Black Texture on Degenerate Surface Regions

[TiagoMLucio]

Describe the bug

The texture does not render correctly on certain TexturedSurfaces (Sphere, Cone, Disk3D). In specific areas it appears completely black.

Code:

from manimlib import *


class SurfaceExample(Scene):
    CONFIG = {
        "camera_class": ThreeDCamera,
    }

    def construct(self):
        frame = self.camera.frame
        frame.set_euler_angles(
            theta=55 * DEGREES,
            phi=70 * DEGREES,
        )

        sphere = Sphere(radius=2.5, resolution=(4, 4))

        texture = "https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Whole_world_-_land_and_oceans.jpg/1280px-Whole_world_-_land_and_oceans.jpg"
        earth = TexturedSurface(sphere, texture, opacity=0.9)

        sphere.shift(LEFT * 3)
        earth.shift(RIGHT * 3)

        self.play(FadeIn(sphere))
        self.play(FadeIn(earth))

        self.wait()

Wrong display or Error traceback:

It is mainly noticeable in surfaces with low resolution:

Resolution (4, 4):

Resolution (11, 11):

Additional context

The issue occurs at surface points where one of the derivative vectors is degenerate:

point − du_point = 0 or point − dv_point = 0

Potential Fix

I added a conditional check in the texture_surface shader to detect du or dv with zero lengths, in which case the unit_normal must be calculated in a different way.

shaders/textured_surface/vert.glsl:

#version 330

in vec3 point;
in vec3 du_point;
in vec3 dv_point;
in vec2 im_coords;
in float opacity;

out vec3 v_point;
out vec3 v_unit_normal;
out vec2 v_im_coords;
out float v_opacity;

#INSERT emit_gl_Position.glsl
#INSERT get_unit_normal.glsl

void main(){
    v_point = point;
    vec3 du = du_point - point;
    vec3 dv = dv_point - point;

    if(length(dv) < 1e6 || length(du) < 1e-6){
        v_unit_normal = normalize(point);
    } else {
        v_unit_normal = normalize(cross(normalize(du), normalize(dv)));
    }

    v_im_coords = im_coords;
    v_opacity = opacity;
    emit_gl_Position(point);
}

The analytical normal of a sphere was used in the fallback case.

Results

Resolution (4, 4):

Resolution (11, 11):

Conclusion

I am not sure if this was the ideal solution, but it is working perfectly for various resolutions and surfaces now!

[Varniex]

I guess this has been resolved with the latest commit. I tested your code with the resolution (4, 4), and I didn't encounter any black space / texture.