[FIXED] Saving a figure vis matplotlib by adapting inches to pixels

Issue

I am trying to save an image which is supposed to have the dimension (230, 256) in pixels. However, since the calculations are within inches I'm trying to find a way to convert the following image into that dimension:

label = np.zeros((230,256))


my_dpi = 96
# Create a figure. Equal aspect so circles look circular
fig,ax = plt.subplots(1, figsize=(4.5, 3.27))
ax.set_aspect('equal')
ax.set_axis_off()
# Show the image
ax.imshow(label)

circ1 = Circle((28,64),10, color='white')
circ1 = Circle((26,65),13, color='white')
circ2 = Circle((100,65),13, color='white')
circ3 = Circle((172,65),11, color='white')
circ4 = Circle((247,65),11, color='white')
circ5 = Circle((315,65),10, color='white')
ax.add_patch(circ1)
ax.add_patch(circ2)
ax.add_patch(circ3)
ax.add_patch(circ4)
ax.add_patch(circ5)
# Show the image
plt.show()


fig.savefig(nimage[23].split('.')[0]+'_gt.jpg', bbox_inches='tight', dpi=my_dpi)

I also tried to adapt the dpi but it was not outputing the expected dimension. Can anybody explain how to convert from dpi to pixel values in matplotlib?

Solution

Your DPI is directly related to figsize (which is defined in inches). DPI (dots per inch) multiplied by your width/height from figsize will let you calculate the size in terms of pixels. The snipper below would generate a figure with your required dimensions. Please note, the aspect ratio is NOT the same between what you had in your figsize and label.

label = np.zeros((230,256))


my_dpi = 96

w = 230./my_dpi
h = 256./my_dpi

# Create a figure. Equal aspect so circles look circular
fig = plt.figure()
ax = fig.add_subplot(111)
ax.set_aspect('equal')
ax.set_axis_off()
# Show the image
ax.imshow(label)

circ1 = Circle((28,64),10, color='white')
circ1 = Circle((26,65),13, color='white')
circ2 = Circle((100,65),13, color='white')
circ3 = Circle((172,65),11, color='white')
circ4 = Circle((247,65),11, color='white')
circ5 = Circle((315,65),10, color='white')
ax.add_patch(circ1)
ax.add_patch(circ2)
ax.add_patch(circ3)
ax.add_patch(circ4)
ax.add_patch(circ5)
# Show the image
plt.show()

fig.set_size_inches(w, h, forward=True)
fig.tight_layout()
fig.savefig(nimage[23].split('.')[0]+'_gt.jpg', dpi=my_dpi)

Answered By - Neviem