[FFmpeg-user] Output tiles onto a black background

[Molly Millions]

I have the following command, which converts a video and also generates a
10x9 tileset. Currently it gets the longer side and either generate a 90px
wide or 45px tall tiles. My problem is that each tile has to be 90x45
large, so in essence what I need is a properly resized and centered tile
inside a black bounding box. How can I achieve this?

The resulting tile:
http://i.imgur.com/NzRiC8o.jpg

What I would like to achieve:
http://i.imgur.com/cgH55xl.jpg

ffmpeg -i K/vertical.mp4 .... vertical.out.mp4
-an -vsync vfr -vf
select="isnan(prev_selected_t)+gte(t-prev_selected_t\,2)",scale="'if(gt(iw,ih),90,-1)':'if(gt(iw,ih),-1,45)'",tile="10x9"
-qscale:v 3 './f/f%03d.jpg'