Adding PatchCollection with Affine Transformations

Question

I have some patches on which I apply different Affine2D transformations in matplotlib. Is there a possibility to add them as a collections.PatchCollection? Somehow I am only able to draw them if I call ax.add_patch() separately for each of them.

from matplotlib import pyplot as plt, patches, collections, transforms

fig, ax = plt.subplots()

trafo1 = transforms.Affine2D().translate(+0.3, -0.3).rotate_deg_around(0, 0, 45) + ax.transData
trafo2 = transforms.Affine2D().translate(+0.3, -0.3).rotate_deg_around(0, 0, 65) + ax.transData

rec1 = patches.Rectangle(xy=(0.1, 0.1), width=0.2, height=0.3, transform=trafo1, color='blue')
rec2 = patches.Rectangle(xy=(0.2, 0.2), width=0.3, height=0.2, transform=trafo2, color='green')

ax.add_collection(collections.PatchCollection([rec1, rec2], color='red', zorder=10))

# ax.add_patch(rec1)
# ax.add_patch(rec2)

Answer 1

It looks like PatchCollection does not support individually transformed elements. From the Matplotlib documentation, we can read a Collection is a

class for the efficient drawing of large collections of objects that share most properties, e.g., a large number of line segments or polygons.

You can understand this with creating the collection without any individually transformed patches:

rec1 = patches.Rectangle(xy=(0.1, 0.1), width=0.2, height=0.3, color='blue')
rec2 = patches.Rectangle(xy=(0.2, 0.2), width=0.3, height=0.2, color='green')
col = collections.PatchCollection([rec1, rec2], color='red', zorder=10)
print(col.get_transform())

That prints IdentityTransform() for the last statement, and correctly displays the (non-transformed) patches. These patches can be transformed all-at-once from the PatchCollection, without individual specification.

On the contrary, when you apply the individual transform for each patch (like in your case), the .get_transform() method returns an empty list. This is probably due to the fact that PatchCollection classes are made to gather patches with a lot of common attributes in order to accelerate the drawing efficiency (as mentioned above), including the transform attribute.

Note: on this answer, you can find a workaround with a patch to path conversion, then to a PathCollection with an increase drawing efficiency compared to individual patch draw.