WebAgg: _png.write_png raises TypeError

[mdboom]

This looks like a regression after 1.4.x.
Trying to view the plots results in

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/tornado/websocket.py", line 303, in wrapper
    return callback(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg.py", line 229, in on_message
    manager.handle_json(message)
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 361, in handle_json
    self.canvas.handle_event(content)
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 192, in handle_event
    self.draw()
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 90, in draw
    self.manager.refresh_all()
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 365, in refresh_all
    diff = self.canvas.get_diff_image()
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 147, in get_diff_image
    self._png_buffer)
TypeError: Cannot cast array data from dtype('uint32') to dtype('uint8') according to the rule 'safe'

looks like this happened after write_png() API changed.

[WeatherGod]

Which version of numpy are you using? Also, is this independent of any
plotting command (i.e., it doesn't matter if it is imshow() or plot(), etc?)

On Fri, Nov 7, 2014 at 9:45 AM, Vadim Markovtsev notifications@github.com
wrote:

This looks like a regression after 1.4.x.
Trying to view the plots results in

Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/tornado/websocket.py", line 303, in wrapper
return callback(_args, *_kwargs)
File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg.py", line 229, in on_message
manager.handle_json(message)
File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 361, in handle_json
self.canvas.handle_event(content)
File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 192, in handle_event
self.draw()
File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 90, in draw
self.manager.refresh_all()
File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 365, in refresh_all
diff = self.canvas.get_diff_image()
File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_webagg_core.py", line 147, in get_diff_image
self._png_buffer)TypeError: Cannot cast array data from dtype('uint32') to dtype('uint8') according to the rule 'safe'

looks like this happened after write_png() API change.

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#3763.

[mdboom]

Fix attached.

[tacaswell]

@vmarkovtsev I'll merge this when you confirm that it fixes the problem.

Exactly which commit are you working off of? Trying to sort out if this needs to be back-ported to the 1.4.x branch, or just on master.

[mdboom]

My guess is just master -- as I think this has to do with the C++ refactor. But @vmarkovtsev should confirm in case there is another bug lurking on 1.4.x.

[vmarkovtsev]

I confirm that there is no issue with 1.4.x. I am using matplotlib daily builds ppa for Ubuntu and apart from upgrading it, nothing changes, e.g. numpy (1.8.2).