Figure resize when saving a plot

Bug report

Bug summary

The print_figure function in the FigureCanvasBase class records the current figure's dpi, sets it to the requested dpi for the saved figure, and then resets it to the original. Setting the dpi invokes a property function, _set_dpi that calls the figure's set_size_inches method, which then triggers a resize of the figure manager.

This resize is not evident when I use the standard Pyplot commands within a Qt console, presumably because the plot size doesn't change. However, I have embedded the Matplotlib figure in a PyQt GUI where I don't have the toolbar in the same widget, and it causes a noticeable size change for a fraction of a second before the second dpi reset restores the old size.

Possible solution

I would like to suggest that the FigureCanvasBase print_figure function should not invoke the dpi property, but set and then reset the figure's hidden variable, _dpi, which will not trigger the redundant canvas resize. I can submit a pull request, but I wanted to check if there would be side effects.

Matplotlib version

Matplotlib Version: 2.0.2
Operating System: Mac OS 10.12.5
Python Version: 2.7.11
PyQt 4.8.7

This is a side-effect of the work to support high-dpi screens and the need to temporarily double the dpi.

What is the dpi on the screen vs the dpi you are saving at? Is it a big change (factor of 2) or a small one? I suspect that this may actually be a bug in the figure size not round-tripping properly rather than in the dpi handling per-say. I am also not sure off the top of my head what is forcing the re-draw in the GUI. Are you using pyplot + embedding?

If we are going to go down the route of disabling the auto-redraw can it be done as a context manager (this is one of many things where we should be using context managers, but the code was written before context managers existed 😜 ).

I would say that it is an approximately 10% reduction in width during the temporary resize. The requested dpi of the PNG file I am saving is 100, and a little googling shows that the Thunderbolt display I am using has a dpi of 109, so that could be consistent. It's puzzling that the value of _dpi was already 100 before the resize, so another possible fix is to make sure that _set_dpi only triggers a resize if the new value is different (or is that something that context managers handle?).

The GUI subclasses FigureManagerQT, so it is using the save_figure function in backend_qt5.py. Matplotlib 2 must have added the _status_and_tool_height parameter, which I have set to 0, since my toolbar is in a different widget. It was while investigating why the parameter was needed that I encountered this issue.

If there is a long-term plan that will render this issue moot, then it can probably wait. It's just disconcerting seeing the window suddenly change size, and it took me a while to persuade myself that it reverted back to the same size.

Probably related to roughly related to #8717

I've finally realized what the problem is. When setting the dpi figure property, it restores the figure manager window to the canvas height and width, not the original window's height and width. There is a fudge in the FigureManagerQT resize function that adds the toolbar height to the canvas height, which works within Matplotlib, but I would argue that it screws up any attempt to embed a Matplotlib canvas in another GUI as I am doing. It would be much cleaner to confine canvas resizes to the canvas widget, and let PyQt handle the window resizes.

As a follow-up, I had already subclassed FigureManagerQT for other reasons. If I add the following resize function to the class, then my problems disappear, i.e., there is no discernible resize of the plotting window, temporary or permanent. I don't think it is related to screen resolutions.

def resize(self, width, height):
    extra_width = self.window.width() - self.canvas.width()
    extra_height = self.window.height() - self.canvas.height()
    self.window.resize(width+extra_width, height+extra_height)

You could presumably do the same thing in FigureManagerQT, since canvas is still its own widget, embedded in the main window.

that is a very experimentalist approach, I like it 👍

@rayosborn

exact work:

replace https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/backends/backend_qt5.py#L535 with the code @rayosborn provided above
sort out if we still need to carry around self._status_and_tool_height

FIX: prevent the canvas from jump sizes due to DPI changes

Hello! Can you please tell me how I can solve this issue? I have the exact same problem: I am saving a figure over and over again, so that I can animate it afterwards but the figure keeps shriniking in size after successive saves...

I have set a custom dpi, which I though was the "fix" mentioned in #8805, but it did not make any difference.

What version of Matplotlib are you using? This should have been fixed in the 2.2 series.

Yes you are right, sorry for the spam. Turns out I wasn't up to date with the latest version. Now everything is working properly!

@Datseris

I'm having the same issue as above, particularly like what @Datseris described.

I have a tkinter GUI for doing some plotting which has a tab displaying a single Figure with Axes that get cleared and updated when new plots are created/reselected. When saving one file, (using savefig), there is no issue, but when saving multiple plots iteratively (i.e. save the figure, clear axes, update, repeat...), the graph shrinks (both in the GUI and in the saved images). Here's a gif displaying the issue. The Figure is rendered at 150 DPI in the application, but I am trying to save at 300 DPI (specified when calling savefig, if I leave out the dpi argument I don't have the issue).

I'm using matplotlib v 3.2.1. Any ideas for a fix or workaround? I don't quite follow all of the thread above... but let me know if I can provide any other information. Thanks!

I added lines to explicitly update the canvas and the application every loop, and that seemed to fix the issue. NVM!

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Figure resize when saving a plot #8736

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Figure resize when saving a plot #8736

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tacaswell commented on Jun 9, 2017

rayosborn commented on Jun 9, 2017

tacaswell commented on Jun 12, 2017

rayosborn commented on Jun 13, 2017

rayosborn commented on Jun 13, 2017

tacaswell commented on Jun 13, 2017

tacaswell commented on Jun 13, 2017

Datseris commented on Apr 6, 2018

tacaswell commented on Apr 6, 2018

Datseris commented on Apr 6, 2018

earnestt1234 commented on Jun 4, 2020

earnestt1234 commented on Jun 5, 2020

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