The alpha parameter in geom_imshow()

import numpy as np
from lets_plot import *

LetsPlot.setup_html()

# Load an image

from PIL import Image
import requests
from io import BytesIO

response = requests.get('https://github.com/JetBrains/lets-plot-docs/raw/master/source/examples/cookbook/images/fisher_boat.png')

image = Image.open(BytesIO(response.content))
im_arr = np.asarray(image)
im_arr.shape

(225, 225, 3)

p = ggplot() + geom_point(x=112, y=112, color="dark-blue", size=100, shape=15) + ggsize(450, 450)
p + ggtitle("The background blue square") + ggsize(400, 300)

1. alpha parameter and color image

pair1 = GGBunch()
pair1.add_plot(p + geom_imshow(im_arr) + ggtitle("no alpha (opaque image)"), 0, 0)
pair1.add_plot(p + geom_imshow(im_arr, alpha=0.7) + ggtitle("alpha=0.7"), 500, 0)
pair1

2. alpha parameter and grayscale image

im_arr_gs = im_arr[:,:,0]
im_arr_gs.shape

(225, 225)

pair2 = GGBunch()
pair2.add_plot(p + geom_imshow(im_arr_gs) + ggtitle("no alpha (opaque image)"), 0, 0)
pair2.add_plot(p + geom_imshow(im_arr_gs, alpha=0.7) + ggtitle("alpha=0.7"), 500, 0)
pair2

3. alpha parameter and pseudo-color image

pair3 = GGBunch()
pair3.add_plot(p + geom_imshow(im_arr_gs, "magma") + ggtitle("no alpha (opaque image)"), 0, 0)
pair3.add_plot(p + geom_imshow(im_arr_gs, "magma", alpha=0.7) + ggtitle("alpha=0.7"), 500, 0)
pair3

4. alpha parameter and grayscale image with transparent pixels (NaN values)

im_arr_gs_nan=im_arr_gs.copy()
im_arr_gs_nan=im_arr_gs_nan.astype(np.float64)
im_arr_gs_nan[im_arr_gs_nan < 80] = np.nan

pair4 = GGBunch()
pair4.add_plot(p + geom_imshow(im_arr_gs_nan, "magma", norm=False) + ggtitle("no alpha (opaque image)"), 0, 0)
pair4.add_plot(p + geom_imshow(im_arr_gs_nan, "magma", norm=False, alpha=0.7) + ggtitle("alpha=0.7"), 500, 0)
pair4