Re: [Matplotlib-users] Change a few pixels color

[Paul I. <piv...@gm...>]

Hi Xavier,

Xavier Gnata, on 2011-04-23 02:33,  wrote:
> Imagine you have this code:
> 
> import  numpy  as  np
> import  matplotlib.cm  as  cm
> import  matplotlib.mlab  as  mlab
> import  matplotlib.pyplot  as  plt
> 
> delta  =  0.25
> x  =  y  =  np.arange(-3.0,  3.0,  delta)
> X,  Y  =  np.meshgrid(x,  y)
> Z1  =  mlab.bivariate_normal(X,  Y,  1.0,  1.0,  0.0,  0.0)
> Z2  =  mlab.bivariate_normal(X,  Y,  1.5,  0.5,  1,  1)
> Z  =  Z2-Z1   # difference of Gaussians
> 
> plt.imshow(Z,  interpolation='nearest',  cmap=cm.gray,  origin='lower',  extent=[-3,3,-3,3])

> Then you want to change the color of a few pixels to red.
> You have a list of coordinates (i,j) and each pixel in this list should 
> now be red.
> 
> I could play with masked arrays like in:
> http://matplotlib.sourceforge.net/examples/pylab_examples/image_masked.html
> but I would prefer a simple "display this pixel (i,j) in red whatever 
> his value is" function.

Since you're using a gray color map for that image, you won't be
able to set a particular pixel to red. You'll have to either
overlay a new image that would be masked out everywhere except
for the pixels you want to change, as you mentioned, or create
new image patches at the corresponding positions like this:

  idx2im = lambda i,j: (x[i],x[j+1],y[i],y[j+1] )
  plt.imshow([[.9]], extent=idx2im(12,12), cmap =cm.jet, origin='lower',vmin=0,vmax=1)

or something like this:

  plt.Rectangle((x[10],y[10]),width=delta,height=delta,color='red')
  ax = plt.gca()
  ax.add_artist(r)
  plt.draw()


best,
-- 
Paul Ivanov
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