Using pi2 from Python

Pi2 can be used from Python scripts and from iPython console. In order to initialize Pi2, the package pi2py must be imported and a Pi2 object created:

import sys
sys.path.append('path/to/pi2/folder')

from pi2py2 import *
pi = Pi2()

A common pitfall is that the pi2py.py file must be in the Python path so that the Python interpreter finds it. The easiest way is to execute the above commands in the directory that contains the pi2py.py file. Pi2py is written in Python 3.6, and might or might not work in other Python versions.

After initalization, all the functionality of Pi2 can be accessed through the Pi2 object:

img = pi.newimage(ImageDataType.UINT8, 100, 100, 100)
pi.noise(img, 100, 25)
pi.writetif(img, './noise')

For reference on the available functionality, use either iPython help or the Command reference page. There are many Python examples available here: https://github.com/arttumiettinen/pi2/blob/master/python_scripts/pi2py2_examples.py Most of them are documented in the Examples page.

The Python bindings in the pi2py.py file are self-generating so they are always up-to-date.

Pi2 and NumPy

NumPy arrays can be used together with Pi2 in a few ways.

1. NumPy arrays can be passed to Pi2 functions as input images. Outputting to NumPy arrays is not supported at the moment. Additionally, 3-element NumPy arrays can be used in all occasions where a 3-element vector is required.

2. Image data can be retrieved as a Numpy array using Pi2Image.get_data() function. The function returns the data in the image as NumPy array of appropriate shape and data type. The NumPy array is a copy of the original image data so changes made to it are not reflected in the Pi2 system.

Retrieving image data as a NumPy array causes it to be read into RAM in its entirety. This might not be desirable in distributed computing mode, if the image is large.

3. NumPy array can be copied into a Pi2 image using Pi2Image.set_data(array) function. The data of the array is copied from NumPy into the Pi2 system, so changes made by Pi2 are not reflected in the NumPy array.

For ‘power users’ there is also a method Pi2Image.get_data_pointer() that returns reference to the image data in the Pi2 system as a NumPy array. Changes made to the array are reflected in the Pi2 system and vice versa. However, please note that Pi2 commands that change the size of the image will re-allocate the memory reserved for the image and this process invalidates any NumPy arrays returned by the get_data_pointer() method. Accessing invalidated array might result in program crash. Arrays returned by the get_data() method are not affected by this problem.

If Pi2 is in distributed computing mode, calling Pi2Image.get_data_pointer() will cause the image to be read into RAM in its entirety. In this mode, changes made to the NumPy array are not reflected to the image until Pi2Image.flush_pointer() method is called.