Fixed alpha operations on (PNG) images with transparency

[NicoWeio]

Overview: What does this pull request change?

Methods like set_opacity, fade and set_color now preserve transparent areas in the image instead of overriding them.

Note: This depends on #2924.

Motivation and Explanation: Why and how do your changes improve the library?

The previous behavior was probably not intended in the first place, thus a bug.
If I add a PNG icon with transparency, for example, I don't expect the formerly transparent background to become (mostly) black when calling set_opacity(0.9).

Further Information and Comments

My Discord message

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[kolibril13]

Hi @NicoWeio,
thanks a lot for this pull request, that's a great improvement to the library.
I've just tested the code locally and it works, and also the implementation looks good to me!
One question:
Is there a reason why you use once

int(self.orig_alpha_pixel_array * alpha)

and the other time

(self.orig_alpha_pixel_array * alpha).astype(int)

?
From my side, it can be merged like this, I just keep the pr open so that other potential reviewers can have a look.
One nice extra to this pr would be a new example in the docs, I was thinking of e.g. this:

class Example1(Scene):
    def construct(self):
        im1 = ImageMobject("translate.png").scale(5)
        self.add(im1)
        self.wait(0.7)
        im1.set_opacity(0.7)

        self.play(im1.animate.set_color(YELLOW))
        self.play(FadeOut(im1))

Example1@2022-08-15@18-26-18.mp4

[NicoWeio]

Thanks for taking a look at it!

In fact, int(self.orig_alpha_pixel_array * alpha) didn't even work, since self.orig_alpha_pixel_array isn't a scalar. Secondly, I didn't include it in my test. Oops. 😄
Now, I rely on implicit conversion to int, which definitely works.

I'm not sure how to reference our PNG file in the example, otherwise I'd gladly include it.

[kolibril13]

You can use either this NumPy array then:
https://docs.manim.community/en/stable/reference/manim.mobject.types.image_mobject.ImageMobject.html#imagefromarray
or alternatively you can add assets to the docs like here https://docs.manim.community/en/stable/reference/manim.scene.scene.Scene.html?highlight=Soundexample#soundexample

[NicoWeio]

Thanks for the pointers.

While creating this PR, I noticed another bug, which I just opened #2924 for.
And while investigating said bug, I found that pixel_array is explicitly excluded from hashing.
Now that I introduced orig_alpha_pixel_array in this PR, I think we should wait with merging it until #2924 is sorted out conceptually, so that either orig_alpha_pixel_array is exempt from hashing, too, or pixel_array no longer is.

[github-actions]

This PR/issue depends on:

• ImageMobjects are not properly hashed #2924
  By Dependent Issues (🤖). Happy coding!

[behackl]

Closing this as stale.