Function heif_image_handle_decode_image_tile 

pub unsafe extern "C" fn heif_image_handle_decode_image_tile(
    in_handle: *const heif_image_handle,
    out_img: *mut *mut heif_image,
    colorspace: heif_colorspace,
    chroma: heif_chroma,
    options: *const heif_decoding_options,
    tile_x: u32,
    tile_y: u32,
) -> heif_error

The tile position is given in tile indices, not in pixel coordinates. If the image transformations are processed (option->ignore_image_transformations==false), the tile position is given in the transformed coordinates.