修改matplotlib颜色图

Question

I'm trying to produce a similar version of this image using Python:

I'm close but can't quite figure out how to modify a matplotlib colormap to make values <0.4 go to white. I tried masking those values and using set_bad but I ended up with a real blocky appearance, losing the nice smooth contours seen in the original image.

• Result with continuous colormap (problem: no white):

  

• Result with set_bad (problem: no smooth transition to white):

  

Code so far:

from netCDF4 import Dataset as NetCDFFile
import matplotlib.pyplot as plt
import numpy as np
from mpl_toolkits.basemap import Basemap
nc = NetCDFFile('C:/myfile1.nc')
nc1 = NetCDFFile('C:/myfile2.nc')
lat = nc.variables['lat'][:]
lon = nc.variables['lon'][:]
time = nc.variables['time'][:]
uwnd = nc.variables['uwnd'][:]
vwnd = nc1.variables['vwnd'][:]
map = Basemap(llcrnrlon=180.,llcrnrlat=0.,urcrnrlon=340.,urcrnrlat=80.)
lons,lats = np.meshgrid(lon,lat)
x,y = map(lons,lats)
speed = np.sqrt(uwnd*uwnd+vwnd*vwnd)
#speed = np.ma.masked_where(speed < 0.4, speed)
#cmap = plt.cm.jet
#cmap.set_bad(color='white')
levels = np.arange(0.0,3.0,0.1)
ticks = np.arange(0.0,3.0,0.2)
cs = map.contourf(x,y,speed[0],levels, cmap='jet')
vw = plt.quiver(x,y,speed)
cbar = plt.colorbar(cs, orientation='horizontal', cmap='jet', spacing='proportional',ticks=ticks)
cbar.set_label('850 mb Vector Wind Anomalies (m/s)')
map.drawcoastlines()
map.drawparallels(np.arange(20,80,20),labels=[1,1,0,0], linewidth=0.5)
map.drawmeridians(np.arange(200,340,20),labels=[0,0,0,1], linewidth=0.5)
#plt.show()
plt.savefig('phase8_850wind_anom.png',dpi=600)

Answer 1

The answer to get the result smooth lies in constructing your own colormap. To do this one has to create an RGBA-matrix: a matrix with on each row the amount (between 0 and 1) of Red, Green, Blue, and Alpha (transparency; 0 means that the pixel does not have any coverage information and is transparent).

As an example the distance to some point is plotted in two dimensions. Then:

• For any distance higher than some critical value, the colors will be taken from a standard colormap.
• For any distance lower than some critical value, the colors will linearly go from white to the first color of the previously mentioned map.

The choices depend fully on what you want to show. The colormaps and their sizes depend on your problem. For example, you can choose different types of interpolation: linear, exponential, ...; single- or multi-color colormaps; etc..

The code:

import numpy             as np
import matplotlib        as mpl
import matplotlib.pyplot as plt

from mpl_toolkits.axes_grid1 import make_axes_locatable

# create colormap
# ---------------

# create a colormap that consists of
# - 1/5 : custom colormap, ranging from white to the first color of the colormap
# - 4/5 : existing colormap

# set upper part: 4 * 256/4 entries
upper = mpl.cm.jet(np.arange(256))

# set lower part: 1 * 256/4 entries
# - initialize all entries to 1 to make sure that the alpha channel (4th column) is 1
lower = np.ones((int(256/4),4))
# - modify the first three columns (RGB):
#   range linearly between white (1,1,1) and the first color of the upper colormap
for i in range(3):
  lower[:,i] = np.linspace(1, upper[0,i], lower.shape[0])

# combine parts of colormap
cmap = np.vstack(( lower, upper ))

# convert to matplotlib colormap
cmap = mpl.colors.ListedColormap(cmap, name='myColorMap', N=cmap.shape[0])

# show some example
# -----------------

# open a new figure
fig, ax = plt.subplots()

# some data to plot: distance to point at (50,50)
x,y = np.meshgrid(np.linspace(0,99,100),np.linspace(0,99,100))
z   = (x-50)**2. + (y-50)**2.

# plot data, apply colormap, set limit such that our interpretation is correct
im = ax.imshow(z, interpolation='nearest', cmap=cmap, clim=(0,5000))

# add a colorbar to the bottom of the image
div  = make_axes_locatable(ax)
cax  = div.append_axes('bottom', size='5%', pad=0.4)
cbar = plt.colorbar(im, cax=cax, orientation='horizontal')

# save/show the image
plt.savefig('so.png')
plt.show()

The result: