import numpy as np

import matplotlib.pyplot as plt
from matplotlib.testing.decorators import image_comparison


@image_comparison(baseline_images=['agg_filter_alpha'],
                  extensions=['png', 'pdf'])
def test_agg_filter_alpha():
    ax = plt.axes()
    x, y = np.mgrid[0:7, 0:8]
    data = x**2 - y**2
    mesh = ax.pcolormesh(data, cmap='Reds', zorder=5)

    def manual_alpha(im, dpi):
        im[:, :, 3] *= 0.6
        print('CALLED')
        return im, 0, 0

    # Note: Doing alpha like this is not the same as setting alpha on
    # the mesh itself. Currently meshes are drawn as independent patches,
    # and we see fine borders around the blocks of color. See the SO
    # question for an example: https://stackoverflow.com/questions/20678817
    mesh.set_agg_filter(manual_alpha)

    # Currently we must enable rasterization for this to have an effect in
    # the PDF backend.
    mesh.set_rasterized(True)

    ax.plot([0, 4, 7], [1, 3, 8])