Save image in original resolution

Hi! I want to save my plot with the ScaleBar, here is my code:

fig, ax = plt.subplots()
ax.axis("off")
ax.imshow(my_slice3, cmap="gray")
scalebar = ScaleBar(650, "nm", length_fraction=0.25, location= "lower right")
ax.add_artist(scalebar)
plt.savefig('foo.png')

When I save the image, the resolution is not anymore the resolution of the original image. At Stack Overflow I found the hint, that one can use imsave , but that is not working with the Scalebar. https://stackoverflow.com/questions/31544130/saving-an-imshow-like-image-while-preserving-resolution I know that I can increase the resolution e.g. with plt.rcParams['figure.dpi'] = 300 . But that is not leading to the exact pixel-by-pixel image. Any idea how to solve this?

[kolibril13]

Here is a solution I found:

1. Find out display dpi here: https://dpi.lv/
2. Run this script:

import matplotlib.pyplot as plt
import numpy as np

def export_figure_matplotlib(arr, f_name, dpi=200, resize_fact=1, plt_show=False):
    """
    Export array as figure in original resolution
    :param arr: array of image to save in original resolution
    :param f_name: name of file where to save figure
    :param resize_fact: resize facter wrt shape of arr, in (0, np.infty)
    :param dpi: dpi of your screen
    :param plt_show: show plot or not
    """
    fig = plt.figure(frameon=False)
    fig.set_size_inches(arr.shape[1]/dpi, arr.shape[0]/dpi)
    ax = plt.Axes(fig, [0., 0., 1., 1.])
    ax.set_axis_off()
    fig.add_axes(ax)
    ax.imshow(arr)
    scalebar = ScaleBar(650, "nm", length_fraction=0.25, location=  "lower right")
    ax.add_artist(scalebar)
    plt.savefig(f_name, dpi=(dpi * resize_fact))
    if plt_show:
        plt.show()
    else:
        plt.close()
        
export_figure_matplotlib(my_slice3, "hello2.pdf", dpi=298, resize_fact=1, plt_show=True)

Inspired by https://stackoverflow.com/a/51438809/6933377

[hakonanes]

As these images have a resolution about 2000x2000 pixel, I don’t want to lose image quality when adding the scalebar.

Aha, the scalebar would not be pixelated, then, wrong assumption on my end…

As I am making the poster with a vector program, I can simply save the image as a pdf. The image is then a raster image, and the Scalebar is still in vector format.

I would increase the dots per inch (DPI), as you do, to ensure that you don’t loose much pixel resolution. Apart from that, I don’t know what else can help, unfortunately. It might be that just adding a custom scalebar in your program yourself is the simplest solution when you want to keep the exact image resolution.