python - Is there a way to use Pillows "Image.convert()" on an existing variable?

Question

Hello,

maybe this question looks stupid, but I try to use Pillows Image.convert() to convert an image to grayscale. This image I have stored in a variable img because I already pre-processed it, but not with Pillow (type: numpy.ndarray). So I type:

img = Image.convert('LA')

But it does not seem to work, as it says:

AttributeError: module 'PIL.Image' has no attribute 'convert'

If I type img = Image.open("picture.jpg").convert('LA') it works, but I want to use it on a variable that already exists. I also do not want to save the preprocessed image just to open and convert it with the previous command because this is even more inefficient (in terms of speed and CPU-power). So: Is there a proper way to do this?

Thanks for the help in advance!

Answer 1

Whilst you could perfectly well convert your Numpy array to a PIL Image and then convert that to greyscale and then convert back to a Numpy array like this:

PILImage = Image.fromarray(Numpyimg)
PILgrey  = PILImage.convert('L')
Numpygrey= np.array(PILgrey)

You might as well just do the ITU-R 601-2 luma transform yourself, i.e.

L = 0.299 * Red + 0.587 * Green + 0.114 * Blue

So, you would get:

Numpygrey = np.dot(Numpyimg[...,:3], [0.299, 0.587, 0.114]).astype(np.uint8)