Question

Currently I’m using PIL and NumPy. I have a colored png image and I want to:

1. Read it in in grayscale
2. Convert to NumPy array
3. Perform a FFT on array
4. Display the image

This is what I’m trying (in IPython w/ --pylab flag):

In [1]: import Image

In [2]: img = Image.open('ping.png').convert('LA')

In [3]: img_as_np = np.asarray(img)

In [4]: img_as_np
Out[4]: array(<Image.Image image mode=LA size=1000x1000 at 0x105802950>, dtype=object)

In [5]: img_fft = fft.fft2(img_as_np) // IndexError: index out of range for array

Answer 1

You want to use the mode ‘L’ instead of ‘LA’ as the parameter to the convert() method. ‘LA’ leaves an alpha channel and then the numpy.asarray doesn’t work as you intended. If you need the alpha channel, then you will need a different method to convert to a numpy array. Otherwise, use mode ‘L’.