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Honouring the alpha attribute when creating composite images. #1955

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Merged
pelson merged 4 commits into from
May 13, 2013
Merged

Honouring the alpha attribute when creating composite images. #1955

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2f10e68
Take Image object's alpha attribute into account when compositing mul…
Westacular Apr 25, 2013
547cb7b
Fix Cairo backend so Images are actually visible for output formats o…
Westacular Apr 26, 2013
04ee71e
Add a test case for compositing images with an alpha attribute
Westacular Apr 27, 2013
baeb6de
Updated test, and added a check for tuple length
Westacular May 8, 2013
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Add a test case for compositing images with an alpha attribute
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@Westacular
Westacular committed May 3, 2013
commit 04ee71e886d4ff8e85d133dd811b587e342326a7
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260 changes: 260 additions & 0 deletions lib/matplotlib/tests/baseline_images/test_image/image_composite_alpha.svg
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20 changes: 20 additions & 0 deletions lib/matplotlib/tests/test_image.py
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Expand Up @@ -235,6 +235,26 @@ def test_image_composite_background():
ax.set_axis_bgcolor((1, 0, 0, 0.5))
ax.set_xlim([0, 12])

@image_comparison(baseline_images=['image_composite_alpha'], remove_text=True)
def test_image_composite_alpha():
"""
Tests that the alpha value is recognized and correctly applied in the
process of compositing images together.
"""
fig = plt.figure()
ax = fig.add_subplot(111)
arr = np.arange(12).reshape(4, 3)
ax.imshow(arr, extent=[0, 2, 15, 0], alpha=0.25)
ax.imshow(arr, extent=[4, 6, 15, 0], alpha=0.5)
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@pelson pelson May 7, 2013

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What would happen if this image was RGBA? Would the alpha value still have any effect?

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@Westacular Westacular May 8, 2013

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The image's alpha channel would be multiplied by the alpha value. So, an image with alphas ranging from 0.2 to 0.8 combined with alpha=0.5 would effectively be treated like an image with alphas in the 0.1 to 0.4 range. This seems reasonable, and it's already the way the backends that don't require this compositing behave.

But now that you point it out, this test case should probably involve that. I'll update it.

ax.imshow(arr, extent=[8, 10, 15, 0])
ax.imshow(arr.T, extent=[0, 12, 2, 4], alpha=0.25)
ax.imshow(arr.T, extent=[0, 12, 6, 8], alpha=0.5)
ax.imshow(arr.T, extent=[0, 12, 10, 12], alpha=1.0)
ax.set_axis_bgcolor((0, 0, 0, 0.5))
ax.set_xlim([0, 12])
ax.set_ylim([15, 0])


if __name__=='__main__':
import nose
nose.runmodule(argv=['-s','--with-doctest'], exit=False)