Matplotlib save image with specific colours only

Question

I am using the following example code to create image masks. The masks need to have one of 6 colours - 5 given colours and white background. However when I look at the unique colours of an image saved using code below, there are 25 colours generated. Using matplotlib is there a way to limit the colours generated on save?

def rect(x,y,w,h,c):
    ax = plt.gca()
    polygon = plt.Rectangle((x,y),w,h,color=c)
    ax.add_patch(polygon)

X = np.arange(0,10,0.1)   
F = np.zeros(int(len(X)/5))
for f in range(1,5):
    Fn=np.full(int(len(X)/5), f, dtype=int)
    F = np.append(F, Fn, axis=0)

Y = np.sin(X)
plt.plot(X,Y, alpha=0.0)
plt.xlim([0, 10])
dx = X[1]-X[0]

ccc = ['#996633','blue','yellow','red','green']
cmap = colors.ListedColormap(ccc[0:len(ccc)], 'indexed')
for n, (x,y, f) in enumerate(zip(X,Y,F)):
    color = cmap(int(f))
    rect(x,0,dx,y,color)
plt.axis('off')
plt.savefig(f'5_colours_only.png', transparent = False, bbox_inches = 'tight', pad_inches = 0)

code to test image colours:

def get_unique_colours(img_name):
    im = pil_image.open(img_name)
    by_color = defaultdict(int)
    for pixel in im.getdata():
        by_color[pixel] += 1
    return by_color

Image added, bulk of colours are the 5 generated plus white but there are 160 pixels or so of nearly the same colour but not exacltly as specified. — proximacentauri
– proximacentauri, Commented Dec 8, 2019 at 8:24
Unless I’m mistaken they are occurring at the boundaries between colors? — William Miller
– William Miller, Commented Dec 8, 2019 at 8:27
Yes, may be due to antialiasing? I couldn't find a way to disable in matplotlib — proximacentauri
– proximacentauri, Commented Dec 8, 2019 at 8:29
matplotlib.patches.Patch objects have the bool keyword antialiased, assuming rect here is a matplotlib.patches.Rectangle object (whose base class is matplotlib.patches.Patch) then you should be able to pass antialiased=False in their construction — William Miller
– William Miller, Commented Dec 8, 2019 at 8:35

William Miller · Accepted Answer · 2019-12-08 08:54:37Z

1

It would appear the extra colors are generated by the antialiasing, matplotlib.patches.Patch objects (which matplotlib.patches.Rectangle objects inherit) support disabling this via the bool keyword antialising, so

plt.Rectangle((x,y),w,h,color=c,antialiased=False)

should prevent this.

This can also be done after construction via the method

polygon.set_antialiased()

answered Dec 8, 2019 at 8:54

William Miller

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