SDL to view color planes for YUV4:2:0


SDL is great for showing YUV420 using overlay together with SDL_YV12_OVERLAY.
But now I need to show the three color-planes in separated areas, i.e.
one Y, one Cb and one Cr
as depicted below in ascii-art:±------+
| |
| Y |
| U | V |

(I hope gmail renders it correctly using monospace font…)
Y = w x h
U = V = w/2 x h/2

The Y-part is just pure luminance data and is easy to solve, but how
do I do with the U and V part?
Can this be done using YUV directly or do I need to move to another
format like RGB?