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Fix array alpha to multiply (not replace) existing RGBA alpha #30795

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Merged
timhoffm merged 1 commit into from
Mar 2, 2026
Merged

Fix array alpha to multiply (not replace) existing RGBA alpha #30795

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11 changes: 7 additions & 4 deletions doc/api/next_api_changes/behavior/28437-CH.rst
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Original file line number Diff line number Diff line change
@@ -1,8 +1,11 @@
*alpha* parameter handling on images
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When passing and array to ``imshow(..., alpha=...)``, the parameter was silently ignored
if the image data was an RGB or RBGA image or if :rc:`image.interpolation_stage`
resolved to "rbga".
Prior to Matplotlib 3.10.1, when passing an array to ``imshow(..., alpha=...)``, the
parameter was silently ignored if the image data was an RGB or RGBA image or if
:rc:`image.interpolation_stage` resolved to "rbga".

This is now fixed, and the alpha array overwrites any previous transparency information.
Matplotlib 3.10.1 changed this to apply the alpha array as the alpha channel,
overwriting any existing transparency information in the image data. Matplotlib v3.11.0
further fixes the handling for RGBA images: the existing alpha channel is now multiplied
by the alpha array, consistent with how scalar alpha values are handled.
6 changes: 4 additions & 2 deletions lib/matplotlib/image.py
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Expand Up @@ -512,8 +512,10 @@ def _make_image(self, A, in_bbox, out_bbox, clip_bbox, magnification=1.0,
if A.shape[2] == 3: # image has no alpha channel
A = np.dstack([A, np.ones(A.shape[:2])])
elif np.ndim(alpha) > 0: # Array alpha
# user-specified array alpha overrides the existing alpha channel
A = np.dstack([A[..., :3], alpha])
if A.shape[2] == 3: # RGB: use array alpha directly
A = np.dstack([A, alpha])
else: # RGBA: multiply existing alpha by array alpha
A = np.dstack([A[..., :3], A[..., 3] * alpha])
else: # Scalar alpha
if A.shape[2] == 3: # broadcast scalar alpha
A = np.dstack([A, np.full(A.shape[:2], alpha, np.float32)])
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7 changes: 4 additions & 3 deletions lib/matplotlib/tests/test_image.py
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Expand Up @@ -1857,7 +1857,7 @@ def test_interpolation_stage_rgba_respects_alpha_param(fig_test, fig_ref, intp_s
axs_ref[0][2].imshow(im_rgba, interpolation_stage=intp_stage)

# When the image already has an alpha channel, multiply it by the
# scalar alpha param, or replace it by the array alpha param
# alpha param (both scalar and array alpha multiply the existing alpha)
axs_tst[1][0].imshow(im_rgba)
axs_ref[1][0].imshow(im_rgb, alpha=array_alpha)
axs_tst[1][1].imshow(im_rgba, interpolation_stage=intp_stage, alpha=scalar_alpha)
Expand All @@ -1869,7 +1869,8 @@ def test_interpolation_stage_rgba_respects_alpha_param(fig_test, fig_ref, intp_s
new_array_alpha = np.random.rand(ny, nx)
axs_tst[1][2].imshow(im_rgba, interpolation_stage=intp_stage, alpha=new_array_alpha)
axs_ref[1][2].imshow(
np.concatenate( # combine rgb channels with new array alpha
(im_rgb, new_array_alpha.reshape((ny, nx, 1))), axis=-1
np.concatenate( # combine rgb channels with multiplied array alpha
(im_rgb, array_alpha.reshape((ny, nx, 1))
* new_array_alpha.reshape((ny, nx, 1))), axis=-1
), interpolation_stage=intp_stage
)
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