matplotlib · NelleV · Oct 15, 2017 · Sep 30, 2017 · Oct 1, 2017 · Oct 15, 2017
diff --git a/examples/images_contours_and_fields/image_transparency_blend.py b/examples/images_contours_and_fields/image_transparency_blend.py
@@ -0,0 +1,125 @@
+"""
+===========================================
+Blend transparency with color in 2-D images
+===========================================
+
+Blend transparency with color to highlight parts of data with imshow.
+
+A common use for :func:`matplotlib.pyplot.imshow` is to plot a 2-D statistical
+map. While ``imshow`` makes it easy to visualize a 2-D matrix as an image,
+it doesn't easily let you add transparency to the output. For example, one can
+plot a statistic (such as a t-statistic) and color the transparency of
+each pixel according to its p-value. This example demonstrates how you can
+achieve this effect using :class:`matplotlib.colors.Normalize`. Note that it is
+not possible to directly pass alpha values to :func:`matplotlib.pyplot.imshow`.
+
+First we will generate some data, in this case, we'll create two 2-D "blobs"
+in a 2-D grid. One blob will be positive, and the other negative.
+"""
+# sphinx_gallery_thumbnail_number = 3
+import numpy as np
+import matplotlib.pyplot as plt
+from matplotlib.colors import Normalize
+
+
+def normal_pdf(x, mean, var):
+    return np.exp(-(x - mean)**2 / (2*var))
+
+
+# Generate the space in which the blobs will live
+xmin, xmax, ymin, ymax = (0, 100, 0, 100)
+n_bins = 100
+xx = np.linspace(xmin, xmax, n_bins)
+yy = np.linspace(ymin, ymax, n_bins)
+
+# Generate the blobs. The range of the values is roughly -.0002 to .0002
+means_high = [20, 50]
+means_low = [50, 60]
+var = [150, 200]
+
+gauss_x_high = normal_pdf(xx, means_high[0], var[0])
+gauss_y_high = normal_pdf(yy, means_high[1], var[0])
+
+gauss_x_low = normal_pdf(xx, means_low[0], var[1])
+gauss_y_low = normal_pdf(yy, means_low[1], var[1])
+
+weights_high = np.array(np.meshgrid(gauss_x_high, gauss_y_high)).prod(0)
+weights_low = -1 * np.array(np.meshgrid(gauss_x_low, gauss_y_low)).prod(0)
+weights = weights_high + weights_low
+
+# We'll also create a grey background into which the pixels will fade
+greys = np.ones(weights.shape + (3,)) * 70
+
+# First we'll plot these blobs using only ``imshow``.
+vmax = np.abs(weights).max()
+vmin = -vmax
+cmap = plt.cm.RdYlBu
+
+fig, ax = plt.subplots()
+ax.imshow(greys)
+ax.imshow(weights, extent=(xmin, xmax, ymin, ymax), cmap=cmap)
+ax.set_axis_off()
+
+###############################################################################
+# Blending in transparency
+# ========================
+#
+# The simplest way to include transparency when plotting data with
+# :func:`matplotlib.pyplot.imshow` is to convert the 2-D data array to a
+# 3-D image array of rgba values. This can be done with
+# :class:`matplotlib.colors.Normalize`. For example, we'll create a gradient
+# moving from left to right below.
+
+# Create an alpha channel of linearly increasing values moving to the right.
+alphas = np.ones(weights.shape)
+alphas[:, 30:] = np.linspace(1, 0, 70)
+
+# Normalize the colors b/w 0 and 1, we'll then pass an MxNx4 array to imshow
+colors = Normalize(vmin, vmax, clip=True)(weights)
+colors = cmap(colors)
+
+# Now set the alpha channel to the one we created above
+colors[..., -1] = alphas
+
+# Create the figure and image
+# Note that the absolute values may be slightly different
+fig, ax = plt.subplots()
+ax.imshow(greys)
+ax.imshow(colors, extent=(xmin, xmax, ymin, ymax))
+ax.set_axis_off()
+
+###############################################################################
+# Using transparency to highlight values with high amplitude
+# ==========================================================
+#
+# Finally, we'll recreate the same plot, but this time we'll use transparency
+# to highlight the extreme values in the data. This is often used to highlight
+# data points with smaller p-values. We'll also add in contour lines to
+# highlight the image values.
+
+# Create an alpha channel based on weight values
+# Any value whose absolute value is > .0001 will have zero transparency
+alphas = Normalize(0, .3, clip=True)(np.abs(weights))
+alphas = np.clip(alphas, .4, 1)  # alpha value clipped at the bottom at .4
+
+# Normalize the colors b/w 0 and 1, we'll then pass an MxNx4 array to imshow
+colors = Normalize(vmin, vmax)(weights)
+colors = cmap(colors)
+
+# Now set the alpha channel to the one we created above
+colors[..., -1] = alphas
+
+# Create the figure and image
+# Note that the absolute values may be slightly different
+fig, ax = plt.subplots()
+ax.imshow(greys)
+ax.imshow(colors, extent=(xmin, xmax, ymin, ymax))
+
+# Add contour lines to further highlight different levels.
+ax.contour(weights[::-1], levels=[-.1, .1], colors='k', linestyles='-')
+ax.set_axis_off()
+plt.show()
+
+ax.contour(weights[::-1], levels=[-.0001, .0001], colors='k', linestyles='-')
+ax.set_axis_off()
+plt.show()