如何在灰度中转换 Cartopy 瓷砖？

Question

I'm trying to display tiles downloaded via Cartopy's img_tiles module in gray scale. That's because I would like to overlay color-coded satellite data and I would prefer a background as little distracting as possible.

The various tiles providers as documented in the source code allow the use of a desired_tile_form during initialization. The documentation is not entirely clear about the possible values one could assign to that parameter (only "RGB" is mentioned), but digging a bit in the code I see that it is passed to img.convert , which is a PIL.image method.

Now the PIL documentation says:

The current version supports all possible conversions between “L”, “RGB” and “CMYK.” The matrix argument only supports “L” and “RGB”.

When translating a color image to black and white (mode “L”), the library uses the ITU-R 601-2 luma transform: [...]

So it would appear that using the "L" mode would turn the image in black and white, however that doesn't really seem to work in my case.

Example:

Standard "RGB" mode:

import matplotlib.pyplot as plt    
import cartopy.crs as ccrs
import cartopy.io.img_tiles as cimgt

figure, ax = plt.subplots(figsize=(15, 10))
request = cimgt.StamenTerrain()
ax = plt.axes(projection=request.crs)
ax.set_extent([94, 102, 0, 6])
ax.add_image(request, 8)
plt.show()

(Not working) Black and white mode:

figure, ax = plt.subplots(figsize=(15, 10))
request = cimgt.StamenTerrain(desired_tile_form="L")

ax = plt.axes(projection=request.crs)
ax.set_extent([94, 102, 0, 6])
ax.add_image(request, 8)
plt.show()

I had similar results with GoogleTiles() .

What is the correct way to turn tiles to black and white?

Answer 1

I'm not sure if it's by design, but it looks like the default colormap is being used here. You can pass a different colormap to the add_image() method, eg:

ax.add_image(request, 8, cmap='gray')

Here's a list of colormaps available in Matplotlib.