Fixed alpha operations on (PNG) images with transparency

Previously, calling set_opacity(alpha) would have replaced the original alpha channel, thus making previously invisible parts visible.

Author: NicoWeio

manim/mobject/types/image_mobject.py

@@ -189,6 +189,7 @@ def __init__(
         )
         if self.invert:
             self.pixel_array[:, :, :3] = 255 - self.pixel_array[:, :, :3]
+        self.orig_alpha_pixel_array = self.pixel_array[:, :, 3].copy()
         super().__init__(scale_to_resolution, **kwargs)
 
     def get_pixel_array(self):
@@ -199,7 +200,7 @@ def set_color(self, color, alpha=None, family=True):
         rgb = color_to_int_rgb(color)
         self.pixel_array[:, :, :3] = rgb
         if alpha is not None:
-            self.pixel_array[:, :, 3] = int(255 * alpha)
+            self.pixel_array[:, :, 3] = int(self.orig_alpha_pixel_array * alpha)
         for submob in self.submobjects:
             submob.set_color(color, alpha, family)
         self.color = color
@@ -214,7 +215,7 @@ def set_opacity(self, alpha):
             The alpha value of the object, 1 being opaque and 0 being
             transparent.
         """
-        self.pixel_array[:, :, 3] = int(255 * alpha)
+        self.pixel_array[:, :, 3] = (self.orig_alpha_pixel_array * alpha).astype(int)
         self.fill_opacity = alpha
         self.stroke_opacity = alpha
         return self