+value_at v3d i j k
+ | i < 0 = value_at v3d 0 j k
+ | j < 0 = value_at v3d i 0 k
+ | k < 0 = value_at v3d i j 0
+ | xsize <= i = value_at v3d xsize j k
+ | ysize <= j = value_at v3d i ysize k
+ | zsize <= k = value_at v3d i j zsize
+ | otherwise = idx v3d i j k