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Add alpha-array support to _rgb_to_rgba #26520

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Add alpha-array support to _rgb_to_rgba #26520

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9f51cda
Add alpha-array support to _rgb_to_rgba
AALAM98mod100 Aug 14, 2023
ce6efd3
Remove hardcoded value used in testing
AALAM98mod100 Aug 14, 2023
848100f
Added test for alpha array
AALAM98mod100 Aug 14, 2023
5240fc4
Add svg baseline image
AALAM98mod100 Aug 15, 2023
f77ac82
Combine alpha for rgba images.
AALAM98mod100 Aug 18, 2023
499949b
Merge branch 'main' into alpha-array-support-imshow
AALAM98mod100 Jul 21, 2024
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17 changes: 11 additions & 6 deletions lib/matplotlib/image.py
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Original file line number Diff line number Diff line change
Expand Up @@ -215,17 +215,16 @@
return out


def _rgb_to_rgba(A):
def _rgb_to_rgba(A, alpha=None):
"""
Convert an RGB image to RGBA, as required by the image resample C++
extension.
"""
rgba = np.zeros((A.shape[0], A.shape[1], 4), dtype=A.dtype)
if alpha is None or np.ndim(alpha) == 0:
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what input is np.ndim(alpha) == 0 addressing?

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Scalar alpha input should remaing unaffected by new code. This ensures just that

alpha = 255 if A.dtype == np.uint8 else 1.0
rgba[:, :, :3] = A
if rgba.dtype == np.uint8:
rgba[:, :, 3] = 255
else:
rgba[:, :, 3] = 1.0
rgba[:, :, 3] = alpha
return rgba


Expand Down Expand Up @@ -582,7 +581,13 @@
else:
if A.ndim == 2: # interpolation_stage = 'rgba'
self.norm.autoscale_None(A)
A = self.to_rgba(A)
A = self.to_rgba(A, alpha=self.get_alpha())
elif A.shape[2] == 3:
A = _rgb_to_rgba(A, alpha=self.get_alpha())
elif A.shape[2] == 4:
array_alpha = self.get_alpha()
if array_alpha is not None and np.ndim(array_alpha) != 0:
A[:, :, 3] *= array_alpha # blend alphas of image and param

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This modifies A in place where as in the other branches we get a copy of A back. Because it modifies in in place and we cache A higher up in the call stack it will apply the alpha everytime it draws.

alpha = self._get_scalar_alpha()
if A.shape[2] == 3:
# No need to resample alpha or make a full array; NumPy will expand
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Binary file added lib/matplotlib/tests/baseline_images/test_image/image_alpha_array.png
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215 changes: 215 additions & 0 deletions lib/matplotlib/tests/baseline_images/test_image/image_alpha_array.svg
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16 changes: 16 additions & 0 deletions lib/matplotlib/tests/test_image.py
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Expand Up @@ -255,6 +255,22 @@ def test_image_alpha():
ax3.imshow(Z, alpha=0.5, interpolation='nearest')


@image_comparison(['image_alpha_array'], remove_text=True)
def test_image_alpha_array():
_, ax = plt.subplots()

# create a 2x2 grid. If alpha is applying correctly,
# the top half should not match the 0.5 valued pixel
# in the bottom half.
arr = np.array([[.5, .5], [.75, .5]])
cmap = plt.get_cmap('gray')
norm = colors.Normalize()
arr_rgb = cmap(norm(arr))[:, :, :3]
alpha = np.ones_like(arr)
alpha[:1] = 0.2
ax.imshow(arr, alpha=alpha, cmap='gray', interpolation='none')
Comment on lines +258 to +271
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This can be better written as a check_figures_equal test. Compare a (n, m, 3) array plus additional alpha to the same plot with a (n, m, 4) array.

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It is also concerning that the png and svg images are different....



@mpl.style.context('mpl20')
@check_figures_equal(extensions=['png'])
def test_imshow_alpha(fig_test, fig_ref):
Expand Down
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