From the analysis of Yann's program :
- All image should have the same size.
- The dimension of the image should be a multiple of the dimension of the grib
- Mapping of pixel to rollups (files) and accounts are done following these formula :
let output_of x y = let (row, col) = (y / height_cell, x / width_cell) in row * nb_cols + col
let account_of x y = let x0 = x mod width_cell and y0 = y mod height_cell in y0 * width_cell + x0
for a example of 16 rollups and 16 pixel by rollup. The image is split like so to rollups : | 1 2 3 4 | | 5 6 7 8 | | 9 10 11 12| | 13 14 15 16|
and the mapping of pixel to accounts in these regions follow the same rule