matplotlib Fix: Change alpha application for RGBA from overwrite to multiply

PR summary

Closes #26092

Per community consensus in #26092, the behavior for RGBA inputs has been changed from overwriting to blending; the image's intrinsic alpha is now multiplied with the passed alpha value. This change unifies alpha handling for 2D, RGB, and RGBA images, making the behavior consistent and predictable.

PR checklist

[x] "closes #0000" is in the body of the PR description to link the related issue
[x] new and changed code is tested
[N?A] Plotting related features are demonstrated in an example
[N/A] New Features and API Changes are noted with a directive and release note
[N/A] Documentation complies with general and docstring guidelines

Sep 07 '25 03:09 LangQi99

Old Behavior pay attention to the bottom right Figure_2

New Behavior Figure_fix

Sep 08 '25 02:09 LangQi99

Hi @LangQi99 are you still working on this one? If not, we can close in favour of #30795, which is closer to completion, but I do not want to do that if you are actively working on this.

Dec 17 '25 10:12 rcomer

I'm not working on this currently. Feel free to close it.

Dec 17 '25 10:12 LangQi99

Thanks for the quick reply @LangQi99. Closing in favour of #30795.

Dec 17 '25 10:12 rcomer