Update README.md with example

python/README.md

@@ -11,3 +11,79 @@ Required **Python** modules:
 Required libraries
 
  * libde265 (available on most Linux distributions)
+
+## Example
+
+Example reads from `input.ftl` and writes to `output.ftl`. Calibration and
+pose are copied directly (same method can be used for other channels as well).
+The program copies left and right frames of source 0 to new file (and re-encodes
+them in JPG) when both frames are available.
+
+```python
+import ftl
+from ftl import types
+
+reader = ftl.FTLStreamReader("./input.ftl")
+writer = ftl.FTLStreamWriter("./output")
+
+source_id = 0
+fps = 25
+frame_t = int(1000.0/fps)
+timestamp_out = 0
+timestamp_in = 0
+
+im_left = None
+im_right = None
+
+while reader.read():
+    channel = reader.get_channel_type()
+    timestamp = reader.get_timestamp()
+    frame = reader.get_frame()
+    
+    if reader.get_source_id() != source_id:
+        # not interested in this source, skip
+        continue
+    
+    if channel in (types.Channel.Calibration, types.Channel.Configuration):
+        # copy calibration and pose (but replace timestamp with new value)
+        
+        sp, p = reader.get_raw()
+        sp = sp._replace(timestamp=timestamp_out)
+        writer.add_raw(sp, p)
+        continue
+
+    if channel not in (types.Channel.Left, types.Channel.Right):
+        # only interested in left and right frame
+        continue
+
+    if frame is None:
+        # no frame if decoding failed
+        continue
+
+    if timestamp_in != timestamp:
+        # new timestamp, process available frames
+
+        if not (im_left is None or im_right is None):
+            # save frames only if both of them were found for this timestamp
+            
+            # Note: In this expample channel is re-encoded. If channel content
+            # is not modified, lossy channels should be passed directly
+            # (using add_raw() in same way as for calibration/pose) instead of 
+            # re-encoding them.
+            
+            writer.add_frame(timestamp_out, 0, types.Channel.Left, 2,
+                             types.codec_t.JPG, im_left)
+            writer.add_frame(timestamp_out, 0, types.Channel.Right, 2,
+                             types.codec_t.JPG, im_right)
+        
+        
+        timestamp_out += frame_t
+        timestamp_in = timestamp
+        im_left, im_right = None, None
+
+    if channel is types.Channel.Left:
+        im_left = frame
+    else:
+        im_right = frame
+
+```