python - something like plt.matshow but with triangles

Question

Basically, I'd like to make something like the following (triangles not squares as is typically used with plt.matshow).

One could start with four 2D arrays, each representing the values for the colours of a set of triangles: right, left, bottom, top:

import numpy as np
right=np.random.randn(8, 8)
left=np.random.randn(8, 8)
bottom=np.random.randn(8, 8)
top=np.random.randn(8, 8)

But I have no idea about the plotting...

Answer 1

You may indeed use tripcolor to plot a set of triangles. In the code below the function quatromatrix takes 4 2D arrays of values to colormap as input, creates the triangles and rearanges the colors to fit to the respective positions. It is thus very similar to plotting 4 imshow plots.

import matplotlib.pyplot as plt
import numpy as np

def quatromatrix(left, bottom, right, top, ax=None, triplotkw={},tripcolorkw={}):
    if not ax: ax=plt.gca()
    n = left.shape[0]; m=left.shape[1]

    a = np.array([[0,0],[0,1],[.5,.5],[1,0],[1,1]])
    tr = np.array([[0,1,2], [0,2,3],[2,3,4],[1,2,4]])

    A = np.zeros((n*m*5,2))
    Tr = np.zeros((n*m*4,3))

    for i in range(n):
        for j in range(m):
            k = i*m+j
            A[k*5:(k+1)*5,:] = np.c_[a[:,0]+j, a[:,1]+i]
            Tr[k*4:(k+1)*4,:] = tr + k*5

    C = np.c_[ left.flatten(), bottom.flatten(), 
              right.flatten(), top.flatten()   ].flatten()

    triplot = ax.triplot(A[:,0], A[:,1], Tr, **triplotkw)
    tripcolor = ax.tripcolor(A[:,0], A[:,1], Tr, facecolors=C, **tripcolorkw)
    return tripcolor


right=np.random.randn(8, 8)
left=np.random.randn(8, 8)
bottom=np.random.randn(8, 8)
top=np.random.randn(8, 8)

fig, ax=plt.subplots()

quatromatrix(left, bottom, right, top, ax=ax,
             triplotkw={"color":"k", "lw":1},
             tripcolorkw={"cmap": "plasma"}) 

ax.margins(0)
ax.set_aspect("equal")