We detect local maxima in a galaxy image. The image is corrupted by noise, generating many local maxima. To keep only those maxima with sufficient local contrast, we use h-maxima.
import numpy as np import matplotlib.pyplot as plt from skimage.measure import label from skimage import data from skimage import color from skimage.morphology import extrema from skimage import exposure color_image = data.hubble_deep_field() # for illustration purposes, we work on a crop of the image. x_0 = 70 y_0 = 354 width = 100 height = 100 img = color.rgb2grey(color_image)[y_0:(y_0 + height), x_0:(x_0 + width)] # the rescaling is done only for visualization purpose. # the algorithms would work identically in an unscaled version of the # image. However, the parameter h needs to be adapted to the scale. img = exposure.rescale_intensity(img)
# Maxima in the galaxy image are detected by mathematical morphology. # There is no a priori constraint on the density. # We find all local maxima local_maxima = extrema.local_maxima(img) label_maxima = label(local_maxima) overlay = color.label2rgb(label_maxima, img, alpha=0.7, bg_label=0, bg_color=None, colors=[(1, 0, 0)]) # We observed in the previous image, that there are many local maxima # that are caused by the noise in the image. # For this, we find all local maxima with a height of h. # This height is the grey level value by which we need to descent # in order to reach a higher maximum and it can be seen as a local # contrast measurement. # The value of h scales with the dynamic range of the image, i.e. # if we multiply the image with a constant, we need to multiply # the value of h with the same constant in order to achieve the same result. h = 0.05 h_maxima = extrema.h_maxima(img, h) label_h_maxima = label(h_maxima) overlay_h = color.label2rgb(label_h_maxima, img, alpha=0.7, bg_label=0, bg_color=None, colors=[(1, 0, 0)])
# a new figure with 3 subplots fig, ax = plt.subplots(1, 3, figsize=(15, 5)) ax.imshow(img, cmap='gray', interpolation='none') ax.set_title('Original image') ax.axis('off') ax.imshow(overlay, interpolation='none') ax.set_title('Local Maxima') ax.axis('off') ax.imshow(overlay_h, interpolation='none') ax.set_title('h maxima for h = %.2f' % h) ax.axis('off') plt.show()
Total running time of the script: ( 0 minutes 0.296 seconds)