imshow(..., alpha=0.5) produces different results in 2.x

[QuLogic]

I think I have whittled this down sufficiently from the Cartopy example that is failing. Running imshow(..., alpha=0.5) produces a different result in 2.0.0b1 vs. 1.5.1 (both installed via conda.)

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

# mpl.style.use('classic')

im = np.zeros((100, 100, 3), dtype=np.uint8)
plt.imshow(im, extent=(1, 10, 1, 10), alpha=0.5)
plt.xlim(0, 11)
plt.ylim(0, 11)

plt.savefig('imshow_alpha.png')

Using 1.5.1:

Using 2.0.0b1:

Using 2.0.0b1 with the classic style:

GIMP tells me that the first is 50%, while the latter two are 75%.

[WeatherGod]

@mdboom, is this a result of the image architecture overhaul?

On Mon, Jun 6, 2016 at 1:47 AM, Elliott Sales de Andrade <
notifications@github.com> wrote:

I think I have whittled this down sufficiently from the Cartopy example
that is failing. Running imshow(..., alpha=0.5) produces a different
result in 2.0.0b1 vs. 1.5.1 (both installed via conda.)

import numpy as npimport matplotlib.pyplot as pltimport matplotlib as mpl

mpl.style.use('classic')

im = np.zeros((100, 100, 3), dtype=np.uint8)
plt.imshow(im, extent=(1, 10, 1, 10), alpha=0.5)
plt.xlim(0, 11)
plt.ylim(0, 11)

plt.savefig('imshow_alpha.png')

Using 1.5.1:
[image: imshow_alpha_1 5 1]
https://cloud.githubusercontent.com/assets/302469/15812915/443b626e-2b88-11e6-89ca-14c0afbf4648.png
Using 2.0.0b1:
[image: imshow_alpha_2 0]
https://cloud.githubusercontent.com/assets/302469/15812917/4d8bc746-2b88-11e6-8e9f-68a37cd456d6.png
Using 2.0.0b1 with the classic style:
[image: imshow_alpha_2 0_classic]
https://cloud.githubusercontent.com/assets/302469/15812919/543d038e-2b88-11e6-9607-e80ae97955f6.png

GIMP tells me that the first is 50%, while the latter two are 75%.

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[WeatherGod]

I would be curious to know how the result compares with output from
gtkcairo.

On Tue, Jun 7, 2016 at 11:32 AM, Benjamin Root ben.v.root@gmail.com wrote:

@mdboom, is this a result of the image architecture overhaul?

On Mon, Jun 6, 2016 at 1:47 AM, Elliott Sales de Andrade <
notifications@github.com> wrote:

I think I have whittled this down sufficiently from the Cartopy example
that is failing. Running imshow(..., alpha=0.5) produces a different
result in 2.0.0b1 vs. 1.5.1 (both installed via conda.)

import numpy as npimport matplotlib.pyplot as pltimport matplotlib as mpl

mpl.style.use('classic')

im = np.zeros((100, 100, 3), dtype=np.uint8)
plt.imshow(im, extent=(1, 10, 1, 10), alpha=0.5)
plt.xlim(0, 11)
plt.ylim(0, 11)

plt.savefig('imshow_alpha.png')

Using 1.5.1:
[image: imshow_alpha_1 5 1]
https://cloud.githubusercontent.com/assets/302469/15812915/443b626e-2b88-11e6-89ca-14c0afbf4648.png
Using 2.0.0b1:
[image: imshow_alpha_2 0]
https://cloud.githubusercontent.com/assets/302469/15812917/4d8bc746-2b88-11e6-8e9f-68a37cd456d6.png
Using 2.0.0b1 with the classic style:
[image: imshow_alpha_2 0_classic]
https://cloud.githubusercontent.com/assets/302469/15812919/543d038e-2b88-11e6-9607-e80ae97955f6.png

GIMP tells me that the first is 50%, while the latter two are 75%.

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[QuLogic]

I tried GTK3Cairo, but it's current broken:

Traceback (most recent call last):
  File ".../lib/matplotlib/backends/backend_gtk3cairo.py", line 45, in on_draw_event
    self._render_figure(w, h)
  File ".../lib/matplotlib/backends/backend_gtk3cairo.py", line 34, in _render_figure
    self.figure.draw (self._renderer)
  File ".../lib/matplotlib/artist.py", line 63, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File ".../lib/matplotlib/figure.py", line 1129, in draw
    renderer, self, dsu, self.suppressComposite)
  File ".../lib/matplotlib/image.py", line 139, in _draw_list_compositing_images
    a.draw(renderer)
  File ".../lib/matplotlib/artist.py", line 63, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File ".../lib/matplotlib/axes/_base.py", line 2373, in draw
    mimage._draw_list_compositing_images(renderer, self, dsu)
  File ".../lib/matplotlib/image.py", line 163, in _draw_list_compositing_images
    flush_images()
  File ".../lib/matplotlib/image.py", line 147, in flush_images
    image_group[0].draw(renderer)
  File ".../lib/matplotlib/artist.py", line 63, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File ".../lib/matplotlib/image.py", line 472, in draw
    renderer.draw_image(gc, l, b, im)
  File ".../lib/matplotlib/backends/backend_cairo.py", line 180, in draw_image
    im.shape[1]*4)
TypeError: expected a writeable buffer object

[tacaswell]

It looks like alpha is getting double applied?