libpng error: Invalid IHDR data

[tcrombie]

Occasionally we get errors with plt.imsave when in debug mode.

Processing 10_A_0002.jpg
        Center: 956.9,1183.5
        Radius: 964.0
        Cropping plate
        Circular crop
libpng warning: Image height is zero in IHDR
libpng error: Invalid IHDR data
Traceback (most recent call last):
  File "/Users/XXX/conda_envs/chemotaxis_py2.7.12/bin/ct", line 11, in <module>
    load_entry_point('ct==0.0.1', 'console_scripts', 'ct')()
  File "/Users/XXX/conda_envs/chemotaxis_py2.7.12/lib/python2.7/site-packages/ct-0.0.1-py2.7.egg/ct/command.py", line 84, in main
    fp_sigma=args.fp_sigma)
  File "/Users/XXX/conda_envs/chemotaxis_py2.7.12/lib/python2.7/site-packages/ct-0.0.1-py2.7.egg/ct/__init__.py", line 51, in decorated
    return f(*args, **kwargs)
  File "/Users/XXX/conda_envs/chemotaxis_py2.7.12/lib/python2.7/site-packages/ct-0.0.1-py2.7.egg/ct/__init__.py", line 117, in crop_and_filter_plate
    plt.imsave("debug/" + fname + ".05_crop.png", img)
  File "/Users/XXX/conda_envs/chemotaxis_py2.7.12/lib/python2.7/site-packages/matplotlib/pyplot.py", line 2378, in imsave
    return _imsave(*args, **kwargs)
  File "/Users/XXX/conda_envs/chemotaxis_py2.7.12/lib/python2.7/site-packages/matplotlib/image.py", line 1419, in imsave
    image.write_png(fname)
  File "/Users/XXX/conda_envs/chemotaxis_py2.7.12/lib/python2.7/site-packages/matplotlib/image.py", line 628, in write_png
    _png.write_png(im, fname)
RuntimeError: libpng signaled error

We could include a catch in the crop_and_filter_plate function to skip the save and report error.