[Matplotlib-users] Prevent matplotlib (imshow) from interpolating at all

[Rutger K. <ka...@gm...>]

Dear list,

I am trying to plot an image using imshow for which i want to have full
control of the interpolation taking place. I would rather not have any
interpolation at all, but if the input image is very small, it may be
interpolated (nearest neighbor) with an integer factor of my choice (2x,3x
etc.).

To achieve this i decided that the first and most simple case would be to
output exactly the image which i load by using plt.imread(). If this
succeeds i would expand it by adding a buffer around the image to allow
some annotation like for example a title and a colorbar.

Here is my initial try:

import matplotlib.pyplot as plt

img = plt.imread('D:\\input_image.png')

# get x and y dimension of the image
xpixels = img.shape[1]
ypixels = img.shape[0]

# calculate the dimensions in inch for a given dpi
zoomfactor = 1
dpi = 72
xinch = ( xpixels * zoomfactor) / dpi
yinch = (ypixels * zoomfactor) / dpi

# create the figure
fig = plt.figure(figsize=(xinch,yinch))

# stretch the axes to the full extent of the figure
ax = plt.axes([0.0, 0.0, 1.0, 1.0])

ax.imshow(img, interpolation='nearest')

# hide axis stuff
ax.axes.get_yaxis().set_visible(False)
ax.axes.get_xaxis().set_visible(False)

# remove the border of the axes
plt.setp(ax, frame_on=False)

# save the figure with the given dpi
plt.savefig('D:\\output_image.png', dpi=dpi)


The resulting output image is equal in dimensions to the input image, so
far so good. However, when i overlay them in a graphics program, i can
clearly see that some nearest neighbor interpolation has taken place. It
appears as if the top-right corner of the image is plotted 1 or 2 pixels
beyond the edge of the figure. Does anyone have any idea how to get this
right?

Thanks in advance!


Regards,
Rutger