I got the reason why the pictures locate in different position.

"rotation" causes the problem.

I defined domino as follows.

It is not the exact code but just show the form of the codes.

...

def domino (a, b, c, d  )

save pic;

picture pic;

draw something 1;

draw something 2;

pic := currentpicture;

draw pic rotated 90 shifted ( c, d );

enddef;

and call it using

\startbuffer[domino3]

domino (1, 2, 3, 4);

domino (2, 1, 1, 2);

domino (3, 3, 2, 3);

\stopbuffer

and \processMPbuffer[].

it draw the first domino at the right place, but it is rotated when the second domino is drawn. Hence, it actually rotated 180 degree. Also two dominos rotated again when the third domino is drawn. 

Previous drawings are rotated again again when the function domino called.

How to avoid this problem? How to draw one figure and move to wanted position and draw another figure and move to another location without changing the previous figure?

If you want to see the whole codes, please see the attachment in the previous mail in ntg-context Digest, Vol 84, Issue 94.

Thank you.

Best regards,

Dalyoung