[Matplotlib-users] 2d arrays issues

Benjamin Root ben.v.root at gmail.com
Wed Dec 2 16:47:27 EST 2015

You might want to check out xray.

On Wed, Dec 2, 2015 at 4:40 PM, Sameer Grover <sameer.grover.1 at gmail.com>

> I was using matplotlib to plot 2d arrays (or equivalently images), getting
> the user to choose some points, getting the x and y coordinates of those
> points, and then doing calculations. Keeping track of indices proved to be
> a source of frustration.
> This is because, given a 2d array, two different interpretations are
> possible. In the first one, the first index is the (horizontal) x-axis and
> the second index is the (vertical) y-axis. And in the other one, the first
> index is the row (vertical axis) and the second one is the column
> (horizontal axis).
> Matplotlib has always veered towards the second interpretation, perhaps
> because it has been modelled on matlab, where everything is a 2d matrix (or
> higher dimensional). But in numpy, where 1d arrays are first class objects,
> this can get confusing.
> For example, if I were acquiring data as a function of two variables which
> go on the x-axis and y-axis
> for x in x_vals:
>   for y in y_vals:
>     result[x,y] = f(x,y)
> either I would have to use contourf(x_vals, y_vals, result.T) or use
> result[y,x] = f(x,y). Most programmers would probably be used to this, but
> it's easy to slip up with large programs.
> The situation becomes even more confusing with imshow and imread because
> the origin is placed on the upper left corner by default. I could use the
> keyword origin=lower to avoid this but this changes the image orientation
> as well. So the upshot is that if I load an image with imread(..) and want
> to display the image in the same orientation as imshow(origin=upper) would
> but with the origin placed on the lower left corner, I would have to go
> through various rot90 & flip(ud/rl) operations. (As an aside, IMHO it is
> imread that should have the origin option and not imshow)
> It would be nice if there could be a specialized 2d array class (possibly
> subclassing ndarray) with additional information on how the array is to be
> interpreted and routines to convert from one to the other.
> I wanted other opinions on whether this is desirable/feasible?
> Thanks.
> Sameer
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