[Image-SIG] Inter-converting images and Numeric arrays

[Charles G Waldman]

I'm working on some image analysis software.  For what I'm doing it's
much more efficient to have the pixel data in a Numeric array, rather
than a PIL Image, to avoid the overhead of lots of getpixel/setpixel
calls.

I've found it's pretty straightforward to inter-convert images and
arrays using fromstring/tostring.... but I have a few questions:

1:  Shouldn't PIL support conversion directly to/from arrays?  Using
the intermediate string works, but seems like a hack...

2:  Seems that if I start with an Array, make an Image, then convert
back, the y-axis is inverted.  Is this a feature or a bug?

Example program:

#!/usr/local/bin/python
import Image,ImagePalette
from Numeric import *
a = zeros((4,4),'b')
for y in range(0,4):
    for x in range(0,4):
	a[x,y]=x*10+y
print "before:"
print a
i1 = Image.fromstring("P",(4,4),a.tostring())
a1 = fromstring(i1.tostring(),'b')
a1.shape=(4,4)
print "after:"
print a1

And its output
>>> 
before:
[[ 0  1  2  3]
 [10 11 12 13]
 [20 21 22 23]
 [30 31 32 33]]
after:
[[30 31 32 33]
 [20 21 22 23]
 [10 11 12 13]
 [ 0  1  2  3]]
>>>