OpenGL depth test doesn't like transparent objects, such as glowdots.

[colinrubow]

Description of bug / unexpected behavior

A glowdot can be made by nesting a bunch of gradually more transparent balls. The OpenGL renderer depth tester doesn't behave well with this situation and renders it as a dimmer, dull, no longer glowing dot. (Problems with transparancy is apparently a known common OpenGL problem, not directly a Manim problem).

Expected behavior

A glowing dot that correctly moves in front of and behind other objects. If I have a static camera pose, then I can be more careful about the order I add objects, but if my camera pose is dynamic, this won't work.

How to reproduce the issue

Code for reproducing the problem

class Tester(ThreeDScene):
    def construct(s):
        sphere_1 = OpenGLSurface(lambda u, v: np.array([np.cos(u)*np.sin(v), np.sin(u)*np.sin(v), -np.cos(v)]), (0, TAU), (0, PI), color=BLUE_D)

        glowdot = GlowDot(point = 2*RIGHT, radius=70, num_layers=20, color_fade=[YELLOW, YELLOW_E])

        glowdot_bad = GlowDot(point = 4*RIGHT, radius=70, num_layers=20, color_fade=[YELLOW, YELLOW_E])
        glowdot_bad.apply_depth_test()

        s.add(glowdot, glowdot_bad, sphere_1)

        # orbit animation
        orbit_1 = Circle(radius=2).rotate(PI/2, RIGHT)
        orbit_2 = Circle(radius=4).rotate(PI/2, RIGHT)

        s.play(MoveAlongPath(glowdot, orbit_1, run_time=3), MoveAlongPath(glowdot_bad, orbit_2, run_time=6), rate_func=rate_functions.linear)
        s.pause()

class GlowDot(OpenGLPGroup):
    def __init__(
            self,
            point: np.ndarray = ORIGIN,
            radius: float = 1,
            num_layers: int = 80,
            color_fade: ManimColor = [YELLOW, YELLOW_E],
            **kwargs
    ) -> None:
        self.point = point
        self.radius = radius
        self.num_layers = num_layers
        self.color_fade = color_fade
        super().__init__(**kwargs)
        self.init_orbs()

    def init_orbs(self):
        color_list = color_gradient(self.color_fade, self.num_layers)
        opacity_func = lambda t: 1500 * (1 - abs(t - 0.009) ** 0.0001)
        rate_func = lambda t: t**2

        for i in range(self.num_layers):
            orb = OpenGLPMobject(
                stroke_width=self.radius*rate_func((0.5 + i)/self.num_layers),
                color=color_list[i],
                opacity=opacity_func(rate_func(i/self.num_layers))
            )
            orb.add_points(self.point)
            self.add(orb)

Patch Solution I Made

The issue seems to be with the OpenGL depth tester in regards to transparent objects. My bad idea patch solution is in my scene construct to set depth testing for all opaque objects but disable depth testing for transparent objects. Then I modified the update_frame method of class OpenGLRenderer to sort the mobjects in order of farthest to nearest to the camera. This performs expected behavior almost always. Why do depth testing for opaque objects then? For some reason some objects look better with it on (e.g. textured surfaces).

code for my bad idea patch

def update_frame(self, scene):
        self.frame_buffer_object.clear(*self.background_color)
        self.refresh_perspective_uniforms(scene.camera)

        ### MY CODE FIXME ###
        def z_key(mob):
            # Assign a number to a three dimensional mobjects
            # based on how close it is to the camera
            return np.linalg.norm(self.camera.get_position() - mob.get_center())
        scene.mobjects = sorted(scene.mobjects, key=z_key, reverse=True)
        ##############

        for mobject in scene.mobjects:
            if not mobject.should_render:
                continue
            self.render_mobject(mobject)

        for obj in scene.meshes:
            for mesh in obj.get_meshes():
                mesh.set_uniforms(self)
                mesh.render()

        self.animation_elapsed_time = time.time() - self.animation_start_time

Additional media files

Images/GIFs

Both glowdots orbit the planet. The outer glowdot is depth tested. It doesn't glow, has weird border issues, and even blocks the planet. The inner glowdot glows, but due to the order added, only shows behind the planet.

Logs

Terminal output

The debug output contains the following repeated many times:

DEBUG    \surface\geom.glsl does not exist.        shader_wrapper.py:35
DEBUG    C:\Users\colin\AppData\Local\Programs\Pyt shader_wrapper.py:35
         hon\Python312\Lib\site-packages\manim\ren
         derer\shaders\surface\geom.glsl does not
         exist.

System specifications

System Details

• OS: Windows 10 Home v2022
• RAM: 8GB
• Python version: 3.12.9
• Installed modules:

Package                 Version
----------------------- -----------
aiohappyeyeballs        2.4.6
aiohttp                 3.11.12
aiortsp                 1.4.0
aiosignal               1.3.2
annotated-types         0.7.0
asttokens               3.0.0
attrs                   25.1.0
av                      13.1.0
beaupy                  3.10.1
beautifulsoup4          4.13.3
Brotli                  1.1.0
click                   8.1.8
cloup                   3.0.5
colorama                0.4.6
contourpy               1.3.1
cycler                  0.12.1
decorator               5.1.1
dpkt                    1.9.8
emoji                   2.14.1
executing               2.2.0
fonttools               4.56.0
frozenlist              1.5.0
glcontext               3.0.0
idna                    3.10
ifaddr                  0.2.0
ipython                 8.32.0
isosurfaces             0.1.2
jedi                    0.19.2
kiwisolver              1.4.8
manim                   0.19.0
ManimPango              0.6.0
mapbox_earcut           1.0.3
markdown-it-py          3.0.0
matplotlib              3.10.0
matplotlib-inline       0.1.7
mdurl                   0.1.2
moderngl                5.12.0
moderngl-window         3.1.1
multidict               6.1.0
networkx                3.4.2
numpy                   2.2.2
packaging               24.2
pandas                  2.2.3
parso                   0.8.4
pillow                  11.1.0
pip                     25.0.1
pl-neon-recording       0.1.12
prompt_toolkit          3.0.50
propcache               0.2.1
protobuf                5.29.3
pupil_labs_realtime_api 1.3.6
pure_eval               0.2.3
pycairo                 1.27.0
pydantic                2.10.6
pydantic_core           2.27.2
pydub                   0.25.1
pyglet                  2.1.2
pyglm                   2.8.0
Pygments                2.19.1
PyOpenGL                3.1.9
pyparsing               3.2.1
PyQt5                   5.15.11
PyQt5-Qt5               5.15.2
PyQt5_sip               12.17.0
pyqtgraph               0.13.7
pyserial                3.5
python-dateutil         2.9.0.post0
python-yakh             0.4.1
pytz                    2025.1
questo                  0.4.1
rich                    13.9.4
scipy                   1.15.1
screeninfo              0.8.1
six                     1.17.0
skia-pathops            0.8.0.post2
soupsieve               2.6
srt                     3.5.3
stack-data              0.6.3
structlog               25.1.0
svgelements             1.9.6
tqdm                    4.67.1
traitlets               5.14.3
typing_extensions       4.12.2
tzdata                  2025.1
watchdog                6.0.0
wcwidth                 0.2.13
websockets              14.2
yarl                    1.18.3
zeroconf                0.144.1