added function to convert to sparse data

creader/SparseFile.py

@@ -0,0 +1,14 @@
+import numpy as np
+
+def to_sparse(data, th = 0):
+    """
+    Convert frames stack ndarray[frame, row, col] to sparse array.
+    Warning: this function drops the frame numbers and only keeps row, col, energy
+    """
+    sparse_dt = [('row', np.int16), ('col', np.int16), ('energy', data.dtype)]
+    size = (data>th).sum()
+    sparse = np.zeros(size, sparse_dt)
+    frames, rows, cols = np.where(data>th)
+    for i,(f,r,c) in enumerate(zip(frames, rows, cols)):
+        sparse[i] = (r, c, data[f,r,c])
+    return sparse

creader/__init__.py

@@ -5,3 +5,5 @@ from .file_utils import open_file
 from .ClusterFile import ClusterFile
 from .enums import DetectorType
 from .RawFile import RawFile
+
+from .SparseFile import to_sparse