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Bob,

Try doing

b = axis()
ax1.plot([0.0],[0.0],'wo')
axis(b)

That saves the axis range set by imshow and restores it after the call 
to plot.

Eric

Robert Simpson wrote:
> Hi,
> 
> Using matplotlib 0.87.2, I've been trying to plot points on top of an 
> image.  The image without points fills axes of the extent requested, but 
> using plot to add a point on top of the image changes the axes lengths 
> and gives unwanted white margins around the edges.  Here's an example.  
> Any help would be greatly appreciated!
> 
> Bob Simpson
> 
> from pylab import *
> 
> # Sample image.
> image = rand(12,6)
> 
> # This choice for extent works fine... It produces no white margins.
> #extent = [-80, 60, -60, 40]
> 
> # This choice gives unwanted white margins,
> #   except when no point is plotted.
> extent = [-80.,64., -74.,48.]
> 
> ax1 = axes()
> ax1.im = imshow(image, origin='lower', extent=extent)
> 
> # Plot point on image.
> hold(True)
> ax1.plot([0.0],[0.0],'wo')
> 
> show()
> 
> 
> 
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Hi,

Using matplotlib 0.87.2, I've been trying to plot points on top of an 
image.  The image without points fills axes of the extent requested, 
but using plot to add a point on top of the image changes the axes 
lengths and gives unwanted white margins around the edges.  Here's an 
example.  Any help would be greatly appreciated!

Bob Simpson

from pylab import *

# Sample image.
image = rand(12,6)

# This choice for extent works fine... It produces no white margins.
#extent = [-80, 60, -60, 40]

# This choice gives unwanted white margins,
#   except when no point is plotted.
extent = [-80.,64., -74.,48.]

ax1 = axes()
ax1.im = imshow(image, origin='lower', extent=extent)

# Plot point on image.
hold(True)
ax1.plot([0.0],[0.0],'wo')

show()

Wouldn't it be simpler to use the extent key to shift the image?

Let x and y be the pixel centers
imshow(M, extent =3D [x[0]-dx, x[-1]-dx, y[0]-dy, y[-1]-dy])
where dx and dy is half the distance between pixels.

David


2006/5/2, Gary Ruben <gr...@bi...>:
>
> Just looking back through the thread, I guess the problem is that the
> coords returned by JH's example don't correspond to the centre of the
> pixel. I'm pretty sure you'll have to do the transformation yourself
> using values from the axes and figure classes.
>
> Gary R.
>
> Gary Ruben wrote:
> > I don't understand what you're asking here. Is there some unexpected
> > interpolation going on? A small example might be useful to clarify if
> > noone else steps forward with an answer.
> >
> > Gary R.
> >
> > Tommy Grav wrote:
> >> Hi,
> >>
> >>    sorry for all the newbie questions. I have looked though the
> >> documentation but have not found anything on how to handle the fact
> >> that matplotlib takes the array indices as the lower left corner
> >> position for the value, while an image
> >> has its pixel values at the center of the pixel. Is there anything
> >> written on how to handle this?
> >>
> >> Cheers
> >>
> >> Tommy
>
>
>
> -------------------------------------------------------
> Using Tomcat but need to do more? Need to support web services, security?
> Get stuff done quickly with pre-integrated technology to make your job
> easier
> Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronim=
o
> http://sel.as-us.falkag.net/sel?cmd=3Dlnk&kid=3D120709&bid=3D263057&dat=
=3D121642
> _______________________________________________
> Matplotlib-users mailing list
> Mat...@li...
> https://lists.sourceforge.net/lists/listinfo/matplotlib-users
>

Just looking back through the thread, I guess the problem is that the 
coords returned by JH's example don't correspond to the centre of the 
pixel. I'm pretty sure you'll have to do the transformation yourself 
using values from the axes and figure classes.

Gary R.

Gary Ruben wrote:
> I don't understand what you're asking here. Is there some unexpected 
> interpolation going on? A small example might be useful to clarify if 
> noone else steps forward with an answer.
> 
> Gary R.
> 
> Tommy Grav wrote:
>> Hi,
>>
>>    sorry for all the newbie questions. I have looked though the 
>> documentation but have not found anything on how to handle the fact 
>> that matplotlib takes the array indices as the lower left corner 
>> position for the value, while an image
>> has its pixel values at the center of the pixel. Is there anything 
>> written on how to handle this?
>>
>> Cheers
>>
>> Tommy