How do I specify and save a Matplotlib figure with a precise pixel size?

Specifying and Saving a Figure with Exact Size in Pixels: A Comprehensive Guide

Matplotlib, a popular Python library for data visualization, offers robust capabilities for displaying and saving figures. However, when it comes to specifying and saving figures with an exact size in pixels, the process can be nuanced. This article addresses this challenge, providing a detailed explanation of how to achieve desired pixel dimensions using Matplotlib.

Understanding Matplotlib's Units and Coordinate System

Unlike some image-saving libraries, Matplotlib operates using a different coordinate system and unit system. It employs physical sizes and DPI (dots per inch) to manage the dimensions of figures. Consequently, specifying the size of a figure in pixels requires an intermediary step of converting pixels to inches.

Determining the DPI of Your Monitor

To accurately convert pixels to inches, you must ascertain the DPI of your monitor. This value represents the number of pixels per inch that your monitor can display. Several online tools and software can automatically detect your monitor's DPI.

Converting Pixels to Inches

Once you know your monitor's DPI, convert the desired width and height of the figure in pixels to inches using the following formula:

width_in_inches = width_in_pixels / DPI
height_in_inches = height_in_pixels / DPI

Displaying a Figure with a Specific Pixel Size

To display a figure with a specific pixel size on the screen, you can use the figsize and dpi arguments in the pyplot.figure() function. The figsize argument takes a tuple of width and height in inches, while the dpi argument specifies the DPI of the figure.

For example, to display a figure with a width of 800 pixels and a height of 800 pixels on a monitor with a DPI of 96, you would use the following code:

import matplotlib.pyplot as plt

plt.figure(figsize=(800/96, 800/96), dpi=96)

Saving a Figure with a Specific Pixel Size

To save a figure with a specific pixel size, you must adjust the DPI of the saved image. By default, Matplotlib's backends do not support setting the DPI directly. However, the savefig function provides the dpi argument, which allows you to specify the DPI to use when saving the figure.

To save the same figure as a PNG file with a resolution of 8000x8000 pixels, use the following code:

plt.savefig('my_fig.png', dpi=8000)

Note: Some backends may not support setting the DPI directly. In such cases, you can manipulate the DPI by scaling the figure's physical size using the figsize argument.

By following these steps, you can specify and save figures with exact pixel sizes using Matplotlib, enabling you to achieve precise control over the dimensions of your graphical representations.

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