PIL : How to write array to image ???

[Mart.]

On Oct 4, 9:47 am, Martin <Mar... at Hvidberg.net> wrote:
> On Oct 3, 11:56 pm, Peter Otten <__pete... at web.de> wrote:
>
>
>
>
>
> > Martin wrote:
> > > Dear group
>
> > > I'm trying to use PIL to write an array (a NumPy array to be exact) to
> > > an image.
> > > Peace of cake, but it comes out looking strange.
>
> > > I use the below mini code, that I wrote for the purpose. The print of
> > > a looks like expected:
>
> > > [[ 200.  200.  200. ...,    0.    0.    0.]
> > >  [ 200.  200.  200. ...,    0.    0.    0.]
> > >  [ 200.  200.  200. ...,    0.    0.    0.]
> > >  ...,
> > >  [   0.    0.    0. ...,  200.  200.  200.]
> > >  [   0.    0.    0. ...,  200.  200.  200.]
> > >  [   0.    0.    0. ...,  200.  200.  200.]]
>
> > > But the image looks nothing like that.
>
> > > Please see the images on:
> > >http://hvidberg.net/Martin/temp/quat_col.png
> > >http://hvidberg.net/Martin/temp/quat_bw.png
>
> > > or run the code to see them locally.
>
> > > Please – what do I do wrong in the PIL part ???
>
> > > :-? Martin
>
> > > import numpy as np
> > > from PIL import Image
> > > from PIL import ImageOps
>
> > > maxcol = 100
> > > maxrow = 100
>
> > > a = np.zeros((maxcol,maxrow),float)
>
> > > for i in range(maxcol):
> > >     for j in range(maxrow):
> > >         if (i<(maxcol/2) and j<(maxrow/2)) or (i>=(maxcol/2) and j>=
> > > (maxrow/2)):
> > >             a[i,j] = 200
> > >         else:
> > >             a[i,j] = 0
>
> > > print a
>
> > > pilImage = Image.fromarray(a,'RGB')
> > > pilImage.save('quat_col.png')
> > > pilImage = ImageOps.grayscale(pilImage)
> > > pilImage.save('quat_bw.png')
>
> > The PIL seems to copy the array contents directly from memory without any
> > conversions or sanity check. In your example The float values determine the
> > gray value of 8 consecutive pixels.
>
> > If you want a[i,j] to become the color of the pixel (i, j) you have to use
> > an array with a memory layout that is compatible to the Image.
> > Here are a few examples:
>
> > >>> import numpy
> > >>> from PIL import Image
> > >>> a = numpy.zeros((100, 100), numpy.uint8)
> > >>> a[:50, :50] = a[50:, 50:] = 255
> > >>> Image.fromarray(a).save("tmp1.png")
> > >>> b = numpy.zeros((100, 100, 3), numpy.uint8)
> > >>> b[:50, :50, :] = b[50:, 50:, :] = [255, 0, 0]
> > >>> Image.fromarray(b).save("tmp2.png")
> > >>> c = numpy.zeros((100, 100), numpy.uint32)
> > >>> c[:50, :50] = c[50:, 50:] = 0xff808000
> > >>> Image.fromarray(c, "RGBA").save("tmp3.png")
>
> > Peter
>
> Thanks All - That helped a lot...
>
> The working code ended with:
>
>         imga = np.zeros((imgL.shape[1],imgL.shape[0]),np.uint8)
>         for ro in range(imgL.shape[1]):
>             for co in range(imgL.shape[0]):
>                 imga[ro,co] = imgL[ro,co]
>         Image.fromarray(imga).save('_a'+str(lev)+'.png')

Without knowing how big your image is (can't remember if you said!).
Perhaps rather than looping in the way you might in C for example, the
numpy where might be quicker if you have a big image. Just a
thought...