Sensor data visualization/display

Choosing the display backend

Matplotlib is used to plot and display the sensor data. Image-specific display utility functions are implemented in client/python/projectairsim/src/projectairsim/image_utils.py. The backend is strictly enforced to be TkAgg for consistency and reliability. Other backends may work but GTK has been reported to cause issues.

The backend used for rendering the plot can be configured by changing the MPL backend value offered by Matplotlib. The list of supported backends can be found here: https://matplotlib.org/stable/tutorials/introductory/usage.html#the-builtin-backends

Relevant details about how to choose the backend with Matplotlib is reproduced below:

There are three ways to configure your backend:

  1. The backend parameter in your matplotlibrc file

  2. The MPLBACKEND environment variable

  3. The function matplotlib.use

Below is a more detailed description.

If there is more than one configuration present, the last one from the list takes precedence; e.g. calling matplotlib.use() will override the setting in your matplotlibrc.

Without a backend explicitly set, Matplotlib automatically detects a usable backend based on what is available on your system and on whether a GUI event loop is already running. On Linux, if the environment variable DISPLAY is unset, the “event loop” is identified as “headless”, which causes a fallback to a noninteractive backend (agg); in all other cases, an interactive backend is preferred (usually, at least tkagg will be available).

Here is a detailed description of the configuration methods:

1. Setting backend in your matplotlibrc file::

backend : qt5agg   # use pyqt5 with antigrain (agg) rendering.

2. Setting the MPLBACKEND environment variable:

You can set the environment variable either for your current shell or for a single script.

On Unix:

> export MPLBACKEND=qt5agg
> python simple_plot.py

> MPLBACKEND=qt5agg python sensor_data_plot.py

On Windows, only the former is possible:

> set MPLBACKEND=qt5agg
> python sensor_data_plot.py

Setting this environment variable will override the backend parameter in any matplotlibrc, even if there is a matplotlibrc in your current working directory. Therefore, setting MPLBACKEND globally, e.g. in your .bashrc or .profile, is discouraged as it might lead to counter-intuitive behavior.

3. If your script depends on a specific backend you can use the function

matplotlib.use

import matplotlib
matplotlib.use('qt5agg')

This should be done before any figure is created, otherwise Matplotlib may fail to switch the backend and raise an ImportError.

Using ~matplotlib.use will require changes in your code if users want to use a different backend. Therefore, you should avoid explicitly calling ~matplotlib.use unless absolutely necessary.

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