Re: [Matplotlib-users] imshow question

[John H. <jdh...@ac...>]

>>>>> "Rein" == Rein van den Boomgaard (UvA) <re...@sc...> writes:

    Rein> Dear All, as i'm new to this list let me first introduce
    Rein> myself before posting a question.

    Rein> I'm working in the field of image processing and computer
    Rein> vision for a long time now and i am a long time matlab
    Rein> user. Up to now i didn't like any of the matlab clones but
    Rein> it seems that numarray+matplotlib might be a winner here.

I've worked some with octave in the past and found it unsatisfying
(though a very impressive one man feat!), in part because I wasn't
happy with gnuplot, which it used for graphics at the time (and
perhaps still does), and in part because it really aimed at being a
matlab clone but very few of my m-files ran w/o alteration.
matplotlib tries to solve the first problem by producing better
graphics than matlab and the second by not trying to be a drop in
replacement for matlab.

I'm curious about your experiences, what you tried, and what you found
lacking.

    Rein> I started using numarray+matplotlib by coding the Gaussian
    Rein> derivative convolutions. Calculating a derivative (by
    Rein> convolving with the derivative of the Gaussian function)
    Rein> will lead to an image(array) with both positive and negative
    Rein> values. Although imshow should deal with that (as far as i
    Rein> understand the code: beware i am a Python beginner) the
    Rein> display turns black for all derivatives (except for the
    Rein> 'zero order' derivative of course).

    Rein> What is going on? Is it me (probably) or is it imshow?

In matplotlib-0.54.2, the images must be normalized to the unit
interval before color mapping or plotting.  For example, in
examples/image_demo2.py, notice that I do 

  A *= 1.0/max(A)

Your images are black, I think, because you haven't normalized them.

I've done a lot of work on matplotlib images since 0.54.2, with new
fixes and features.  Normalization is handled by default in the 0.60
release candidate.  When I run your (very nice) example with my
development version of matplotlib, it produces this image for figure 2
( I added "show" at the end of your script, but it was otherwise
unaltered)

  http://nitace.bsd.uchicago.edu:8080/files/share/fig2.png

which looks right to me.  I'm always interested in nice screenshots
for the web site, so please consider donating this example!

I've uploaded the 0.60d release candidate to 

  http://nitace.bsd.uchicago.edu:8080/files/share/matplotlib-0.60d.tar.gz

Can you compile matplotlib yourself or are you using a binary
distribution?  If you can try the development version linked above,
that would solve two problems: you'll get enhanced image support and
I'll get a tester for my image changes.  If you do so, see the updated
help for the image related commands: imshow, figimage, clim, jet, gray
and the new matplotlibrc parameters image.*

In the figure 2 image from your script, I notice that there the titles
overlap the images above.  Here is how you can control this (requires
matplotlib 0.60d)

        # pass in these keywor args to title.  The y location is the y
        # text coordinate in axes coords (0,0 is lower left, 1,1 is
        # upper right).  You can use **somedict in place of keyword
        # args in python
        offsets = {'y':1.0, 'fontsize':10}
        _gDplot(1,0,0)
        title(r'$G$', **offsets)
        _gDplot(5,1,0)
        title(r'$G_x$', **offsets)
        _gDplot(6,0,1)
        title(r'$G_y$', **offsets)
        _gDplot(9,2,0)
        title(r'$G_{xx}$', **offsets)
        _gDplot(10,1,1)
        title(r'$G_{xy}$', **offsets)
        _gDplot(11,0,2)
        title(r'$G_{yy}$', **offsets)
        _gDplot(13,3,0)
        title(r'$G_{xxx}$', **offsets)
        _gDplot(14,2,1)
        title(r'$G_{xxy}$', **offsets)
        _gDplot(15,1,2)
        title(r'$G_{xyy}$', **offsets)
        _gDplot(16,0,3)
        title(r'$G_{yyy}$', **offsets)

Cheers,
JDH