ImagePlot.draw generates a fatal error when the window showing a plot was resized too small

[sallenEnth]

Problem Description
I have a plot shown in a window. There are a few tools added to the plot: a hovering tool, a selection tool, a zoom tool. When I resize the window so small that the plot does not show the application crashes. According to the traceback the error comes from Chaco.

Reproduction Steps:

I tried to reproduce with simple plots, but I cannot. My first thought is that the bug is related to the presence of the tools. I can share the traceback

Traceback (most recent call last):
  File "lib/python3.6/site-packages/enable/qt4/base_window.py", line 213, in paintEvent
    self.handler.paintEvent(event)
  File "lib/python3.6/site-packages/enable/qt4/base_window.py", line 65, in paintEvent
    self._enable_window._paint(event)
  File lib/python3.6/site-packages/enable/abstract_window.py", line 471, in _paint
    self.component.draw(gc, view_bounds=(0, 0, size[0], size[1]))
  File "lib/python3.6/site-packages/enable/component.py", line 427, in draw
    self._draw(gc, view_bounds, mode)
  File "lib/python3.6/site-packages/enable/component.py", line 779, in _draw
    self._dispatch_draw(layer, gc, view_bounds, mode)
  File "lib/python3.6/site-packages/enable/container.py", line 273, in _dispatch_draw
    component._dispatch_draw(layer, gc, new_bounds, mode)
  File "lib/python3.6/site-packages/enable/component.py", line 799, in _dispatch_draw
    handler(gc, view_bounds, mode)
  File "lib/python3.6/site-packages/chaco/base_2d_plot.py", line 204, in _draw_image
    self._render(gc)
  File "lib/python3.6/site-packages/chaco/image_plot.py", line 124, in _render
    self._compute_cached_image()
  File "lib/python3.6/site-packages/chaco/image_plot.py", line 255, in _compute_cached_image
    col_min, col_max, row_min, row_max = index_bounds
TypeError: 'NoneType' object is not iterable
Fatal Python error: Aborted

Expected behavior:

I would expect the plot to simply not show when the window is too small and that it would not crash my application. I would also expect that the tools won't be available.

OS, Python version: [Enter OS name and Python version]
The bug can be reproduced on Linux and MacOS, Python 3.6

[sallenEnth]

The same bug (same trace at least) happens when I use a ZoomTool on a plot and that I use a box to select an area on which to zoom. If that box has a size of 0 in one direction then I got the same fatal error. I'll try to create a simple code sample that reproduce the bug.

[sallenEnth]

By testing some more the initial bug I posted, I realized that it is not due to resizing, but it is a bug that happens when I try to have a specific size for the window. A width of 201 is what makes my UI crash. That explains why when I do a quick resize I do not see the bug. Someone else tried on MacOS and reproduce the bug at the same width of 201. I expect that the specific width at which the bug appear is related to my specific plot. It is at the limit when the plot is displayed or not displayed.

[ievacerny]

Bringing this up again as the workaround we had no longer works.

We had subclassed Plot.draw and added a condition to avoid drawing when view_bounds reach the values where the bug would be triggered. However, with the introduction of "HiDPI support for the Qt toolkit" in enable v5.0.1, the view_bounds vary depending on the display and their limiting values can no longer be hardcoded.

I also found a way to replicate the bug. If resizing the example examples/demo/image_plot_origin_and_orientation.py to the point where the window is too small, it crashes with the same error:

Traceback (most recent call last):
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/enable/qt4/base_window.py", line 243, in paintEvent
    self.handler.paintEvent(event)
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/enable/qt4/base_window.py", line 78, in paintEvent
    self._enable_window._paint(event)
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/enable/abstract_window.py", line 506, in _paint
    self.component.draw(gc, view_bounds=(0, 0, size[0], size[1]))
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 432, in draw
    self._draw(gc, view_bounds, mode)
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 803, in _draw
    self._dispatch_draw(layer, bb, view_bounds, mode)
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/enable/container.py", line 280, in _dispatch_draw
    component._dispatch_draw(layer, gc, new_bounds, mode)
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/enable/container.py", line 280, in _dispatch_draw
    component._dispatch_draw(layer, gc, new_bounds, mode)
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 831, in _dispatch_draw
    handler(gc, view_bounds, mode)
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/base_2d_plot.py", line 204, in _draw_image
    self._render(gc)
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/image_plot.py", line 118, in _render
    self._compute_cached_image()
  File ".edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/image_plot.py", line 250, in _compute_cached_image
    col_min, col_max, row_min, row_max = index_bounds
TypeError: 'NoneType' object is not iterable
Abort trap: 6

I ran the example in the chaco test environment from current master (2d36abd) with the addition of a scipy v1.4.1 package. Runtime Python 3.6, toolkit pyqt5.

Note that the resizing has to happen slowly so that the specific size value which causes the error is triggered.

I believe that index_bounds become None in this condition:

chaco/chaco/image_plot.py

Lines 304 to 305 in 2297206

	if 0 in (image_width, image_height) or 0 in self.bounds:
	return (None, None)

[rahulporuri]

If resizing the example examples/demo/image_plot_origin_and_orientation.py to the point where the window is too small, it crashes with the same error:

@ievacerny thanks for discovering a way to reproduce the issue! I am able to reproduce the error on my windows machine with pyqt5 + python 3.6.

Ideally, I guess we want to prevent the window from shrinking to such a size in the first place - or failing that, ensure that the window resizes dont crash chaco internals.

[ievacerny]

Ideally, I guess we want to prevent the window from shrinking to such a size in the first place

That would be great, but we couldn't find a straight forward way to do that. Any suggestions?

[sallenEnth]

@rahulporuri, I am not convinced that the solution is to prevent shrinking to such a size. From our experience, it was not very small. The solution might be to replace the figure with a small icon that would indicate that the figure is too small to be displayed. I think that it needs to be dealt with inside Chaco. When _calc_zoom_coord returns (None, None) the Chaco caller function should correctly deal with that results rather than trying to continue the process using (None, None).

[rahulporuri]

@sallenEnth @ievacerny can you check that the solution in #589 fixes the problem in your apps?