Video from image with same image as background I 'd like to create a 16/9 video from an image that is not exactly 16/9; 
I can add black lines aside, to fit 16/9, but I'd like to create a background with same image, with a boxblur applied; the result will be the original image, fit at 16/9, with the same image, blurry, visible instead of blank side lines.

I'm trying to do with split filter, something similar to this:

[0:v]split=2[v0i][v1i];\
[v0i]scale=iw*3:ih*3,boxblur=5[v0f];\
[v1i][v0f]overlay=(W-w/2):(H-h)/2[v0];\
[v0]\
scale=iw*min(640/iw\,360/ih):ih*min(640/iw\,360/ih),
pad=640:360:(640-iw)/2:(360-ih)/2\
[v00];\


but I think I miss something ... can someone help me? thanks ...
Use

[0:v]split=2[vblur][vmain];\
[vblur]scale=640:360:force_original_aspect_ratio=increase,boxblur=5,crop=640:360[vblurred];\
[vmain]scale=640:360:force_original_aspect_ratio=decrease[vmain];\
[vblurred][vmain]overlay=(W-w)/2:(H-h)/2[out]