How to Specify Exact Pixel Dimensions for Matplotlib Images?

Specifying Image Size with Exact Pixels for Matplotlib

Understanding the Issue

Matplotlib, a widely used Python library for data visualization, requires figure sizes to be specified in inches and dots per inch (DPI). This can be inconvenient when the desired outcome is an image of a specific pixel size.

Overcoming Pixel-to-Inch Conversion

To avoid potential accuracy loss from pixel-to-inch conversions, Matplotlib provides an alternative solution. Instead of specifying inches, you can directly specify the pixel dimensions.

Setting Pixel Size for Figure

To set the figure size based on pixel dimensions:

<code class="python">import matplotlib.pyplot as plt

# Pixel dimensions of the figure
w = 7195
h = 3841

# Create a figure without axes or titles
fig = plt.figure(frameon=False)
ax = plt.Axes(fig, [0., 0., 1., 1.])
ax.set_axis_off()
fig.add_axes(ax)</code>

Setting Pixel Size for Image Data

To display an image of the desired pixel size:

<code class="python">ax.imshow(im_np, aspect='normal')</code>

Saving with Specific Pixel Size

To save the figure as a high-resolution image with the exact pixel dimensions, adjust the DPI correspondingly:

<code class="python">dpi = 1000

fig.savefig('some_path.png', dpi=dpi)</code>

Note: Matplotlib's support for specifying DPI depends on the backend used. While the PNG backend uses DPI, other backends like PDF and PS have different interpretations.

Example:

To obtain a 3841 x 7195 pixel image:

<code class="python">plt.figure(figsize=(3.841, 7.195), dpi=100)
(your code ...)
plt.savefig('myfig.png', dpi=1000)</code>

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