how to use 'extent' in matplotlib.pyplot.imshow

Question

I managed to plot my data and would like to add a background image (map) to it. Data is plotted by the long/lat values and I have the long/lat values for the image's three corners (top left, top right and bottom left) too.

I am trying to figure out how to use 'extent' option with imshow. However, the examples I found don't explain how to assign x and y for each corner ( in my case I have the information for three corners).

How can I assign the location of three corners for the image when adding it to the plot?

Thanks

Check the origin and extent in imshow tutorial.

ImportanceOfBeingErnest
– ImportanceOfBeingErnest

2019-08-12 16:55:03 +00:00
Commented Aug 12, 2019 at 16:55 — ImportanceOfBeingErnest
– ImportanceOfBeingErnest, Commented Aug 12, 2019 at 16:55

ProfDFrancis · Accepted Answer · 2019-09-20 13:09:21Z

55

Specify, in the coordinates of your current axis, the corners of the rectangle that you want the image to be pasted over

Extent defines the left and right limits, and the bottom and top limits. It takes four values like so: extent=[horizontal_min,horizontal_max,vertical_min,vertical_max].

Assuming you have longitude along the horizontal axis, then use extent=[longitude_top_left,longitude_top_right,latitude_bottom_left,latitude_top_left]. longitude_top_left and longitude_bottom_left should be the same, latitude_top_left and latitude_top_right should be the same, and the values within these pairs are interchangeable.

If your first element of your image should be plotted in the lower left, then use the origin='lower' imshow option as well, otherwise the 'upper' default is what you want.

edited Sep 20, 2019 at 13:09

ProfDFrancis

9,4171 gold badge23 silver badges33 bronze badges

answered Aug 9, 2011 at 17:38

Yann

35.8k9 gold badges84 silver badges71 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Victor Eijkhout Over a year ago

Still not clear. What do you mean by "max of the horizontal values"? There are no values that are horizontal: there is a 2d matrix. Do you mean the maximum in each row? But there is a bunch of them. The maximum over those then? But then the horizontal and vertical are the same.

Yann Over a year ago

The max is the number written at the right edge of the matrix in the plot, the min is the number at the left edge.

Dave X · Accepted Answer · 2015-07-30 18:42:00Z

Here's an example based on http://matplotlib.org/examples/pylab_examples/image_demo3.html showing use of extent.

#!/usr/bin/env python
from pylab import *
try:
    from PIL import Image
except ImportError, exc:
    raise SystemExit("PIL must be installed to run this example")

import matplotlib.cbook as cbook

datafile = cbook.get_sample_data('ada.png')
h = Image.open(datafile)
dpi = rcParams['figure.dpi']
figsize = h.size[0]/dpi, h.size[1]/dpi

figure(figsize=figsize)
ax = axes([0,0,1,1], frameon=False)
ax.set_axis_off()
ax.set_xlim(0,2)
ax.set_ylim(0,2)
im = imshow(h, origin='upper',extent=[-2,4,-2,4])  # axes zoom in on portion of image
im2 = imshow(h, origin='upper',extent=[0,.5,0,.5]) # image is a small inset on axes

show()

If you don't set your axis limits, they become your extents & then don't seem to have any effect.

Collectives™ on Stack Overflow

how to use 'extent' in matplotlib.pyplot.imshow

2 Answers 2

Specify, in the coordinates of your current axis, the corners of the rectangle that you want the image to be pasted over

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Specify, in the coordinates of your current axis, the corners of the rectangle that you want the image to be pasted over

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related