that intermediate function already exists, it's called _dask_block:

Lines 940 to 962 in eb4bf8f

    
           def _dask_block(self, copy: bool, block_id: tuple[int]) -> np.ndarray: 
        
               if isinstance(block_id, np.ndarray): 
        
                   return None 
        
               with self._lock: 
        
                   was_closed = self.closed 
        
                   if self.closed: 
        
                       self.open() 
        
                   try: 
        
                       ncoords = len(self._coord_shape) 
        
                       idx = self._seq_index_from_coords(block_id[:ncoords]) 
        
                       if idx == self._NO_IDX: 
        
                           if any(block_id):  # pragma: no cover 
        
                               raise ValueError( 
        
                                   f"Cannot get chunk {block_id} for single frame image." 
        
                               ) 
        
                           idx = 0 
        
                       data = self.read_frame(int(idx))  # type: ignore 
        
                       data = data.copy() if copy else data 
        
                       return data[(np.newaxis,) * ncoords] 
        
                   finally: 
        
                       if was_closed: 
        
                           self.close()

so, what additionally needs to happen is:

the to_dask method needs to accept a frame_chunks parameter that lets the user define how they would like each CYX plane to be chunked. It would modify this line
the _dask_block function needs to add a block_info keyword argument as described in the dask map_blocks docs.
the array-location key in the block_info dict would then need to be used to slice the buffer returned from read_frame here, before the copy operation:

nd2/src/nd2/nd2file.py

Lines 957 to 959 in eb4bf8f

    
           data = self.read_frame(int(idx))  # type: ignore 
        
           data = data.copy() if copy else data 
        
           return data[(np.newaxis,) * ncoords]

	def _dask_block(self, copy: bool, block_id: tuple[int]) -> np.ndarray:
	if isinstance(block_id, np.ndarray):
	return None
	with self._lock:
	was_closed = self.closed
	if self.closed:
	self.open()
	try:
	ncoords = len(self._coord_shape)
	idx = self._seq_index_from_coords(block_id[:ncoords])

	if idx == self._NO_IDX:
	if any(block_id): # pragma: no cover
	raise ValueError(
	f"Cannot get chunk {block_id} for single frame image."
	)
	idx = 0
	data = self.read_frame(int(idx)) # type: ignore
	data = data.copy() if copy else data
	return data[(np.newaxis,) * ncoords]
	finally:
	if was_closed:
	self.close()

Implement sub-frame read/chunking #85

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