Writing tiles partially

Hi,

I have a question regarding tiled files.
It seems that the tiles need to be aligned on the origin (0,0) of the 
data window to be readable.

Can anyone confirm this ?

Here's the story on how I came to ask this question:
I want to export a file with a data window larger than the visible 
window. In my application, tiles are aligned on the origin of the 
visible window:
Exemple: If I write a 640 * 360 image with a 10% overscan on each side, 
my visible window is {0,0,640,360}, and the data window is {-64,-36, 
704, 396 }.
Using  64x64 tiles, the image seems corrupt because the data window 
starts in the middle of the first row of tiles.