How do you directly overlay a scatter plot on top of a jpg image in matplotlib / Python?


I need to rapidly plot jpg frames that result as the output of a tracking algorithm. Companion with the jpg frames are text files containing simple (x,y) data locating the image targets that are being tracked. I would like to use matplotlib to plot the jpg images, then overlay a scatter plot of the (x,y) data which gets read from the text file and stored into a Pythonic list. Below is code that will plot the jpg image, but in all of the scouring I have done of matplotlib, scipy, and PIL manuals and help pages, I cannot find anything that explains how to maintain this plot window and simply overlay a scatter plot of simple markers at various (x,y) locations in the image. Any help is greatly appreciated.

import matplotlib.pyplot as plt;
im = plt.imread(image_name);
implot = plt.imshow(im);
2/22/2011 3:01:40 AM

Accepted Answer

The pyplot.scatter() function was tailor made for this reason:

import matplotlib.pyplot as plt
im = plt.imread(image_name)
implot = plt.imshow(im)

# put a blue dot at (10, 20)
plt.scatter([10], [20])

# put a red dot, size 40, at 2 locations:
plt.scatter(x=[30, 40], y=[50, 60], c='r', s=40)

See the documentation for more info.

2/22/2011 2:47:43 AM

this should work:

import matplotlib.pyplot as plt
im = plt.imread('test.png')
implot = plt.imshow(im)

keep in mind that each pixel in the image is one unit on the x,y axes. The 'o' is a shorthand way of getting the plot function to use circles instead of lines.

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