performance of imshow and numarray

Community matplotlib-users
1

Hi guys,

Just to add to this issue, I have been profiling my own code which embeds matplotlib inside a Cocoa app, and uses Agg to draw to an NSView. (I posted a howto on this recently to the user list). I am drawing a grayscale image (an array with values 0-255), and then drawing lines over the top.

I have noticed that most of the time is spend in the ma.asarray(x) method (specifically the ma.__init__ method) and discounted this as too hard for me to figure on. But these posts point out that this is a slow point in the code. Now it is faster for RGB images. I'm wondering if there is a way to optimise this method for the (fairly common, I imagine) case of using imshow on grayscale, non-masked numpy arrays. Can this be done?

Your help is appreciated.
Josh

Eric Firing wrote:

ยทยทยท

Jeff,

I really blew it on that one--it looked so simple! OK, I think this
version is correct; it will land in CVS in a few minutes.

     def to_rgba(self, x, alpha=1.0):
         '''Return a normalized rgba array corresponding to x.
         If x is already an rgb or rgba array, return it unchanged.
         '''
         if hasattr(x, 'shape') and len(x.shape)>2: return x
         x = ma.asarray(x)
         x = self.norm(x)
         x = self.cmap(x, alpha)
         return x
Eric

Jeff Whitaker wrote:

Eric Firing wrote:

Ray,

You tripped over a bug in cm.py: when I added masked array support, I
put the "x = ma.asarray(x)" too early in the to_rgba() method of
ScalarMappable. It is fixed now in CVS. Thanks for finding the problem.

If you want to try the fix in your version, in place of your
workaround, here is the revised method:

   def to_rgba(self, x, alpha=1.0):
        # assume normalized rgb, rgba
        if len(x.shape)>2: return x
        x = ma.asarray(x)
        x = self.norm(x)
        x = self.cmap(x, alpha)
        return x

Eric

Hi Eric: This breaks some of the basemap demos which do this

CS = m.contour(x,y,hgt,15,linewidths=0.5,colors='k')

to draw solid black contour lines. I now get

File "/sw/lib/python2.4/site-packages/matplotlib/cm.py", line 52, in
to_rgba
   if len(x.shape)>2: return x
AttributeError: 'list' object has no attribute 'shape'

It appears that colors now has to be an array?

-Jeff