`FadeOut` and `.animate.set_opacity()` are not working correctly on `ImageMobject`

[chopan050]

Description of bug / unexpected behavior

After adding an ImageMobject:

• When applying .animate.set_opacity(0.1), the opacity goes up from 0.0 to 0.1 instead of going down from 1.0 to 0.1.
• Applying .animate.set_opacity(0.5) works as expected.
• When applying FadeOut, the opacity of the image goes to 1.0 instead of 0.0.
• Applying FadeIn works as it should: it starts with an opacity of 0.0, and it goes up to 0.5: the last opacity we set before.

Expected behavior

• When applying .animate.set_opacity(0.1), the opacity should have gone down from 1.0 to 0.1.
• When applying FadeOut, the opacity of the image should have gone down to 0.0.

How to reproduce the issue

class ImageScene(Scene):
    def construct(self):
        array = np.zeros((400, 400, 4), dtype=np.uint8)
        array[:, :, 1] = 255 # Green
        array[:, :, 3] = 255 # Opaque
        image = ImageMobject(array)

        self.add(image)
        self.play(image.animate.set_opacity(0.1))
        self.play(image.animate.set_opacity(0.5))
        self.play(FadeOut(image))
        self.play(FadeIn(image))

Additional media files

Current version (with bug):

ImageScene.mp4

Without this bug, it should look like this instead (I rendered this video on an earlier version of Manim):

ImageScene.mp4

Additional comments

This bug is recent. I switched to the branch for my PR #3742, last updated in July, and the error was not present. I haven't yet found the cause of this. I looked at the fading.py, transform.py, animation.py and image_mobject.py, but I didn't find any relevant change: most of them were added typehints.

NOTE: I had to change a np.float_ to np.float64 in manim.utils.iterables and convert the config.frame_rate float to a Fraction in manim.scene.scene_file_writer in order to be able to render the scene in that older branch.