this croma 
image against another video with chromakey, that way, there won't be any 
outer borders showing up in the finished product. But I haven't been 
able to find the way to keep things crisp, even when forcing the pix_fmt.

Can anyone help me out? Thank you for your time.
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