So, I manage to get the arrow on the rod by tweaking:

#local dir_y  = vaxis_rotate (lookat-cam_loc, vcross (cam_d, cam_up), 
(0.483-screen_y_position*0.01)*cam_ang/image_width*image_height);
#local dir_xy = vaxis_rotate (dir_y, cam_up, 
(0.535-screen_x_position*0.01)*cam_mirror*cam_ang);

replacing the 0.5 value by respectively 0.483 in dir_y and 0.535 in dir_xy.

Of course, this is entirely by just dumb trial and error, and I highly 
doubt if it is significant. I wonder now if it holds for other scenes... 
tests to do...

-- 
Thomas

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