I have just done some image processing using the Python image library (PIL) using a post I found earlier to perform fourier transforms of images and I can't get the save function to work. The whole code works fine but it just wont save the resulting image:

```
from PIL import Image
import numpy as np
i = Image.open("C:/Users/User/Desktop/mesh.bmp")
i = i.convert("L")
a = np.asarray(i)
b = np.abs(np.fft.rfft2(a))
j = Image.fromarray(b)
j.save("C:/Users/User/Desktop/mesh_trans",".bmp")
```

The error I get is the following:

```
save_handler = SAVE[string.upper(format)] # unknown format
KeyError: '.BMP'
```

How can I save an image with Pythons PIL?

The error regarding the file extension has been handled, you either use `bmp`

(without the dot), or pass the output name with the extension already. Now to handle the error you need to properly modify your data in frequency domain to be saved as an integer image, `PIL`

is telling you that it doesn't accept float data to save as bmp.

Here is a suggestion (with other minor modifications, like using `fftshift`

and `numpy.array`

instead of `numpy.asarray`

) for doing the conversion for proper visualization:

```
import sys
import numpy
from PIL import Image
img = Image.open(sys.argv[1]).convert('L')
im = numpy.array(img)
fft_mag = numpy.abs(numpy.fft.fftshift(numpy.fft.fft2(im)))
visual = numpy.log(fft_mag)
visual = (visual - visual.min()) / (visual.max() - visual.min())
result = Image.fromarray((visual * 255).astype(numpy.uint8))
result.save('out.bmp')
```

You should be able to simply let PIL get the filetype from extension, i.e. use:

```
j.save("C:/Users/User/Desktop/mesh_trans.bmp")
```

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