Ken Schutte wrote:
> Hi,
>
> If I display a simple matrix with imshow(), e.g:
>
> a = zeros((3,3))
> a[0,0] = 1
> imshow(a,interpolation='nearest')
>
> the axes extend from 0.0 to 3.0 in such a way that elements are centered
> at 0.5, 1.5,... For example, the point at (x,y)=(.9,.9) is red, whereas
> nearest-interp. should give a[1,1]=0. Would it make more sense for the
> default to be like this (as is in matlab): ?
>
> imshow(a,interpolation='nearest',extent=(-0.5, 2.5, -0.5, 2.5))
Ken,
Actually, the matlab nearest equivalent would be
imshow(a, interpolation='nearest', extent=(-0.5, 2.5, 2.5, -0.5))
so that the y-axis increases down and both axes correspond to the array
indices. I agree that this makes good sense. It seems to me that in
the long run it would make imshow easier to explain and easier to use.
>
> Is there a way to get behavior this automatically?
Not at present without defining your own little function to set the
extent. It would be easy to change imshow to work this way, so the
question is whether enough other users would prefer it and whether it
would cause too much disruption. It could also be made optional via a
new kwarg or a new interpretation of an old kwarg. For example, extent
could take a string to indicate this option, something like
"extent='indices'". An rc param could control the default.
Some discussion (and/or a ruling by John) is needed. My preference is
that if a change is made, it be a change in basic default behavior, not
additional options, because any non-default behavior can be achieved
easily using the extent kwarg in its present form, and at some point
adding options and more complicated handling of kwargs causes more
trouble (complex code and documentation) than it is worth. I don't see
any advantage of the present behavior over the matlab-equivalent
behavior--but that may be because I am ignorant of the original rationale.
If imshow behavior is changed, a corresponding change will be needed in
the image-matching capability of contour. Maybe matshow would also
change; I haven't checked.
Eric
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