Matplotlib - unable to save image in same resolution as original image

Question

I am unable to save the image without the white borders and at the initial resolution (1037x627)

import numpy as np
import matplotlib.pyplot as          


        

Answer 1

There are two factors at play here:

1. An Axes doesn't take up the entire Figure by default
2. In matplotlib, the Figure's size is fixed, and the contents are stretched/squeezed/interpolated to fit the figure. You want the Figure's size to be defined by its contents.

To do what you want to do, there are three steps:

1. Create a figure based on the size of the image and a set DPI
2. Add a subplot/axes that takes up the entire figure
3. Save the figure with the DPI you used to calculate figure's size

Let's use a random Hubble image from Nasa http://www.nasa.gov/sites/default/files/thumbnails/image/hubble_friday_12102015.jpg. It's a 1280x1216 pixel image.

Here's a heavily commented example to walk you through it:

import matplotlib.pyplot as plt

# On-screen, things will be displayed at 80dpi regardless of what we set here
# This is effectively the dpi for the saved figure. We need to specify it,
# otherwise `savefig` will pick a default dpi based on your local configuration
dpi = 80

im_data = plt.imread('hubble_friday_12102015.jpg')
height, width, nbands = im_data.shape

# What size does the figure need to be in inches to fit the image?
figsize = width / float(dpi), height / float(dpi)

# Create a figure of the right size with one axes that takes up the full figure
fig = plt.figure(figsize=figsize)
ax = fig.add_axes([0, 0, 1, 1])

# Hide spines, ticks, etc.
ax.axis('off')

# Display the image.
ax.imshow(im_data, interpolation='nearest')

# Add something...
ax.annotate('Look at This!', xy=(590, 650), xytext=(500, 500),
            color='cyan', size=24, ha='right',
            arrowprops=dict(arrowstyle='fancy', fc='cyan', ec='none'))

# Ensure we're displaying with square pixels and the right extent.
# This is optional if you haven't called `plot` or anything else that might
# change the limits/aspect.  We don't need this step in this case.
ax.set(xlim=[-0.5, width - 0.5], ylim=[height - 0.5, -0.5], aspect=1)

fig.savefig('test.jpg', dpi=dpi, transparent=True)
plt.show()

The saved test.jpg will be exactly 1280x1216 pixels. Of course, because we're using a lossy compressed format for both input and output, you won't get a perfect pixel match due to compression artifacts. If you used lossless input and output formats you should, though.